This is the MySQL reference manual; it documents MySQL Version 3.23.41. As MySQL is work in progress, the manual gets updated frequently. There is a very good chance that this version is out of date, unless you are looking at it online. The most recent version of this manual is available at http://www.mysql.com/documentation/ in many different formats. If you have a hard time finding information in the manual, you can try the searchable PHP version at http://www.mysql.com/documentation/manual.php.
MySQL is a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server.
MySQL is free software. It is licensed with the GNU GENERAL PUBLIC LICENSE http://www.gnu.org/. See section 1.3 MySQL Licensing and Support.
The MySQL home page provides the latest information about MySQL.
The following list describes some useful sections of the manual:
IMPORTANT:
Reports of errors (often called bugs), as well as questions and comments,
should be sent to the mailing list at mysql@lists.mysql.com.
See section 1.2.22.3 How to Report Bugs or Problems.
The mysqlbug
script should be used to generate bug reports.
For source distributions, the mysqlbug
script can be found in the
`scripts' directory. For binary distributions, mysqlbug
can
be found in the `bin' directory. If you have found a sensitive
security bug in MySQL, you should send an email to
security@mysql.com.
If you have any suggestions concerning additions or corrections to this manual, please send them to the manual team at docs@mysql.com.
This is a reference manual; it does not provide general instruction on SQL or relational database concepts. If you want general information about SQL, see section 1.2.2 General SQL Information and Tutorials. For books that focus more specifically on MySQL, see section 1.2.1 Books About MySQL.
MySQL, the most popular Open Source SQL database, is provided by MySQL AB. MySQL AB is a commercial company that builds its business providing services around the MySQL database. See section 1.1.2 What Is MySQL AB.
The official way to pronounce MySQL is ``My Ess Que Ell'' (not MY-SEQUEL). But we try to avoid correcting people who say MY-SEQUEL.
MySQL AB is the Swedish company owned and run by the MySQL founders and main developers. We are dedicated to developing MySQL and spreading our database to new users. MySQL AB owns the copyright to the MySQL server source code and the MySQL trademark. A significant amount of revenues from our services goes to developing MySQL. See section 1.1.1 What Is MySQL.
MySQL AB has been profitable providing MySQL from the start. We don't get any outside funding, but have earned all our money ourselves.
We are searching after partners that would like to support our development of MySQL so that we could accelerate the development pace. If you are interested in doing this, you can email partner@mysql.com about this!
MySQL AB has currently 20+ people (http://www.mysql.com/development/team.html) on its payroll and is growing rapidly.
Our main sources of income are:
The MySQL core values show our dedication to MySQL and Open Source.
We want MySQL to be:
MySQL AB and the people of MySQL AB:
This manual is currently available in Texinfo, plain text, Info, HTML,
PostScript, and PDF versions. The primary document is the Texinfo file.
The HTML version is produced automatically using a modified version of
texi2html
. The plain text and Info versions are produced with
makeinfo
. The Postscript version is produced using texi2dvi
and dvips
. The PDF version is produced with pdftex
.
This manual is written and maintained by David Axmark, Michael (Monty) Widenius, Jeremy Cole, and Paul DuBois. For other contributors, see section F Credits.
This manual uses certain typographical conventions:
constant
mysqladmin
works, invoke it with the
--help
option.''
When commands are shown that are meant to be executed by a particular
program, the program is indicated by a prompt shown before the command. For
example, shell>
indicates a command that you execute from your login
shell, and mysql>
indicates a command that you execute from the
mysql
client program:
shell> type a shell command here mysql> type a mysql command here
Shell commands are shown using Bourne shell syntax. If you are using a
csh
-style shell, you may need to issue commands slightly differently.
For example, the sequence to set an environment variable and run a command
looks like this in Bourne shell syntax:
shell> VARNAME=value some_command
For csh
, you would execute the sequence like this:
shell> setenv VARNAME value shell> some_command
Often, database, table, and column names must be substituted into commands. To
indicate that such substitution is necessary, this manual uses
db_name
, tbl_name
and col_name
. For example, you might
see a statement like this:
mysql> SELECT col_name FROM db_name.tbl_name;
This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this:
mysql> SELECT author_name FROM biblio_db.author_list;
SQL statements may be written in uppercase or lowercase. When this manual
shows a SQL statement, uppercase is used for particular keywords if those
keywords are under discussion (to emphasize them) and lowercase is used for
the rest of the statement. For example, you might see the following in a
discussion of the SELECT
statement:
mysql> SELECT count(*) FROM tbl_name;
On the other hand, in a discussion of the COUNT()
function, the
same statement would be written like this:
mysql> select COUNT(*) from tbl_name;
If no particular emphasis is intended, all keywords are written uniformly in uppercase.
In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses:
DROP TABLE [IF EXISTS] tbl_name
When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices may be chosen, the alternatives are listed within square brackets (`[' and `]'):
TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'):
{DESCRIBE | DESC} tbl_name {col_name | wild}
We once started out with the intention of using mSQL
to connect to our
tables using our own fast low-level (ISAM) routines. However, after some
testing we came to the conclusion that mSQL
was not fast enough nor
flexible enough for our needs. This resulted in a new SQL interface to our
database but with almost the same API interface as mSQL
. This API was
chosen to ease porting of third-party code.
The derivation of the name MySQL is not perfectly clear. Our base directory and a large number of our libraries and tools have had the prefix ``my'' for well over 10 years. However, Monty's daughter (some years younger) is also named My. Which of the two gave its name to MySQL is still a mystery, even for us.
The following list describes some of the important characteristics of MySQL:
FLOAT
, DOUBLE
, CHAR
, VARCHAR
,
TEXT
, BLOB
, DATE
, TIME
, DATETIME
,
TIMESTAMP
, YEAR
, SET
, and ENUM
types.
See section 6.2 Column Types.
SELECT
and WHERE
parts of queries. For example:
mysql> SELECT CONCAT(first_name, " ", last_name) FROM tbl_name WHERE income/dependents > 10000 AND age > 30;
GROUP BY
and ORDER BY
clauses. Support
for group functions (COUNT()
, COUNT(DISTINCT ...)
,
AVG()
, STD()
, SUM()
, MAX()
and MIN()
).
LEFT OUTER JOIN
and RIGHT OUTER JOIN
with ANSI
SQL and ODBC syntax.
CHAR
or VARCHAR
field.
INSERT
to insert a
subset of a table's columns; those columns that are not explicitly given
values are set to their default values.
myisamchk
, a very fast utility for table checking,
optimization, and repair. All of the functionality of myisamchk
is also available through the SQL interface as well.
See section 4 MySQL Database Administration.
DELETE
, INSERT
, REPLACE
, and UPDATE
return
the number of rows that were changed (affected). It is possible to return
the number of rows matched instead by setting a flag when connecting to the
server.
ABS
is a valid column name. The only restriction is that for a
function call, no spaces are allowed between the function name and the
`(' that follows it. See section 6.1.6 Is MySQL Picky About Reserved Words?.
--help
or -?
options to obtain online assistance.
SHOW
command can be used to retrieve
information about databases, tables, and indexes. The EXPLAIN
command
can be used to determine how the optimizer resolves a query.
This section addresses the questions ``How stable is MySQL?'' and ``Can I depend on MySQL in this project?'' We will try to clarify some issues and to answer some of the more important questions that seem to concern many people. This section has been put together from information gathered from the mailing list (which is very active in reporting bugs).
At TcX, MySQL has worked without any problems in our projects since mid-1996. When MySQL was released to a wider public, we noticed that there were some pieces of ``untested code'' that were quickly found by the new users who made queries in a manner different than our own. Each new release has had fewer portability problems than the previous one (even though each has had many new features).
Each release of MySQL has been usable, and there have been problems only when users start to use code from the ``gray zones.'' Naturally, outside users don't know what the gray zones are; this section attempts to indicate those that are currently known. The descriptions deal with Version 3.23 of MySQL. All known and reported bugs are fixed in the latest version, with the exception of the bugs listed in the bugs section, which are things that are design-related. See section 1.4.7 Known errors and design deficiencies in MySQL.
MySQL is written in multiple layers and different independent modules. These modules are listed below with an indication of how well-tested each of them is:
mysql
, mysqladmin
, mysqlshow
,
mysqldump
, and mysqlimport
.
fcntl()
). In these cases, you should run the
MySQL daemon with the --skip-locking
flag. Problems are known
to occur on some Linux systems, and on SunOS when using NFS-mounted file
systems.
fcntl()
call, which is
fixed by using the --skip-locking
option to
mysqld
. Some people have reported lockup problems with Version 0.5.
LinuxThreads will need to be recompiled if you plan to use
1000+ concurrent connections. Although it is possible to run that many
connections with the default LinuxThreads (however, you will never go
above 1021), the default stack spacing of 2 MB makes the application
unstable, and we have been able to reproduce a coredump after creating
1021 idle connections. See section 2.6.1 Linux Notes (All Linux Versions).
SELECT
statements are usually done in one time frame so there shouldn't
be a mutex locking/thread juggling.
LOAD DATA ...
, INSERT ... SELECT
-- Stable
ALTER TABLE
-- Stable
mysqlaccess
-- Stable
GRANT
-- Stable
MySQL
. They appear to work good and
can be used after some initial testing.
MERGE
tables is still not that tested. The
other part of the MERGE
code is quite well tested.
MySQL AB provides e-mail support for paying customers, but the MySQL mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release.
MySQL Version 3.22 has a 4G limit on table size. With the new
MyISAM
in MySQL Version 3.23 the maximum table size is
pushed up to 8 million terabytes (2 ^ 63 bytes).
Note, however, that operating systems have their own file size limits. Here are some examples:
Operating System | File Size Limit |
Linux-Intel 32 bit | 2G, 4G or more, depends on Linux version |
Linux-Alpha | 8T (?) |
Solaris 2.5.1 | 2G (possible 4G with patch) |
Solaris 2.6 | 4G |
Solaris 2.7 Intel | 4G |
Solaris 2.7 ULTRA-SPARC | 8T (?) |
On Linux 2.2 you can get bigger tables than 2G by using the LFS patch for the ext2 file system. On Linux 2.4 there exists also patches for ReiserFS to get support for big files.
This means that the table size for MySQL is normally limited by the operating system.
By default, MySQL tables have a maximum size of about 4G. You can
check the maximum table size for a table with the SHOW TABLE STATUS
command or with the myisamchk -dv table_name
.
See section 4.5.5 SHOW
Syntax.
If you need bigger tables than 4G (and your operating system supports
this), you should set the AVG_ROW_LENGTH
and MAX_ROWS
parameter when you create your table. See section 6.5.3 CREATE TABLE
Syntax. You can
also set these later with ALTER TABLE
. See section 6.5.4 ALTER TABLE
Syntax.
If your big table is going to be read-only, you could use
myisampack
to merge and compress many tables to one.
myisampack
usually compresses a table by at least 50%, so you can
have, in effect, much bigger tables. See section 4.7.4 myisampack, The MySQL Compressed Read-only Table Generator.
You can go around the operating system file limit for MyISAM
data
files by using the RAID
option. See section 6.5.3 CREATE TABLE
Syntax.
Another solution can be the included MERGE library, which allows you to handle a collection of identical tables as one. See section 7.2 MERGE Tables.
MySQL itself has no problems with Year 2000 (Y2K) compliance:
2069
; all 2-digit years are regarded to be in the range
1970
to 2069
, which means that if you store 01
in a
year
column, MySQL treats it as 2001
.
YEAR
column type
can store years 0
and 1901
to 2155
in 1 byte and display
them using 2 or 4 digits.
You may run into problems with applications that use MySQL in a
way that is not Y2K-safe. For example, many old applications store
or manipulate years using 2-digit values (which are ambiguous) rather than
4-digit values. This problem may be compounded by applications that use
values such as 00
or 99
as ``missing'' value indicators.
Unfortunately, these problems may be difficult to fix, because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.
Here is a simple demonstration illustrating that MySQL doesn't have any problems with dates until the year 2030:
mysql> DROP TABLE IF EXISTS y2k; Query OK, 0 rows affected (0.01 sec) mysql> CREATE TABLE y2k (date date, date_time datetime, time_stamp timestamp); Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO y2k VALUES -> ("1998-12-31","1998-12-31 23:59:59",19981231235959), -> ("1999-01-01","1999-01-01 00:00:00",19990101000000), -> ("1999-09-09","1999-09-09 23:59:59",19990909235959), -> ("2000-01-01","2000-01-01 00:00:00",20000101000000), -> ("2000-02-28","2000-02-28 00:00:00",20000228000000), -> ("2000-02-29","2000-02-29 00:00:00",20000229000000), -> ("2000-03-01","2000-03-01 00:00:00",20000301000000), -> ("2000-12-31","2000-12-31 23:59:59",20001231235959), -> ("2001-01-01","2001-01-01 00:00:00",20010101000000), -> ("2004-12-31","2004-12-31 23:59:59",20041231235959), -> ("2005-01-01","2005-01-01 00:00:00",20050101000000), -> ("2030-01-01","2030-01-01 00:00:00",20300101000000), -> ("2050-01-01","2050-01-01 00:00:00",20500101000000); Query OK, 13 rows affected (0.01 sec) Records: 13 Duplicates: 0 Warnings: 0 mysql> SELECT * FROM y2k; +------------+---------------------+----------------+ | date | date_time | time_stamp | +------------+---------------------+----------------+ | 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 | | 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 | | 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 | | 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 | | 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 | | 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 | | 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 | | 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 | | 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 | | 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 | | 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 | | 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 | | 2050-01-01 | 2050-01-01 00:00:00 | 00000000000000 | +------------+---------------------+----------------+ 13 rows in set (0.00 sec)
This shows that the DATE
and DATETIME
types will not
give any problems with future dates (they handle dates until the year
9999).
The TIMESTAMP
type, which is used to store the current time, has a
range up to only 2030-01-01
. TIMESTAMP
has a range of
1970
to 2030
on 32-bit machines (signed value). On 64-bit
machines it handles times up to 2106
(unsigned value).
Even though MySQL is Y2K-compliant, it is your responsibility to provide unambiguous input. See section 6.2.2.1 Y2K Issues and Date Types for MySQL's rules for dealing with ambiguous date input data (data containing 2-digit year values).
While this manual is still the right place for up to date technical information, its primary goal is to contain everything there is to know about MySQL. It is sometimes nice to have a bound book to read in bed or while you travel. Here is a list of books about MySQL and related subjects (in English).
By purchasing a book through these hyperlinks provided herein, you are contributing to the development of MySQL.
MySQL
Available | Barnes and Noble |
Publisher | New Riders |
Author | Paul DuBois |
Pub Date | 1st Edition December 1999 |
ISBN | 0735709211 |
Pages | 800 |
Price | $49.99 US |
Downloadable examples |
samp_db distribution
|
Errata | are available here |
Foreword by Michael ``Monty'' Widenius, MySQL Moderator.
In MySQL, Paul DuBois provides you with a comprehensive guide to
one of the most popular relational database systems. Paul has
contributed to the online documentation for MySQL and is an
active member of the MySQL community. The principal MySQL
developer, Monty Widenius, and a network of his fellow developers
reviewed the manuscript, and provided Paul with the kind of insight
no one else could supply.
Instead of merely giving you a general overview of MySQL, Paul
teaches you how to make the most of its capabilities. Through two
sample database applications that run throughout the book, he
gives you solutions to problems you're sure to face. He helps you
integrate MySQL efficiently with third-party tools, such as PHP
and Perl, enabling you to generate dynamic Web pages through
database queries. He teaches you to write programs that access
MySQL databases, and also provides a comprehensive set of
references to column types, operators, functions, SQL syntax,
MySQL programming, C API, Perl DBI
, and PHP API.
MySQL simply gives you the kind of information you won't find
anywhere else.
If you use MySQL, this book provides you with:
DBI
and PHP APIs for developing
command-line and Web-based applications.
DBI
API,
and PHP's MySQL-related functions.
MySQL & mSQL
Available | Barnes and Noble |
Publisher | O'Reilly |
Authors | Randy Jay Yarger, George Reese & Tim King |
Pub Date | 1st Edition July 1999 |
ISBN | 1-56592-434-7, Order Number: 4347 |
Pages | 506 |
Price | $34.95 |
This book teaches you how to use MySQL and mSQL
, two popular
and robust database products that support key subsets of SQL on both Linux
and Unix systems. Anyone who knows basic C, Java, Perl, or Python can
write a program to interact with a database, either as a stand-alone
application or through a Web page. This book takes you through the
whole process, from installation and configuration to programming
interfaces and basic administration. Includes plenty of tutorial
material.
Sams' Teach Yourself MySQL in 21 Days
Available | Barnes and Noble |
Publisher | Sams |
Authors | Mark Maslakowski and Tony Butcher |
Pub Date | June 2000 |
ISBN | 0672319144 |
Pages | 650 |
Price | $39.99 |
Sams' Teach Yourself MySQL in 21 Days is for intermediate Linux users who want to move into databases. A large share of the audience is Web developers who need a database to store large amounts of information that can be retrieved via the Web.
Sams' Teach Yourself MySQL in 21 Days is a practical, step-by-step
tutorial. The reader will learn to design and employ this open source
database technology into his or her Web site using practical, hands-on
examples to follow.
E-Commerce Solutions with MySQL
Available | Barnes and Noble |
Publisher | Prima Communications, Inc. |
Authors | N/A |
Pub Date | January 2000 |
ISBN | 0761524452 |
Pages | 500 |
Price | $39.99 |
No description available.
MySQL and PHP from Scratch
Available | Barnes and Noble |
Publisher | Que |
Authors | N/A |
Pub Date | September 2000 |
ISBN | 0789724405 |
Pages | 550 |
Price | $34.99 |
This book puts together information on installing, setting up, and
troubleshooting Apache, MySQL, PHP3, and IMP into one complete
volume. You also learn how each piece is part of a whole by learning,
step-by-step, how to create a web-based e-mail system. Learn to run
the equivalent of Active Server Pages (ASP) using PHP3, set up an
e-commerce site using a database and the Apache web server, and create
a data entry system (such as sales, product quality tracking, customer
preferences, etc) that no installation in the PC.
Professional MySQL Programming
Available | Barnes and Noble |
Publisher | Wrox Press, Inc. |
Authors | N/A |
Pub Date | Late 2001 |
ISBN | 1861005164 |
Pages | 1000 |
Price | $49.99 |
No description available.
Professional Linux Programming
Available | Barnes and Noble |
Publisher | Wrox Press, Inc. |
Authors | N/A |
Pub Date | September 2000 |
ISBN | 1861003013 |
Pages | 1155 |
Price | $47.99 |
In this follow-up to the best-selling Beginning Linux Programming,
you will learn from the authors' real-world knowledge and experience of
developing software for Linux; you'll be taken through the development
of a sample 'DVD Store' application, with 'theme' chapters addressing
different aspects of its implementation. Meanwhile, individual
``take-a-break'' chapters cover important topics that go beyond the
bounds of the central theme. All focus on the practical aspects of
programming, showing how crucial it is to choose the right tools for
the job, use them as they should be used, and get things right first
time.
PHP and MySQL Web Development
Available | Barnes and Noble |
Publisher | Sams |
Authors | Luke Welling, Laura Thomson |
Pub Date | March 2001 |
ISBN | 0672317842 |
Pages | 700 |
Price | $49.99 |
PHP and MySQL Web Development introduces you to the advantages
of implementing both MySQL and PHP. These advantages are detailed
through the provision of both statistics and several case studies. A
practical web application is developed throughout the book, providing
you with the tools necessary to implement a functional online
database. Each function is developed separately, allowing you the
choice to incorporate only those parts that you would like to
implement. Programming concepts of the PHP language are highlighted,
including functions which tie MySQL support into a PHP script and
advanced topics regarding table manipulation.
Books recommended by the MySQL Developers
SQL-99 Complete, Really
Available | Barnes and Noble |
Publisher | CMP Books |
Authors | Peter Gulutzan, Trudy Pelzer |
Pub Date | April 1999 |
ISBN | 0879305681 |
Pages | 1104 |
Price | $55.96 |
This book contains complete descriptions of the new standards for
syntax, data structures, and retrieval processes of SQL databases. As
an example-based reference manual, it includes all of the CLI
functions, information, schema tables, and status codes, as well as a
working SQL database provided on the companion disk.
C, A reference manual
Available | Barnes and Noble |
Publisher | Prentice Hall |
Authors | Samuel P. Harbison, Guy L. Steele |
Pub Date | September 1994 |
ISBN | 0133262243 |
Pages | 480 |
Price | $35.99 |
A new and improved revision of the bestselling C language
reference. This manual introduces the notion of "Clean C", writing C
code that can be compiled as a C++ program, C programming style that
emphasizes correctness, portability, maintainability, and
incorporates the ISO C Amendment 1 (1994) which specifies new
facilities for writing portable, international programs in C.
C++ for Real Programmers
Available | Barnes and Noble |
Publisher | Academic Press, Incorporated |
Authors | Jeff Alger, Jim Keogh |
Pub Date | February 1998 |
ISBN | 0120499428 |
Pages | 388 |
Price | $39.95 |
C++ For Real Programmers bridges the gap between C++ as described in beginner and intermediate-level books and C++ as it is practiced by experts. Numerous valuable techniques are described, organized into three simple themes: indirection, class hierarchies, and memory management. It also provides in-depth coverage of template creation, exception handling, pointers and optimization techniques. The focus of the book is on ANSI C++ and, as such, is compiler independent.
C++ For Real Programmers is a revision of
Secrets of the C++ Masters and includes a new appendix comparing C++
with Java. The book comes with a 3.5" disk for Windows with source code.
Algorithms in C
Available | Barnes and Noble |
Publisher | Addison Wesley Longman, Inc. |
Authors | Robert Sedgewick |
Pub Date | April 1990 |
ISBN | 0201514257 |
Pages | 648 |
Price | $45.75 |
Algorithms in C describes a variety of algorithms in a number of
areas of interest, including: sorting, searching, string-processing, and
geometric, graph and mathematical algorithms. The book emphasizes
fundamental techniques, providing readers with the tools to confidently
implement, run, and debug useful algorithms.
Multithreaded Programming with Pthreads
Available | Barnes and Noble |
Publisher | Prentice Hall |
Authors | Bil Lewis, Daniel J. Berg |
Pub Date | October 1997 |
ISBN | 0136807291 |
Pages | 432 |
Price | $34.95 |
Based on the best-selling Threads Primer,
Multithreaded Programming with Pthreads gives you a solid
understanding of Posix threads: what they are, how they work, when to use
them, and how to optimize them. It retains the clarity and humor of
Threads Primer, but includes expanded comparisons to Win32 and OS/2
implementations. Code examples tested on all of the major UNIX platforms
are featured along with detailed explanations of how and why they use threads.
Programming the PERL DBI: Database Programming with PERL
Available | Barnes and Noble |
Publisher | O'Reilly & Associates, Incorporated |
Authors | Alligator Descartes, Tim Bunce |
Pub Date | February 2000 |
ISBN | 1565926994 |
Pages | 400 |
Price | $27.96 |
Programming the Perl DBI is coauthored by Alligator Descartes, one of the most active members of the DBI community, and by Tim Bunce, the inventor of DBI. For the uninitiated, the book explains the architecture of DBI and shows you how to write DBI-based programs. For the experienced DBI dabbler, this book explains DBI's nuances and the peculiarities of each individual DBD.
The book includes:
The following book has been recommended by several people on the MySQL mailing list:
Judith S. Bowman, Sandra L. Emerson and Marcy Darnovsky The Practical SQL Handbook: Using Structured Query Language Second Edition Addison-Wesley ISBN 0-201-62623-3 http://www.awl.com
The following book has also received some recommendations by MySQL users:
Martin Gruber Understanding SQL ISBN 0-89588-644-8 Publisher Sybex 510 523 8233 Alameda, CA USA
A SQL tutorial is available on the net at http://w3.one.net/~jhoffman/sqltut.htm
Apart from the following links, you can find and download a lot of MySQL programs, tools and APIs from the Contrib directory.
1.2.4 Tutorials and Manuals
mSQL
.
mSQL
Tcl.
DBI
/DBD
.
DBI
/DBD
modules homepage.
There are also many Web pages that use MySQL. See section C Some MySQL Users. Send any additions to this list to webmaster@mysql.com. We now require that you show a MySQL logo somewhere if you wish your site to be added. It is okay to have it on a ``used tools'' page or something similar.
This section introduces you to the MySQL mailing lists, and gives some guidelines as to how to use them.
To subscribe to the main MySQL mailing list, send a message to the electronic mail address mysql-subscribe@lists.mysql.com.
To unsubscribe from the main MySQL mailing list, send a message to the electronic mail address mysql-unsubscribe@lists.mysql.com.
Only the address to which you send your messages is significant. The subject line and the body of the message are ignored.
If your reply address is not valid, you can specify your address
explicitly. Adding a hyphen to the subscribe or unsubscribe command
word, followed by your address with the `@' character in your
address replaced by a `='. For example, to subscribe
your_name@host.domain
, send a message to
mysql-subscribe-your_name=host.domain@lists.mysql.com
.
Mail to mysql-subscribe@lists.mysql.com or mysql-unsubscribe@lists.mysql.com is handled automatically by the ezmlm mailing list processor. Information about ezmlm is available at The ezmlm Website.
To post a message to the list itself, send your message to
mysql@lists.mysql.com
. However, please do not send mail about
subscribing or unsubscribing to mysql@lists.mysql.com, because any
mail sent to that address is distributed automatically to thousands of other
users.
Your local site may have many subscribers to mysql@lists.mysql.com.
If so, it may have a local mailing list, so that messages sent from
lists.mysql.com
to your site are propagated to the local list. In such
cases, please contact your system administrator to be added to or dropped
from the local MySQL list.
If you wish to have traffic for a mailing list go to a separate mailbox in
your mail program, set up a filter based on the message headers. You can
use either the List-ID:
or Delivered-To:
headers to identify
list messages.
The following MySQL mailing lists exist:
announce-subscribe@lists.mysql.com announce
mysql-subscribe@lists.mysql.com mysql
mysql-digest-subscribe@lists.mysql.com mysql-digest
mysql
list in digest form. That means you get all individual
messages, sent as one large mail message once a day.
bugs-subscribe@lists.mysql.com bugs
mysqlbug
script (if you are running on Windows, you should
include a description of the operating system and the MySQL version).
Preferably, you should test the problem using the latest stable or development
version of MySQL before posting! Anyone should be able to repeat the
bug by just using mysql test < script
on the included test case. All
bugs posted on this list will be corrected or documented in the next
MySQL release! If there are only small code changes involved, we
will also post a patch that fixes the problem.
bugs-digest-subscribe@lists.mysql.com bugs-digest
bugs
list in digest form.
internals-subscribe@lists.mysql.com internals
internals-digest-subscribe@lists.mysql.com internals-digest
internals
list.
java-subscribe@lists.mysql.com java
java-digest-subscribe@lists.mysql.com java-digest
java
list.
win32-subscribe@lists.mysql.com win32
win32-digest-subscribe@lists.mysql.com win32-digest
win32
list.
myodbc-subscribe@lists.mysql.com myodbc
myodbc-digest-subscribe@lists.mysql.com myodbc-digest
myodbc
list.
plusplus-subscribe@lists.mysql.com plusplus
plusplus-digest-subscribe@lists.mysql.com plusplus-digest
plusplus
list.
msql-mysql-modules-subscribe@lists.mysql.com msql-mysql-modules
msql-mysql-modules-digest-subscribe@lists.mysql.com msql-mysql-modules-digest
msql-mysql-modules
list.
You subscribe or unsubscribe to all lists in the same way as described
above. In your subscribe or unsubscribe message, just put the appropriate
mailing list name rather than mysql
. For example, to subscribe to or
unsubscribe from the myodbc
list, send a message to
myodbc-subscribe@lists.mysql.com or
myodbc-unsubscribe@lists.mysql.com.
If you can't get an answer for your questions from the mailing list, one option is to pay for support from MySQL AB, which will put you in direct contact with MySQL developers. See section 1.3.5 Types of Commercial Support.
The following table shows some MySQL mailing in other languages than English. Note that these are not operated by MySQL AB, so we can't guarantee the quality on these.
mysql-france-subscribe@yahoogroups.com A French mailing list
list@tinc.net A Korean mailing list
subscribe mysql your@email.address
to this list.
mysql-de-request@lists.4t2.com A German mailing list
subscribe mysql-de your@email.address
to this list.
You can find information about this mailing list at
http://www.4t2.com/mysql.
mysql-br-request@listas.linkway.com.br A Portugese mailing list
subscribe mysql-br your@email.address
to this list.
mysql-alta@elistas.net A Spanish mailing list
subscribe mysql your@email.address
to this list.
Before posting a bug report or question, please do the following:
If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to mysql@lists.mysql.com.
Writing a good bug report takes patience, but doing it right the first time saves time for us and for you. A good bug report containing a full test case for the bug will make it very likely that we will fix it in the next release. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.
We encourage everyone to use the mysqlbug
script to generate a bug
report (or a report about any problem), if possible. mysqlbug
can be
found in the `scripts' directory in the source distribution, or, for a
binary distribution, in the `bin' directory under your MySQL
installation directory. If you are unable to use mysqlbug
, you should
still include all the necessary information listed in this section.
The mysqlbug
script helps you generate a report by determining much
of the following information automatically, but if something important is
missing, please include it with your message! Please read this section
carefully and make sure that all the information described here is included
in your report.
The normal place to report bugs and problems is
mysql@lists.mysql.com. If you can make a test case that clearly
demonstrates the bug, you should post it to the bugs@lists.mysql.com
list. Note that on this list you should only post a full, repeatable bug
report using the mysqlbug
script. If you are running on Windows,
you should include a description of the operating system and the
MySQL version. Preferably, you should test the problem using
the latest stable or development version of MySQL before
posting! Anyone should be able to repeat the bug by just using
``mysql test < script
'' on the included test case or run the
shell or perl script that is included in the bug report. All bugs
posted on the bugs
list will be corrected or documented in the next
MySQL release! If there are only small code changes involved
to correct this problem, we will also post a patch that fixes the
problem.
Remember that it is possible to respond to a message containing too much information, but not to one containing too little. Often people omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is: if you are in doubt about stating something, state it! It is a thousand times faster and less troublesome to write a couple of lines more in your report than to be forced to ask again and wait for the answer because you didn't include enough information the first time.
The most common errors are that people don't indicate the version number of the MySQL distribution they are using, or don't indicate what platform they have MySQL installed on (including the platform version number). This is highly relevant information, and in 99 cases out of 100 the bug report is useless without it! Very often we get questions like, ``Why doesn't this work for me?'' then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed already in newer MySQL versions. Sometimes the error is platform dependent; in such cases, it is next to impossible to fix anything without knowing the operating system and the version number of the platform.
Remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether or not your problem depends on your compiler, we need to know what compiler is used. Note that every compiling problem should be regarded as a bug report and reported accordingly.
It is most helpful when a good description of the problem is included in the bug report. That is, a good example of all the things you did that led to the problem and the problem itself exactly described. The best reports are those that include a full example showing how to reproduce the bug or problem. See section H.1.6 Making a test case when you experience table corruption.
If a program produces an error message, it is very important to include the message in your report! If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the case should be observed!) You should never try to remember what the error message was; instead, copy and paste the entire message into your report!
If you have a problem with MyODBC, you should try to generate a MyODBC trace file. See section 8.3.7 Reporting Problems with MyODBC.
Please remember that many of the people who will read your report will
do so using an 80-column display. When generating reports or examples
using the mysql
command line tool, you should therefore use
the --vertical
option (or the \G
statement terminator)
for output that would exceed the available width for such a display
(for example, with the EXPLAIN SELECT
statement; see the
example below).
Please include the following information in your report:
mysqladmin version
. mysqladmin
can be
found in the `bin' directory under your MySQL installation
directory.
uname -a
.
mysqld
died, you should also report the query that crashed
mysqld
. You can usually find this out by running mysqld
with
logging enabled. See section H.1.5 Using log files to find cause of errors in mysqld.
mysqldump --no-data db_name tbl_name1 tbl_name2 ...
. This is very easy
to do and is a powerful way to get information about any table in a database
that will help us create a situation matching the one you have.
SELECT
statements, you should
always include the output of EXPLAIN SELECT ...
, and at least the
number of rows that the SELECT
statement produces. The more
information you give about your situation, the more likely it is that someone
can help you! For example, the following is an example of a very good bug
report (it should of course be posted with the mysqlbug
script):
Example run using the mysql
command line tool (note the use of the
\G
statement terminator for statements whose output width would
otherwise exceed that of an 80-column display device):
mysql> SHOW VARIABLES; mysql> SHOW COLUMNS FROM ...\G <output from SHOW COLUMNS> mysql> EXPLAIN SELECT ...\G <output from EXPLAIN> mysql> FLUSH STATUS; mysql> SELECT ...; <A short version of the output from SELECT, including the time taken to run the query> mysql> SHOW STATUS; <output from SHOW STATUS>
mysqladmin variables extended-status processlist
in your mail to
provide some information of how your system is performing!
mysqldump
and create a `README' file
that describes your problem.
Create a compressed archive of your files using
tar
and gzip
or zip
, and use ftp
to transfer the
archive to ftp://support.mysql.com/pub/mysql/secret/. Then send a
short description of the problem to bugs@lists.mysql.com.
ftp
to transfer it to
ftp://support.mysql.com/pub/mysql/secret/. If the data are really top
secret and you don't want to show them even to us, then go ahead and provide
an example using other names, but please regard this as the last choice.
mysqld
daemon and that you use to run any MySQL client programs. The
options to programs like mysqld
and mysql
, and to the
configure
script, are often keys to answers and are very relevant!
It is never a bad idea to include them anyway! If you use any modules, such
as Perl or PHP, please include the version number(s) of those as well.
mysqlaccess
, the output of mysqladmin reload
, and all
the error messages you get when trying to connect! When you test your
privileges, you should first run mysqlaccess
. After this, execute
mysqladmin reload version
and try to connect with the program that
gives you trouble. mysqlaccess
can be found in the `bin'
directory under your MySQL installation directory.
parse error
, please check your syntax closely! If
you can't find something wrong with it, it's extremely likely that your
current version of MySQL doesn't support the query you are
using. If you are using the current version and the manual at
http://www.mysql.com/documentation/manual.php doesn't cover the
syntax you are using, MySQL doesn't support your query. In this
case, your only options are to implement the syntax yourself or e-mail
mysql-licensing@mysql.com and ask for an offer to implement it!
If the manual covers the syntax you are using, but you have an older version
of MySQL, you should check the MySQL change history to see
when the syntax was implemented. In this case, you have the option of
upgrading to a newer version of MySQL. See section G MySQL change history.
myisamchk
or CHECK TABLE
and
REPAIR TABLE
. See section 4 MySQL Database Administration.
mysqld
should NEVER crash a table if nothing killed it in the
middle of an update! If you can find the cause of mysqld
dying,
it's much easier for us to provide you with a fix for the problem!
See section A.1 How to Determine What Is Causing Problems.
If you are a support customer, please cross-post the bug report to mysql-support@mysql.com for higher priority treatment, as well as to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem.
For information on reporting bugs in MyODBC, see section 8.3.4 How to Report Problems with MyODBC.
For solutions to some common problems, see See section A Problems and Common Errors.
When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem!
If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.
Try to summarize the essential part of the question in your reply; don't feel obliged to quote the entire original message.
Please don't post mail messages from your browser with HTML mode turned on! Many users don't read mail with a browser!
This section describes MySQL support and licensing arrangements:
The formal terms of the GPL license can be found at section K GNU GENERAL PUBLIC LICENSE. Basically, our licensing policy and interpretation of the GPL is as follows:
Note that older versions of MySQL are still using a more strict license. See the documentation for that version for more information. If you need a commercial MySQL license, because the GPL license doesn't suit your application, you can buy one at https://order.mysql.com/.
For normal internal use, MySQL costs nothing. You do not have to pay us if you do not want to.
A license is required if:
A license is NOT required if:
GNU Library General Public License
. The mysql
command-line
client includes code from the readline
library that is under
the GPL
.
For circumstances under which a MySQL license is required, you
need a license per machine that runs the mysqld
server. However,
a multiple-CPU machine counts as a single machine, and there is no
restriction on the number of MySQL servers that run on one
machine, or on the number of clients concurrently connected to a server
running on that machine!
If you have any questions as to whether or not a license is required for your particular use of MySQL, please read this again and then contact us. See section 1.3.4.2 Contact Information.
If you require a MySQL license, the easiest way to pay for it is to use the license form on MySQL's secure server at https://order.mysql.com/. Other forms of payment are discussed in section 1.3.4.1 Payment information.
There are several different copyrights on the MySQL distribution:
mysqlclient
library is licensed under the LGPL
and
programs in the `client' directory is GPL. Each file has a header
that shows which copyright is used for that file.
getopt
) library are covered
by the ``GNU LIBRARY GENERAL PUBLIC LICENSE.'' See section L GNU LESSER GENERAL PUBLIC LICENSE.
regexp
library) are covered
by a Berkeley-style copyright.
readline
) library
is covered by the ``GNU GENERAL PUBLIC LICENSE.'' See section K GNU GENERAL PUBLIC LICENSE.
This is also available as the file `COPYING' in the distributions.
One goal is that the SQL client library should be free enough that it is possible to add MySQL support into commercial products without a license. For this reason, we chose the LGPL license for the client code.
This means that you can use MySQL for free with any program that uses any of the free software licenses. MySQL is also free for any end user for his own or company usage.
However, if you use MySQL for something important to you, you may want to help secure its development by purchasing licenses or a support contract. See section 1.3.5 Types of Commercial Support.
Version 3.22 of MySQL is still using a more strict license. See the documentation for that version for more information.
This section describes some situations illustrating whether or not you must license the MySQL server. Generally these examples involve providing MySQL as an integral part of a product.
Note that a single MySQL license covers any number of CPUs and
mysqld
servers on a machine! There is no artificial limit on the
number of clients that connect to the server in any way.
To determine whether or not you need a MySQL license when selling your application, you should ask whether the proper functioning of your application is dependent on the use of MySQL and whether you include the MySQL server with your product. There are several cases to consider:
mysqld
server. For example, if you've
designed your application around MySQL, then you've really made
a commercial product that requires the engine, so you need a license.
Internet Service Providers (ISPs) often host MySQL servers for their customers. With the GPL license this does not require a license.
On the other hand, we do encourage people to use ISPs that have MySQL support, as this will give them the confidence that if they have some problem with their MySQL installation, their ISP will be able to solve the problem for them (in some cases with the help from the MySQL development team).
All ISPs that want to keep themselves up-to-date should subscribe
to our announce
mailing list so that they can be aware of fatal issues
that may be relevant for their MySQL installations.
Note that if the ISP doesn't have a license for MySQL, it should give its customers at least read access to the source of the MySQL installation so that its customer can verify that it is patched correctly.
If you use MySQL in conjunction with a Web server on Unix, you don't have to pay for a license.
This is true even if you run a commercial Web server that uses MySQL, because you are not selling an embedded MySQL version yourself. However, in this case we would like you to purchase MySQL support, because MySQL is helping your enterprise.
Our current license prices are shown below. To make a purchase, please visit https://order.mysql.com/.
If you pay by credit card, the currency is EURO (European Union Euro) so the prices will differ slightly.
Number of licenses | Per copy |
1-9 | 230 EURO |
10-24 | 138 EURO |
25-49 | 117 EURO |
50-99 | 102 EURO |
100-249 | 91 EURO |
250-499 | 76 EURO |
500-999 | 66 EURO |
For high volume (OEM) purchases, please contact sales@mysql.com.
For OEM purchases, you must act as the middle-man for eventual problems or extension requests from your users. We also require that OEM customers have at least an extended e-mail support contract. Note that OEM licenses only apply for products where the user doesn't have direct access to the MySQL server (embedded system). In other words, the MySQL server should only be used with the application that was supplied you.
If you have a low-margin, high-volume product, you can always talk to us about other terms (for example, a percent of the sale price). If you do, please be informative about your product, pricing, market, and any other information that may be relevant.
A full-price license is not a support agreement and includes very minimal support. This means that we try to answer any relevant questions. If the answer is in the documentation, we will direct you to the appropriate section. If you have not purchased a license or support, we probably will not answer at all.
If you discover what we consider a real bug, we are likely to fix it in any case. But if you pay for support we will notify you about the fix status instead of just fixing it in a later release.
More comprehensive support is sold separately. Descriptions of what each level of support includes are given in section 1.3.5 Types of Commercial Support. Costs for the various types of commercial support are shown below. Support level prices are in EURO (European Union Euro). One EURO is about 1.06 USD.
Type of support | Cost per year |
Basic e-mail support. See section 1.3.5.1 Basic E-mail Support. | EURO 200 |
Extended e-mail support See section 1.3.5.2 Extended E-mail Support. | EURO 1000 |
Login support See section 1.3.5.3 Login Support. | EURO 2000 |
Extended login support See section 1.3.5.4 Extended Login Support. | EURO 5000 |
Telephone support See section 1.3.5.5 Telephone Support. | EURO 12000 |
You may upgrade from any lower level of support to a higher level of support for the difference in price between the two support levels.
We do also provide telephone support (mostly emergency support but also 24/7 support). This support option doesn't however have a fixed price but is negotiated for case to case. If you are interested in this option you can email sales@mysql.com and tell us about your needs.
Note that as our sales staff is very busy, it may take some time until your request is handled. Our support staff does however always answer promptly to support questions!
Currently we can take SWIFT payments, checks, or credit cards.
Payment should be made to:
Postgirot Bank AB 105 06 STOCKHOLM, SWEDEN MySQL AB BOX 6434 11382 STOCKHOLM, SWEDEN SWIFT address: PGSI SESS Account number: 96 77 06 - 3
Specify: license and/or support and your name and e-mail address.
In Europe and Japan you can use EuroGiro (that should be less expensive) to the same account.
If you want to pay by check, make it payable to ``MySQL Finland AB'' and mail it to the address below:
MySQL AB BOX 6434, Torsgatan 21 11382 STOCKHOLM, SWEDEN
If you want to pay by credit card over the Internet, you can use MySQL AB's secure license form.
You can also print a copy of the license form, fill it in, and send it by fax to:
+46-8-729 69 05
If you want us to bill you, you can use the license form and write ``bill
us'' in the comment field. You can also mail a message to
sales@mysql.com (not mysql@lists.mysql.com
!)
with your company information and ask us to bill you.
For commercial licensing, please contact the MySQL licensing team. The much preferred method is by e-mail to licensing@mysql.com. Fax is also possible but handling of these may take much longer (Fax +46-8-729 69 05).
If you represent a business that is interested in partnering with MySQL, please send e-mail to partner@mysql.com.
For timely, precise answers to technical questions about MySQL you should order one of our support contracts. MySQL support is provided by the MySQL developers so the standard is extremely high.
If you are interested in placing a banner advertisement on our Web site, please send e-mail to advertising@mysql.com.
If you are interested in any of the jobs listed in our jobs section, please send e-mail to jobs@mysql.com.
For general discussion amongst our many users, please direct your attention to the appropriate mailing list.
For general information inquires, please send e-mail to info@mysql.com.
For questions or comments about the workings or content of the Web site, please send e-mail to webmaster@mysql.com.
The following is true of all support options:
Basic e-mail support is a very inexpensive support option and should be thought of more as a way to support our development of MySQL than as a real support option. We at MySQL do give a lot of free support in all the different MySQL lists, and the money we get from basic e-mail support is largely used to make this possible.
At this support level, the MySQL mailing lists are the preferred means of communication. Questions normally should be mailed to the primary mailing list (mysql@lists.mysql.com) or one of the other regular lists (for example, win32@lists.mysql.com for Windows-related MySQL questions), as someone else already may have experienced and solved the problem you have. See section 1.2.22.2 Asking Questions or Reporting Bugs.
However, by purchasing basic e-mail support, you also have access to the support address mysql-support@mysql.com, which is not available as part of the minimal support that you get by purchasing a MySQL license. This means that for especially critical questions, you can cross-post your message to mysql-support@mysql.com. (If the message contains sensitive data, you should post only to mysql-support@mysql.com.)
REMEMBER! to ALWAYS include your registration number and expiration date when you send a message to mysql-support@mysql.com.
Note that if you have encountered a critical, repeatable bug, and follow the rules outlined in the manual section of how to report bugs and send it to bugs@lists.mysql.com, we promise to try to fix this as soon as possible, regardless of your support level! See section 1.2.22.3 How to Report Bugs or Problems.
Basic e-mail support includes the following types of service:
Extended e-mail support includes everything in basic e-mail support with these additions:
mysqld
for your situation.
Login support includes everything in extended e-mail support with these additions:
kill -9
command).
Extended login support includes everything in login support with these additions:
mysql> select MY_FUNC(col1,col2) from table;
Telephone support includes everything in extended login support with these additions:
MySQL
developers that you can phone when you have a critical
problem.
MySQL
related issues.
To get support for BDB
tables, InnoDB
tables you have
to pay an additional 30% on the standard support price for each of
the table handlers you would like to have support for.
We at MySQL AB
will help you create a proper bug report for the
table handler and submit it to the developers for the specific table
handler. We will also do our best to ensure that you will get a timely
answer or solution from the developers of the table handler.
Even if we are quite confident that we can solve most problems within a timely manner, we can't guarantee a quick solution for any problems you can get with the different table handlers. We will however do our best to help you get the problem solved.
This section describes how MySQL relates to the ANSI SQL standards. MySQL has many extensions to the ANSI SQL standards, and here you will find out what they are, and how to use them. You will also find information about functionality missing from MySQL, and how to work around some differences.
MySQL includes some extensions that you probably will not find in
other SQL databases. Be warned that if you use them, your code will not be
portable to other SQL servers. In some cases, you can write code that
includes MySQL extensions, but is still portable, by using comments
of the form /*! ... */
. In this case, MySQL will parse and
execute the code within the comment as it would any other MySQL
statement, but other SQL servers will ignore the extensions. For example:
SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...
If you add a version number after the '!'
, the syntax will only be
executed if the MySQL version is equal to or newer than the used
version number:
CREATE /*!32302 TEMPORARY */ TABLE (a int);
The above means that if you have Version 3.23.02 or newer, then MySQL
will use the TEMPORARY
keyword.
MySQL extensions are listed below:
MEDIUMINT
, SET
, ENUM
, and the
different BLOB
and TEXT
types.
AUTO_INCREMENT
, BINARY
, NULL
,
UNSIGNED
, and ZEROFILL
.
BINARY
attribute or use the BINARY
cast, which causes
comparisons to be done according to the ASCII order used on the
MySQL server host.
db_name.tbl_name
syntax. Some SQL servers provide
the same functionality but call this User space
.
MySQL doesn't support tablespaces as in:
create table ralph.my_table...IN my_tablespace
.
LIKE
is allowed on numeric columns.
INTO OUTFILE
and STRAIGHT_JOIN
in a SELECT
statement. See section 6.4.1 SELECT
Syntax.
SQL_SMALL_RESULT
option in a SELECT
statement.
EXPLAIN SELECT
to get a description on how tables are joined.
INDEX
or KEY
in a CREATE TABLE
statement. See section 6.5.3 CREATE TABLE
Syntax.
TEMPORARY
or IF NOT EXISTS
with CREATE TABLE
.
COUNT(DISTINCT list)
where 'list' is more than one element.
CHANGE col_name
, DROP col_name
, or DROP
INDEX
, IGNORE
or RENAME
in an ALTER TABLE
statement. See section 6.5.4 ALTER TABLE
Syntax.
RENAME TABLE
. See section 6.5.5 RENAME TABLE
Syntax.
ADD
, ALTER
, DROP
, or CHANGE
clauses in an ALTER TABLE
statement.
DROP TABLE
with the keywords IF EXISTS
.
DROP TABLE
statement.
LIMIT
clause of the DELETE
statement.
DELAYED
clause of the INSERT
and REPLACE
statements.
LOW_PRIORITY
clause of the INSERT
, REPLACE
,
DELETE
, and UPDATE
statements.
LOAD DATA INFILE
. In many cases, this syntax is compatible with
Oracle's LOAD DATA INFILE
. See section 6.4.8 LOAD DATA INFILE
Syntax.
ANALYZE TABLE
, CHECK TABLE
, OPTIMIZE TABLE
, and
REPAIR TABLE
statements.
SHOW
statement.
See section 4.5.5 SHOW
Syntax.
SET OPTION
statement. See section 5.5.6 SET
Syntax.
GROUP BY
part.
This gives better performance for some very specific, but quite normal
queries.
See section M.3 Functions for Use with GROUP BY
Clauses.
ASC
and DESC
with GROUP BY
.
||
and &&
operators to mean
logical OR and AND, as in the C programming language. In MySQL,
||
and OR
are synonyms, as are &&
and AND
.
Because of this nice syntax, MySQL doesn't support
the ANSI SQL ||
operator for string concatenation; use
CONCAT()
instead. Because CONCAT()
takes any number
of arguments, it's easy to convert use of the ||
operator to
MySQL.
CREATE DATABASE
or DROP DATABASE
.
See section 6.5.1 CREATE DATABASE
Syntax.
%
operator is a synonym for MOD()
. That is,
N % M
is equivalent to MOD(N,M)
. %
is supported
for C programmers and for compatibility with PostgreSQL.
=
, <>
, <=
,<
, >=
,>
,
<<
, >>
, <=>
, AND
, OR
, or LIKE
operators may be used in column comparisons to the left of the
FROM
in SELECT
statements. For example:
mysql> SELECT col1=1 AND col2=2 FROM tbl_name;
LAST_INSERT_ID()
function.
See section 8.4.3.126 mysql_insert_id()
.
REGEXP
and NOT REGEXP
extended regular expression
operators.
CONCAT()
or CHAR()
with one argument or more than two
arguments. (In MySQL, these functions can take any number of
arguments.)
BIT_COUNT()
, CASE
, ELT()
,
FROM_DAYS()
, FORMAT()
, IF()
, PASSWORD()
,
ENCRYPT()
, md5()
, ENCODE()
, DECODE()
,
PERIOD_ADD()
, PERIOD_DIFF()
, TO_DAYS()
, or
WEEKDAY()
functions.
TRIM()
to trim substrings. ANSI SQL only supports removal
of single characters.
GROUP BY
functions STD()
, BIT_OR()
, and
BIT_AND()
.
REPLACE
instead of DELETE
+ INSERT
.
See section 6.4.7 REPLACE
Syntax.
FLUSH flush_option
statement.
:=
:
SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg FROM test_table; SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
We try to make MySQL follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL does some things differently:
--
is only a comment if followed by a white space. See section 1.4.4.8 `--' as the Start of a Comment.
VARCHAR
columns, trailing spaces are removed when the value is
stored. See section 1.4.7 Known errors and design deficiencies in MySQL.
CHAR
columns are silently changed to VARCHAR
columns. See section 6.5.3.1 Silent Column Specification Changes.
REVOKE
to revoke privileges for
a table. See section 4.3.1 GRANT
and REVOKE
Syntax.
NULL AND FALSE
will evaluate to NULL
and not to FALSE
.
This is because we don't think it's good to have to evaluate a lot of
extra conditions in this case.
If you start mysqld
with the --ansi
option, the following
behavior of MySQL changes:
||
is string concatenation instead of OR
.
REAL
will be a synonym for FLOAT
instead of a synonym of
DOUBLE
.
SERIALIZABLE
.
See section 6.7.3 SET TRANSACTION
Syntax.
This is the same as using --sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,SERIALIZE,ONLY_FULL_GROUP_BY
.
The following functionality is missing in the current version of MySQL. For a prioritized list indicating when new extensions may be added to MySQL, you should consult the online MySQL TODO list. That is the latest version of the TODO list in this manual. See section 1.6 MySQL and the future (The TODO).
The following will not yet work in MySQL:
SELECT * FROM table1 WHERE id IN (SELECT id FROM table2); SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2); SELECT * FROM table1 WHERE NOT EXISTS (SELECT id FROM table2 where table1.id=table2.id);
However, in many cases you can rewrite the query without a sub-select:
SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id; SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL
For more complicated subqueries you can often create temporary tables
to hold the subquery. In some cases, however this option will not
work. The most frequently encountered of these cases arises with
DELETE
statements, for which standard SQL does not support joins
(except in sub-selects). For this situation there are two options
available until subqueries are supported by MySQL.
The first option is to use a procedural programming language (such as
Perl or PHP) to submit a SELECT
query to obtain the primary keys
for the records to be deleted, and then use these values to construct
the DELETE
statement (DELETE FROM ... WHERE ... IN (key1,
key2, ...)
).
The second option is to use interactive SQL to contruct a set of
DELETE
statements automatically, using the MySQL
extension CONCAT()
(in lieu of the standard ||
operator).
For example:
SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', tab1.pkid, ';') FROM tab1, tab2 WHERE tab1.col1 = tab2.col2;
You can place this query in a script file and redirect input from it to
the mysql
command-line interpreter, piping its output back to a
second instance of the interpreter:
prompt> mysql --skip-column-names mydb < myscript.sql | mysql mydb
MySQL only supports INSERT ... SELECT ...
and
REPLACE ... SELECT ...
Independent sub-selects will probably
be available in Version 4.0. You can now use the function IN()
in
other contexts, however.
SELECT INTO TABLE
MySQL doesn't yet support the Oracle SQL extension:
SELECT ... INTO TABLE ...
. MySQL supports instead the
ANSI SQL syntax INSERT INTO ... SELECT ...
, which is basically
the same thing. See section 6.4.2.1 INSERT ... SELECT
Syntax.
INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID FROM tblTemp1 WHERE tblTemp1.fldOrder_ID > 100;
Alternatively, you can use SELECT INTO OUTFILE...
or CREATE
TABLE ... SELECT
to solve your problem.
As MySQL does nowadays support transactions, the following
discussion is only valid if you are only using the non-transaction-safe
table types. See section 6.7.1 BEGIN/COMMIT/ROLLBACK
Syntax.
The question is often asked, by the curious and the critical, ``Why is MySQL not a transactional database?'' or ``Why does MySQL not support transactions?''
MySQL has made a conscious decision to support another paradigm for data integrity, ``atomic operations.'' It is our thinking and experience that atomic operations offer equal or even better integrity with much better performance. We, nonetheless, appreciate and understand the transactional database paradigm and plan, within the next few releases, to introduce transaction-safe tables on a per table basis. We will be giving our users the possibility to decide if they need the speed of atomic operations or if they need to use transactional features in their applications.
How does one use the features of MySQL to maintain rigorous integrity and how do these features compare with the transactional paradigm?
First, in the transactional paradigm, if your applications are written in a way that is dependent on the calling of ``rollback'' instead of ``commit'' in critical situations, then transactions are more convenient. Moreover, transactions ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved.
MySQL, in almost all cases, allows you to solve for potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such occurs. Note that just by using the MySQL log or even adding one extra log, one can normally fix tables perfectly with no data integrity loss.
Moreover, fatal transactional updates can be rewritten to be
atomic. In fact,we will go so far as to say that all integrity problems
that transactions solve can be done with LOCK TABLES
or atomic updates,
ensuring that you never will get an automatic abort from the database,
which is a common problem with transactional databases.
Not even transactions can prevent all loss if the server goes down. In such cases even a transactional system can lose data. The difference between different systems lies in just how small the time-lap is where they could lose data. No system is 100% secure, only ``secure enough.'' Even Oracle, reputed to be the safest of transactional databases, is reported to sometimes lose data in such situations.
To be safe with MySQL, you only need to have backups and have the update logging turned on. With this you can recover from any situation that you could with any transactional database. It is, of course, always good to have backups, independent of which database you use.
The transactional paradigm has its benefits and its drawbacks. Many
users and application developers depend on the ease with which they can
code around problems where an abort appears to be, or is necessary, and they
may have to do a little more work with MySQL to either think
differently or write more. If you are new to the atomic operations
paradigm, or more familiar or more comfortable with transactions, do not
jump to the conclusion that MySQL has not addressed these
issues. Reliability and integrity are foremost in our minds. Recent
estimates indicate that there are more than 1,000,000 mysqld
servers
currently running, many of which are in production environments. We
hear very, very seldom from our users that they have lost any data, and
in almost all of those cases user error is involved. This is, in our
opinion, the best proof of MySQL's stability and reliability.
Lastly, in situations where integrity is of highest importance,
MySQL's current features allow for transaction-level or better
reliability and integrity. If you lock tables with LOCK TABLES
, all
updates will stall until any integrity checks are made. If you only obtain
a read lock (as opposed to a write lock), then reads and inserts are
still allowed to happen. The new inserted records will not be seen by
any of the clients that have a READ
lock until they release their read
locks. With INSERT DELAYED
you can queue inserts into a local queue,
until the locks are released, without having the client wait for the insert
to complete. See section 6.4.3 INSERT DELAYED
syntax.
``Atomic,'' in the sense that we mean it, is nothing magical. It only means
that you can be sure that while each specific update is running, no other
user can interfere with it, and there will never be an automatic
rollback (which can happen on transaction based systems if you are not
very careful). MySQL also guarantees that there will not be
any dirty reads. You can find some example of how to write atomic updates
in the commit-rollback section. See section 1.4.6 How to Cope Without COMMIT
/ROLLBACK
.
We have thought quite a bit about integrity and performance, and we believe that our atomic operations paradigm allows for both high reliability and extremely high performance, on the order of three to five times the speed of the fastest and most optimally tuned of transactional databases. We didn't leave out transactions because they are hard to do. The main reason we went with atomic operations as opposed to transactions is that by doing this we could apply many speed optimizations that would not otherwise have been possible.
Many of our users who have speed foremost in their minds are not at all
concerned about transactions. For them transactions are not an
issue. For those of our users who are concerned with or have wondered
about transactions vis-a-vis MySQL, there is a ``MySQL
way'' as we have outlined above. For those where safety is more
important than speed, we recommend them to use the BDB
,
or InnoDB
tables for all their critical data. See section 7 MySQL Table Types.
One final note: We are currently working on a safe replication schema that we believe to be better than any commercial replication system we know of. This system will work most reliably under the atomic operations, non-transactional, paradigm. Stay tuned.
A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep reissuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.
A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.
The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.
To see when MySQL might get stored procedures, see section 1.6 MySQL and the future (The TODO).
Note that foreign keys in SQL are not used to join tables, but are used
mostly for checking referential integrity (foreign key constraints). If
you want to get results from multiple tables from a SELECT
statement, you do this by joining tables:
SELECT * from table1,table2 where table1.id = table2.id;
See section 6.4.1.1 JOIN
Syntax. See section 3.5.6 Using Foreign Keys.
The FOREIGN KEY
syntax in MySQL exists only for compatibility
with other SQL vendors' CREATE TABLE
commands; it doesn't do
anything. The FOREIGN KEY
syntax without ON DELETE ...
is
mostly used for documentation purposes. Some ODBC applications may use this
to produce automatic WHERE
clauses, but this is usually easy to
override. FOREIGN KEY
is sometimes used as a constraint check, but
this check is unnecessary in practice if rows are inserted into the tables in
the right order. MySQL only supports these clauses because some
applications require them to exist (regardless of whether or not they
work).
In MySQL, you can work around the problem of ON DELETE
...
not being implemented by adding the appropriate DELETE
statement to
an application when you delete records from a table that has a foreign key.
In practice this is as quick (in some cases quicker) and much more portable
than using foreign keys.
In the near future we will extend the FOREIGN KEY
implementation so
that at least the information will be saved in the table specification file
and may be retrieved by mysqldump
and ODBC. At a later stage we will
implement the foreign key constraints for application that can't easily be
coded to avoid them.
There are so many problems with foreign key constraints that we don't know where to start:
INSERT
and UPDATE
statements, and in this case almost all FOREIGN KEY
constraint
checks are useless because you usually insert records in the right
tables in the right order, anyway.
FOREIGN KEY ... ON DELETE
rules when
one codes an application. It's not unusual that one loses a lot of
important information just because a wrong or misused ON DELETE
rule.
The only nice aspect of FOREIGN KEY
is that it gives ODBC and some
other client programs the ability to see how a table is connected and to use
this to show connection diagrams and to help in building applications.
MySQL will soon store FOREIGN KEY
definitions so that a
client can ask for and receive an answer about how the original
connection was made. The current `.frm' file format does not have
any place for it. At a later stage we will implement the foreign key
constraints for application that can't easily be coded to avoid them.
MySQL doesn't yet support views, but we plan to implement these to about 4.1.
Views are mostly useful for letting users access a set of relations as one table (in read-only mode). Many SQL databases don't allow one to update any rows in a view, but you have to do the updates in the separate tables.
As MySQL is mostly used in applications and on web system where the application writer has full control on the database usage, most of our users haven't regarded views to be very important. (At least no one has been interested enough in this to be prepared to finance the implementation of views).
One doesn't need views in MySQL to restrict access to columns as MySQL has a very sophisticated privilege system. See section 4.2 General Security Issues and the MySQL Access Privilege System.
Some other SQL databases use `--' to start comments. MySQL
has `#' as the start comment character, even if the mysql
command-line tool removes all lines that start with `--'.
You can also use the C comment style /* this is a comment */
with
MySQL.
See section 6.1.5 Comment Syntax.
MySQL Version 3.23.3 and above supports the `--' comment style
only if the comment is followed by a space. This is because this
degenerate comment style has caused many problems with automatically
generated SQL queries that have used something like the following code,
where we automatically insert the value of the payment for
!payment!
:
UPDATE tbl_name SET credit=credit-!payment!
What do you think will happen when the value of payment
is negative?
Because 1--1
is legal in SQL, we think it is terrible that
`--' means start comment.
In MySQL Version 3.23 you can, however, use:
1-- This is a comment
The following discussion only concerns you if you are running a MySQL version earlier than Version 3.23:
If you have a SQL program in a text file that contains `--' comments you should use:
shell> replace " --" " #" < text-file-with-funny-comments.sql \ | mysql database
instead of the usual:
shell> mysql database < text-file-with-funny-comments.sql
You can also edit the command file ``in place'' to change the `--' comments to `#' comments:
shell> replace " --" " #" -- text-file-with-funny-comments.sql
Change them back with this command:
shell> replace " #" " --" -- text-file-with-funny-comments.sql
Entry level SQL92. ODBC levels 0-2.
COMMIT
/ROLLBACK
The following mostly applies only for ISAM
, MyISAM
, and
HEAP
tables. If you only use transaction-safe tables (BDB
,
or InnoDB
tables) in an an update, you can do
COMMIT
and ROLLBACK
also with MySQL.
See section 6.7.1 BEGIN/COMMIT/ROLLBACK
Syntax.
The problem with handling COMMIT
-ROLLBACK
efficiently with
the above table types would require a completely different table layout
than MySQL uses today. The table type would also need extra
threads that do automatic cleanups on the tables, and the disk usage
would be much higher. This would make these table types about 2-4 times
slower than they are today.
For the moment, we prefer implementing the SQL server language (something
like stored procedures). With this you would very seldom really need
COMMIT
-ROLLBACK.
This would also give much better performance.
Loops that need transactions normally can be coded with the help of
LOCK TABLES
, and you don't need cursors when you can update records
on the fly.
We at TcX had a greater need for a real fast database than a 100% general database. Whenever we find a way to implement these features without any speed loss, we will probably do it. For the moment, there are many more important things to do. Check the TODO for how we prioritize things at the moment. (Customers with higher levels of support can alter this, so things may be reprioritized.)
The current problem is actually ROLLBACK
. Without
ROLLBACK
, you can do any kind of COMMIT
action with
LOCK TABLES
. To support ROLLBACK
with the above table
types, MySQL would have to be changed to store all old records
that were updated and revert everything back to the starting point if
ROLLBACK
was issued. For simple cases, this isn't that hard to do
(the current isamlog
could be used for this purpose), but it
would be much more difficult to implement ROLLBACK
for
ALTER/DROP/CREATE TABLE
.
To avoid using ROLLBACK
, you can use the following strategy:
LOCK TABLES ...
to lock all the tables you want to access.
UNLOCK TABLES
to release your locks.
This is usually a much faster method than using transactions with possible
ROLLBACK
s, although not always. The only situation this solution
doesn't handle is when someone kills the threads in the middle of an
update. In this case, all locks will be released but some of the updates may
not have been executed.
You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:
For example, when we are doing updates to some customer information, we
update only the customer data that has changed and test only that none of
the changed data, or data that depend on the changed data, has changed
compared to the original row. The test for changed data is done with the
WHERE
clause in the UPDATE
statement. If the record wasn't
updated, we give the client a message: "Some of the data you have changed
have been changed by another user". Then we show the old row versus the new
row in a window, so the user can decide which version of the customer record
he should use.
This gives us something that is similar to column locking but is actually
even better, because we only update some of the columns, using values that
are relative to their current values. This means that typical UPDATE
statements look something like these:
UPDATE tablename SET pay_back=pay_back+'relative change'; UPDATE customer SET customer_date='current_date', address='new address', phone='new phone', money_he_owes_us=money_he_owes_us+'new_money' WHERE customer_id=id AND address='old address' AND phone='old phone';
As you can see, this is very efficient and works even if another client has
changed the values in the pay_back
or money_he_owes_us
columns.
In many cases, users have wanted ROLLBACK
and/or LOCK
TABLES
for the purpose of managing unique identifiers for some tables. This
can be handled much more efficiently by using an AUTO_INCREMENT
column
and either the SQL function LAST_INSERT_ID()
or the C API function
mysql_insert_id()
. See section 8.4.3.126 mysql_insert_id()
.
At MySQL AB, we have never had any need for row-level locking because we have always been able to code around it. Some cases really need row locking, but they are very few. If you want row-level locking, you can use a flag column in the table and do something like this:
UPDATE tbl_name SET row_flag=1 WHERE id=ID;
MySQL returns 1 for the number of affected rows if the row was
found and row_flag
wasn't already 1 in the original row.
You can think of it as MySQL changed the above query to:
UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1;
The following problems are known and have a very high priority to get fixed:
ANALYZE TABLE
on a BDB table may in some case make the table
unusable until one has restarted mysqld
. When this happens you will
see errors like the following in the MySQL error file:
001207 22:07:56 bdb: log_flush: LSN past current end-of-log
ALTER TABLE
on a BDB
table on which you are
running not completed multi-statement transactions. (The transaction
will probably be ignored).
ANALYZE TABLE
, OPTIMIZE TABLE
and REPAIR TABLE
may
cause problems on tables for which you are using INSERT DELAYED
.
LOCK TABLE ..
and FLUSH TABLES ..
doesn't
guarantee that there isn't a half-finished transaction in progress on the
table.
mysql
client on the
database if you are not using the -A
option or if you are using
rehash
. This is especially notable when you have a big table
cache.
LOAD DATA INFILE
and line terminator characters of more than 1 character.
The following problems are known and will be fixed in due time:
MATCH
only works with SELECT
statements.
SET CHARACTER SET
, one can't use translated
characters in database, table and column names.
DELETE FROM merge_table
used without a WHERE
will only clear the mapping for the table, not delete everything in the
mapped tables
BLOB
values can't ``reliably'' be used in GROUP BY
or
ORDER BY
or DISTINCT
. Only the first max_sort_length
bytes (default 1024) are used when comparing BLOB
bs in these cases.
This can be changed with the -O max_sort_length
option to
mysqld
. A workaround for most cases is to use a substring:
SELECT DISTINCT LEFT(blob,2048) FROM tbl_name
.
BIGINT
or DOUBLE
(both are
normally 64 bits long). It depends on the function which precision one
gets. The general rule is that bit functions are done with BIGINT
precision, IF
, and ELT()
with BIGINT
or DOUBLE
precision and the rest with DOUBLE
precision. One should try to
avoid using bigger unsigned long long values than 63 bits
(9223372036854775807) for anything else than bit fields!
BLOB
and TEXT
columns, automatically
have all trailing spaces removed when retrieved. For CHAR
types this
is okay, and may be regarded as a feature according to ANSI SQL92. The bug is
that in MySQL, VARCHAR
columns are treated the same way.
ENUM
and SET
columns in one table.
safe_mysqld
re-directs all messages from mysqld
to the
mysqld
log. One problem with this is that if you execute
mysqladmin refresh
to close and reopen the log,
stdout
and stderr
are still redirected to the old log.
If you use --log
extensively, you should edit safe_mysqld
to
log to `'hostname'.err' instead of `'hostname'.log' so you can
easily reclaim the space for the old log by deleting the old one and
executing mysqladmin refresh
.
UPDATE
statement, columns are updated from left to right.
If you refer to an updated column, you will get the updated value instead of the
original value. For example:
mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1;will update
KEY
with 2
instead of with 1
.
select * from temporary_table, temporary_table as t2;
RENAME
doesn't work with TEMPORARY
tables.
DISTINCT
differently if you are using
'hidden' columns in a join or not. In a join, hidden columns are
counted as part of the result (even if they are not shown) while in
normal queries hidden columns doesn't participate in the DISTINCT
comparison. We will probably change this in the future to never compare
the hidden columns when executing DISTINCT
An example of this is:
SELECT DISTINCT mp3id FROM band_downloads WHERE userid = 9 ORDER BY id DESC; and SELECT DISTINCT band_downloads.mp3id, FROM band_downloads,band_mp3 WHERE band_downloads.userid = 9 AND band_mp3.id = band_downloads.mp3id ORDER BY band_downloads.id DESC;In the second case you may in MySQL 3.23.x get two identical rows in the result set (because the hidden 'id' column may differ). Note that the this only happens for queries where you don't have the ORDER BY columns in the result, something that is you are not allowed to do in ANSI SQL.
rollback
data) some things
behaves a little different in MySQL than in other SQL servers:
(This is just to ensure that MySQL never need to do a rollback
for a SQL command). This may be a little awkward at times as column
Because MySQL allows you to work with table types that don't
support transactions, and thus can't rollback
data, some things
behave a little differently in MySQL than in other SQL servers.
This is just to ensure that MySQL never need to do a rollback
for a SQL command. This may be a little awkward at times as column
values must be checked in the application, but this will actually give
you a nice speed increase as it allows MySQL to do some
optimizations that otherwise would be very hard to do.
If you set a column to an incorrect value, MySQL will, instead of
doing a rollback, store the best possible value
in the column:
NULL
into a column that doesn't take
NULL
values, MySQL will store 0 or ''
(empty
string) in it instead. (This behavior can, however, be changed with the
-DDONT_USE_DEFAULT_FIELDS compile option).
DATE
and DATETIME
columns. (Like 2000-02-31 or 2000-02-00).
If the date is totally wrong, MySQL will store the special
0000-00-00 date value in the column.
enum
to an unsupported value, it will be set to
the error value 'empty string', with numeric value 0.
PROCEDURE
on a query that returns an empty set,
in some cases the PROCEDURE
will not transform the columns.
MERGE
doesn't check if the underlying
tables are of compatible types.
NaN
, -Inf
and Inf
values in double. Using these will cause problems when trying to export
and import data. We should as an intermediate solution change NaN
to
NULL
(if possible) and -Inf
and Inf
to the
Minimum respective maximum possible double
value.
LIMIT
on negative numbers are treated as big positive numbers.
The following are known bugs in earlier versions of MySQL:
DROP TABLE
on a table that is
one among many tables that is locked with LOCK TABLES
.
LOCK table
with WRITE
FLUSH TABLES
UPDATE
that updated a key with
a WHERE
on the same key may have failed because the key was used to
search for records and the same row may have been found multiple times:
UPDATE tbl_name SET KEY=KEY+1 WHERE KEY > 100;A workaround is to use:
mysql> UPDATE tbl_name SET KEY=KEY+1 WHERE KEY+0 > 100;This will work because MySQL will not use index on expressions in the
WHERE
clause.
For platform-specific bugs, see the sections about compiling and porting.
This section compares MySQL to other popular databases.
This section has been written by the MySQL developers, so it should be read with that in mind. There are no factual errors contained in this section that we know of. If you find something which you believe to be an error, please contact us about it at docs@mysql.com.
For a list of all supported limits, functions, and types, see the
crash-me
Web page at
http://www.mysql.com/information/crash-me.php.
mSQL
mSQL
should be quicker at:
INSERT
operations into very simple tables with few columns and keys.
CREATE TABLE
and DROP TABLE
.
SELECT
on something that isn't an index. (A table scan is very
easy.)
mSQL
(and
most other SQL implementations) on the following:
SELECT
operations.
VARCHAR
columns.
SELECT
with many expressions.
SELECT
on large tables.
mSQL
, once one connection
is established, all others must wait until the first has finished, regardless
of whether the connection is running a query that is short or long. When the
first connection terminates, the next can be served, while all the others wait
again, etc.
mSQL
can become pathologically slow if you change the order of tables
in a SELECT
. In the benchmark suite, a time more than 15000 times
slower than MySQL was seen. This is due to mSQL
's lack of a
join optimizer to order tables in the optimal order. However, if you put the
tables in exactly the right order in mSQL
2 and the WHERE
is
simple and uses index columns, the join will be relatively fast!
See section 5.1.4 The MySQL Benchmark Suite.
ORDER BY
and GROUP BY
.
DISTINCT
.
TEXT
or BLOB
columns.
GROUP BY
and HAVING
.
mSQL
does not support GROUP BY
at all.
MySQL supports a full GROUP BY
with both HAVING
and
the following functions: COUNT()
, AVG()
, MIN()
,
MAX()
, SUM()
, and STD()
. COUNT(*)
is optimized to
return very quickly if the SELECT
retrieves from one table, no other
columns are retrieved, and there is no WHERE
clause. MIN()
and
MAX()
may take string arguments.
INSERT
and UPDATE
with calculations.
MySQL can do calculations in an INSERT
or UPDATE
.
For example:
mysql> UPDATE SET x=x*10+y WHERE x<20;
SELECT
with functions.
MySQL has many functions (too many to list here; see section 6.3 Functions for Use in SELECT
and WHERE
Clauses).
MEDIUMINT
that is 3 bytes long. If you have 100,000,000 records,
saving even one byte per record is very important.
mSQL2
has a more limited set of column types, so it is
more difficult to get small tables.
mSQL
stability, so we cannot say
anything about that.
mSQL
, and is also less expensive than
mSQL
. Whichever product you choose to use, remember to at least
consider paying for a license or e-mail support. (You are required to get
a license if you include MySQL with a product that you sell,
of course.)
mSQL
with
some added features.
mSQL
has a JDBC driver, but we have too little experience
with it to compare.
GROUP BY
, and so on are still not implemented in mSQL
, it
has a lot of catching up to do. To get some perspective on this, you
can view the mSQL
`HISTORY' file for the last year and
compare it with the News section of the MySQL Reference Manual
(see section G MySQL change history). It should be pretty obvious which one has developed
most rapidly.
mSQL
and MySQL have many interesting third-party
tools. Because it is very easy to port upward (from mSQL
to
MySQL), almost all the interesting applications that are available for
mSQL
are also available for MySQL.
MySQL comes with a simple msql2mysql
program that fixes
differences in spelling between mSQL
and MySQL for the
most-used C API functions.
For example, it changes instances of msqlConnect()
to
mysql_connect()
. Converting a client program from mSQL
to
MySQL usually takes a couple of minutes.
mSQL
Tools for MySQL
According to our experience, it would just take a few hours to convert tools
such as msql-tcl
and msqljava
that use the
mSQL
C API so that they work with the MySQL C API.
The conversion procedure is:
msql2mysql
on the source. This requires the
replace
program, which is distributed with MySQL.
Differences between the mSQL
C API and the MySQL C API are:
MYSQL
structure as a connection type (mSQL
uses an int
).
mysql_connect()
takes a pointer to a MYSQL
structure as a
parameter. It is easy to define one globally or to use malloc()
to get
one.
mysql_connect()
also takes two parameters for specifying the user and
password. You may set these to NULL, NULL
for default use.
mysql_error()
takes the MYSQL
structure as a parameter. Just add
the parameter to your old msql_error()
code if you are porting old code.
mSQL
returns only a text error message.
mSQL
and MySQL Client/Server Communications Protocols DifferThere are enough differences that it is impossible (or at least not easy) to support both.
The most significant ways in which the MySQL protocol differs
from the mSQL
protocol are listed below:
mSQL
2.0 SQL Syntax Differs from MySQLColumn types
MySQL
CREATE TABLE
Syntax):
ENUM
type for one of a set of strings.
SET
type for many of a set of strings.
BIGINT
type for 64-bit integers.
UNSIGNED
option for integer columns.
ZEROFILL
option for integer columns.
AUTO_INCREMENT
option for integer columns that are a
PRIMARY KEY
.
See section 8.4.3.126 mysql_insert_id()
.
DEFAULT
value for all columns.
mSQL2
mSQL
column types correspond to the MySQL types shown below:
mSQL type | Corresponding MySQL type |
CHAR(len) | CHAR(len)
|
TEXT(len) | TEXT(len) . len is the maximal length.
And LIKE works.
|
INT | INT . With many more options!
|
REAL | REAL . Or FLOAT . Both 4- and 8-byte versions are available.
|
UINT | INT UNSIGNED
|
DATE | DATE . Uses ANSI SQL format rather than mSQL 's own format.
|
TIME | TIME
|
MONEY | DECIMAL(12,2) . A fixed-point value with two decimals.
|
Index Creation
MySQL
CREATE TABLE
statement.
mSQL
CREATE INDEX
statements.
To Insert a Unique Identifier into a Table
MySQL
AUTO_INCREMENT
as a column type
specifier.
See section 8.4.3.126 mysql_insert_id()
.
mSQL
SEQUENCE
on a table and select the _seq
column.
To Obtain a Unique Identifier for a Row
MySQL
PRIMARY KEY
or UNIQUE
key to the table and use this.
New in Version 3.23.11: If the PRIMARY
or UNIQUE
key consists of only one
column and this is of type integer, one can also refer to it as
_rowid
.
mSQL
_rowid
column. Observe that _rowid
may change over time
depending on many factors.
To Get the Time a Column Was Last Modified
MySQL
TIMESTAMP
column to the table. This column is automatically set
to the current date and time for INSERT
or UPDATE
statements if
you don't give the column a value or if you give it a NULL
value.
mSQL
_timestamp
column.
NULL
Value Comparisons
MySQL
NULL
is always NULL
.
mSQL
mSQL
, NULL = NULL
is TRUE. You
must change =NULL
to IS NULL
and <>NULL
to
IS NOT NULL
when porting old code from mSQL
to MySQL.
String Comparisons
MySQL
BINARY
attribute, which causes comparisons to be done according to the
ASCII order used on the MySQL server host.
mSQL
Case-insensitive Searching
MySQL
LIKE
is a case-insensitive or case-sensitive operator, depending on
the columns involved. If possible, MySQL uses indexes if the
LIKE
argument doesn't start with a wild-card character.
mSQL
CLIKE
.
Handling of Trailing Spaces
MySQL
CHAR
and VARCHAR
columns. Use a TEXT
column if this behavior is not desired.
mSQL
WHERE
Clauses
MySQL
AND
is evaluated
before OR
). To get mSQL
behavior in MySQL, use
parentheses (as shown in an example below).
mSQL
mSQL
query:
mysql> SELECT * FROM table WHERE a=1 AND b=2 OR a=3 AND b=4;To make MySQL evaluate this the way that
mSQL
would,
you must add parentheses:
mysql> SELECT * FROM table WHERE (a=1 AND (b=2 OR (a=3 AND (b=4))));
Access Control
MySQL
mSQL
When reading the following, please note that both products are continually evolving. We at MySQL AB and the PostgreSQL developers are both working on making our respective database as good as possible, so we are both a serious choice to any commercial database.
The following comparison is made by us at MySQL AB. We have tried to be as accurate and fair as possible, but because we don't have a full knowledge of all PostgreSQL features while we know MySQL througly, we may have got some things wrong. We will however correct these when they come to our attention.
We would first like to note that PostgreSQL
and MySQL
are both widely used products, but with different design goals, even if
we are both striving to be ANSI SQL compatible. This means that for
some applications MySQL is more suitable and for others
PostgreSQL
is more suitable. When choosing which database to
use, you should first check if the database's feature set satisfies your
application. If you need speed, MySQL is probably your best
choice. If you need some of the extra features that only PostgreSQL
can offer, you should use PostgreSQL
.
When adding things to MySQL we take pride to do an optimal, definite solution. The code should be so good that we shouldn't have any need to change it in the foreseeable future. We also do not like to sacrifice speed for features but instead will do our utmost to find a solution that will give maximal throughput. This means that development will take a little longer, but the end result will be well worth this. This kind of development is only possible because all server code are checked by one of a few (currently two) persons before it's included in the MySQL server.
We at MySQL AB believe in frequent releases to be able to push out new features quickly to our users. Because of this we do a new small release about every 3 weeks, which a major branch every year. All releases are throughly tested with our testing tools on a lot of different platforms.
PostgreSQL is based on a kernel with lots of contributors. In this setup it makes sense to prioritize adding a lot of new features, instead of implementing them optimally, because one can always optimize things later if there arises a need for this.
Another big difference between MySQL and PostgreSQL is that nearly all of the code in the MySQL server are coded by developers that are employed by MySQL AB and are still working on the server code. The exceptions are the transaction engines and the regexp library.
This is in sharp contrast to the PostgreSQL code where the majority of the code is coded by a big group of people with different backgrounds. It was only recently that the PostgreSQL developers announced that they current developer group had finally had time to take a look at all the code in the current PostgreSQL release.
Both of the above development methods has it's own merits and drawbacks. We here at MySQL AB think of course that our model is better because our model gives better code consistence, more optimal and reusable code and, in our opinion, fewer bugs. Because we are the authors of the MySQL server code we are better able to coordinate new features and releases.
On the crash-me page you can find a list of those database constructs and limits that one can detect automatically with a program. Note however that a lot of the numerical limits may be changed with startup options for respective database. The above web page is however extremely useful when you want to ensure that your applications works with many different databases or when you want to convert your application from one datbase to another.
MySQL offers the following advantages over PostgreSQL:
MySQL
is generally much faster than PostgreSQL.
See section 1.5.2.3 Benchmarking MySQL and PostgreSQL.
MySQL
. PostgreSQL doesn't
yet support 24/7 systems because you have have to run vacuum()
once in a while to reclaim space from UPDATE
and DELETE
commands and to perform statistics analyzes that are critical to get
good performance with PostgreSQL. Vacuum is also needed after adding
a lot of new rows to a table. On a busy system with lots of changes
vacuum must be run very frequently, in the worst cases even many times a
day. During the vacuum()
run, which may take hours if the
database is big, the database is from a production standpoint
practically dead. The PostgreSQL team has fixing this on their TODO,
but we assume that this is not an easy thing to fix permanently.
PostgreSQL
.
ALTER TABLE
.
HEAP
tables or disk based MyISAM
. See section 7 MySQL Table Types.
BDB
and InnoDB
). Because
every transaction engine performs differently under different
conditions, this gives the application writer more options to find an
optimal solution for his/her setup. See section 7 MySQL Table Types.
MERGE
tables gives you a unique way to instantly make a view over
a set of identical tables and use these as one. This is perfectly for
systems where you have log files that you order for example by month.
See section 7.2 MERGE Tables.
INSERT
,
SELECT
, update/delete
grants per user on a database or a
table MySQL allows you to define a full set of different
privileges on database, table and columns level. MySQL also allows
you to specify the privilege on host+user combinations. See section 4.3.1 GRANT
and REVOKE
Syntax.
MySQL
table types (except InnoDB) are implemented as
files (ie: one table per file), which makes it really easy to backup,
move, delete and even symlink databases and tables when the server is
down.
Drawbacks with MySQL compared to PostgreSQL:
MyISAM
tables, is
in many cases faster than page locks, row locks or versioning. The
drawback however is that if one doesn't take into account how table
locks works, a single long-running query can block a table for updates
for a long time. This can usable be avoided when designing the
application. If not, one can always switch the trouble table to use one
of the transactional table types. See section 5.3.2 Table Locking Issues.
DELETE
and multi-table UPDATE
and in MySQL 4.1
with SUB-SELECT
)
PostgreSQL offers currently the following advantages over MySQL:
Note that because we know the MySQL road map, we have included in the following table the version when MySQL should support this feature. Unfortunately we couldn't do this for previous comparison, because we don't know the PostgreSQL roadmap.
Feature | MySQL version |
Subselects | 4.1 |
Foreign keys | 4.0 and 4.1 |
Views. | 4.2 |
Stored procedures in multiple languages | 4.1 |
Extensible type system. | Not planed |
Unions | 4.0. |
Full join. | 4.0 or 4.1. |
Triggers. | 4.1 |
Constrainst | 4.1 |
Cursors | 4.1 or 4.2 |
Extensible index types like R-trees | R-trees are planned to 4.2 |
Inherited tables | Not planned |
Other reasons to use PostgreSQL:
Drawbacks with PostgreSQL compared to MySQL:
Vaccum()
makes PostgreSQL hard to use in a 24/7 environment.
For a complete list of drawbacks, you should also examine the first table in this section.
The only open source benchmark, that we know of, that can be used to benchmark MySQL and PostgreSQL (and other databases) is our own. It can be found at: http://www.mysql.com/information/benchmarks.html.
We have many times asked the PostgreSQL developers and some PostgreSQL users to help us extend this benchmark to make the definitive benchmark for databases, but unfortunately we haven't got any feedback for this.
We, the MySQL developers, have because of this spent a lot of hours to get maximum performance from PostgreSQL for the benchmarks, but because we don't know PostgreSQL intimately we are sure that there are things that we have missed. We have on the benchmark page documented exactly how we did run the benchmark so that it should be easy for anyone to repeat and verify our results.
The benchmarks are usually run with and without the --fast
option. When run with --fast
we are trying to use every trick
the server can do to get the code to execute as fast as possible.
The idea is that the normal run should show how the server would work in
a default setup and the --fast
run shows how the server would do
if the application developer would use extensions in the server to make
his application run faster.
When running with PostgreSQL and --fast
we do a vacuum()
between after every major table update and drop table to make the database
in perfect shape for the following selects. The time for vacuum() is
measured separately.
When running with PostgreSQL 7.1.1 we could however not run with
--fast
because during the insert test, the postmaster (the
PostgreSQL deamon) died and the database was so corrupted that it was
impossible to restart postmaster. (The details about the machine we run
the benchmark can be found on the benchmark page). After this happened
twice, we decided to postpone the --fast
test until next
PostgreSQL release.
Before going to the other benchmarks we know of, We would like to give some background to benchmarks:
It's very easy to write a test that shows ANY database to be best database in the world, by just restricting the test to something the database is very good at and not test anything that the database is not good at; If one after this publish the result with a single figure things is even easier.
This would be like we would measure the speed of MySQL compared to PostgreSQL by looking at the summary time of the MySQL benchmarks on our web page. Based on this MySQL would be more than 40 times faster than PostgreSQL, something that is of course not true. We could make things even worse by just taking the test where PostgreSQL performs worst and claim that MySQL is more than 2000 times faster than PostgreSQL.
The case is that MySQL does a lot of optimizations that PostgreSQL doesn't do and the other way around. An SQL optimizer is a very complex thing and a company could spend years on just making the optimizer faster and faster.
When looking at the benchmark results you should look for things that you do in your application and just use these results to decide which database would be best suited for your application. The benchmark results also shows things a particular database is not good at and should give you a notion about things to avoid and what you may have to do in other ways.
We know of two benchmark tests that claims that PostgreSQL performers better than MySQL. These both where multi-user tests, a test that we here at MySQL AB haven't had time to write and include in the benchmark suite, mainly because it's a big task to do this in a manner that is fair against all databases.
One is the benchmark paid for by Great Bridge.
This is the worst benchmark we have ever seen anyone ever conduct. This was not only tuned to only test what PostgreSQL is absolutely best at, it was also totally unfair against every other database involved in the test.
NOTE: We know that not even some of the main PostgreSQL developers did like the way Great Bridge conducted the benchmark, so we don't blame them for the way the benchmark was made.
This benchmark has been condemned in a lot of postings and newsgroups so we will here just shortly repeat some things that where wrong with it.
Tim Perdue, a long time PostgreSQL fan and a reluctant MySQL user published a comparison on phpbuider.
When we got aware of the comparison, we phoned Tim Perdue about this because there was a lot of strange things in his results. For example, he claimed that MySQL had a problem with five users in his tests, when we know that there are users with similar machines as his that are using MySQL with 2000 simultaneous connections doing 400 queries per second (In this case the limit was the web bandwidth, not the database).
It sounded like he was using a Linux kernel that either had some problems with many threads (Linux kernels before 2.4 had a problem with this but we have documented how to fix this and Tim should be aware of this problem). The other possible problem could have been an old glibc library and that Tim didn't use a MySQL binary from our site, which is linked with a corrected glibc library, but had compiled a version of his own with. In any of the above cases, the symptom would have been exactly what Tim had measured.
We asked Tim if we could get access to his data so that we could repeat the benchmark and if he could check the MySQL version on the machine to find out what was wrong and he promised to come back to us about this. He has not done that yet.
Because of this we can't put any trust in this benchmark either :(
Conclusion:
The only benchmarks that exist today that anyone can download and run against MySQLand PostgreSQL is the MySQL benchmarks. We here at MySQL believe that open source databases should be tested with open source tools! This is the only way to ensure that no one does tests that nobody can reproduce and use this to claim that a database is better than another. Without knowing all the facts it's impossible to answer the claims of the tester.
The thing we find strange is that every test we have seen about PostgreSQL, that is impossible to reproduce, claims that PostgreSQL is better in most cases while our tests, which anyone can reproduce, clearly shows otherwise. With this we don't want to say that PostgreSQL isn't good at many things (It is!) We would just like to see a fair test where they are very good so that we could get some friendly competition going!
For more information about our benchmarks suite see See section 5.1.4 The MySQL Benchmark Suite.
We are working on an even better benchmark suite, including much better documentation of what the individual tests really do and how to add more tests to the suite.
This appendix lists the features that we plan to implement in MySQL.
Everything in this list is approximately in the order it will be done. If you want to affect the priority order, please register a license or support us and tell us what you want to have done more quickly. See section 1.3 MySQL Licensing and Support.
The plan is that we in the future will support the full ANSI SQL99 standard, but with a lot of useful extensions. The challenge is to do this without sacrifying the speed or compromise the code.
We plan to make MySQL Version 4.0 a ``quick'' release where we only add some new stuff to enable others to help us with developing new features into Version 4.1. The MySQL 4.0 version should only take us about a month to make after which we want to stabilize it and start working on Version 4.1. Version 4.0 should have the following new features:
The news section for 4.0 includes a list of the features we have already implemented in the 4.0 tree. See section G.1 Changes in release 4.0.x (Development; Alpha).
.frm
files) This will enable us
to not run out of bits when adding more table options. One will still
be able to use the old .frm
file format with 4.0. All newly created
tables will, however, use the new format.
The new file format will enable us to add new column types, more options
for keys and FOREIGN KEY
support.
mysqld
as a library. This will have the same interface as the
standard MySQL client (with an extra function to just set up
startup parameters) but will be faster (no TCP/IP or socket overhead),
smaller and much easier to use for embedded products.
One will be able to define at link time if one wants to use the
client/server model or a stand-alone application just by defining which
library to link with.
The mysqld
will support all standard MySQL features and
one can use it in a threaded client to run different queries in each
thread.
RAND()
and user variables @var
.
DELETE FROM table_name
will return the number of deleted rows. For
fast execution one should use TRUNCATE table_name
.
DELETE
on MyISAM
tables to use the record cache.
To do this, we need to update the threads record cache when we update
the .MYD
file.
ORDER BY key_name DESC
queries.
SHOW COLUMNS FROM table_name
(used by mysql
client to allow
expansions of column names) should not open the table, but only the
definition file. This will require less memory and be much faster.
SET CHARACTER SET
we should translate the whole query
at once and not only strings. This will enable users to use the translated
characters in database, table and column names.
gethostbyaddr_r()
so that we can change
ip_to_hostname()
to not block other threads while doing DNS lookups.
record_in_range()
method to MERGE
tables to be
able to choose the right index when there is many to choose from. We should
also extend the info interface to get the key distribution for each index,
of analyze
is run on all sub tables.
SET SQL_DEFAULT_TABLE_TYPE=[MyISAM | INNODB | BDB | HEAP]
.
select id from t where grp in (select grp from g where u > 100)
select a.col1, b.col2 from (select max(col1) as col1 from root_table ) a, other_table b where a.col1=b.col1This could be done by automatically creating temporary tables for the derived tables for the duration of the query.
PREPARE
of statements and sending of parameters to mysqld
.
INSERT ... SELECT
to optionally use concurrent inserts.
RENAME DATABASE
. To make this safe for all table handlers,
it should work as follows:
RENAME
command.
SELECT MIN(column)
... GROUP BY
.
long_query_time
with a granularity
in microseconds.
mysql
command line client, with
options like database in use, time and date...
MERGE
tables.
myisampack
code into the server.
INSERT/DELETE/UPDATE
so that we
can gracefully recover if the index file gets full.
ALTER TABLE
on a table that is symlinked to another
disk, create temporary tables on this disk.
DATE/DATETIME
type that handles time zone information
properly, so that dealing with dates in different time zones is easier.
MyISAM
)
without threads.
INSERT SQL_CONCURRENT
and mysqld --concurrent-insert
to do
a concurrent insert at the end of the file if the file is read-locked.
FOREIGN
key definitions in the `.frm' file.
DELETE
lockd
works with modern Linux kernels; If not, we have
to fix lockd
! To test this, start mysqld
with
--enable-locking
and run the different fork* test suits. They shouldn't
give any errors if lockd
works.
LIMIT
, like in LIMIT @a,@b
.
UPDATE
statements. For example:
UPDATE TABLE foo SET @a=a+b,a=@a, b=@a+c
GROUP BY
, as in the following example:
SELECT id, @a:=count(*), sum(sum_col)/@a FROM table_name GROUP BY id
.
DEFAULT
values to columns. Give an error when using
an INSERT
that doesn't contain a column that doesn't have a
DEFAULT
.
SELECT CACHED ...
mysql_query()
commands in a row
without reading results or give a nice error message when one does this.
BIT
type to take 1 bit (now BIT
takes 1 char).
ctime()
doesn't work on some FreeBSD systems.
IMAGE
option to LOAD DATA INFILE
to not update
TIMESTAMP
and AUTO_INCREMENT
fields.
LOAD DATE INFILE.. UPDATE
syntax.
LOAD DATA INFILE ... REPLACE INTO
is now.
LOAD DATA INFILE
understand syntax like:
LOAD DATA INFILE 'file_name.txt' INTO TABLE tbl_name TEXT_FIELDS (text_field1, text_field2, text_field3) SET table_field1=concatenate(text_field1, text_field2), table_field3=23 IGNORE text_field3 This can be used to skip over extra columns in the text file, or update columns based on expressions of the read data...
LOAD DATA INFILE 'file_name' INTO TABLE 'table_name' ERRORS TO err_table_name
This would cause any errors and warnings to be logged into the err_table_name
table. That table would have a structure like:
line_number - line number in data file error_message - the error/warning message and maybe data_line - the line from the data file
VARCHAR
support (There is already support for this in MyISAM).
mysql
to netscape.
LOCK DATABASES
. (with various options)
DECIMAL
and NUMERIC
types can't read exponential numbers;
Field_decimal::store(const char *from,uint len)
must be recoded
to fix this.
mysql.cc
to do fewer malloc()
calls when hashing field
names.
t1 JOIN t2 ON ...
and t1 JOIN t2 USING ...
Currently, you can only use this syntax with LEFT JOIN
.
unsigned long long
type.
show status
. Counts for:
INSERT
/DELETE
/UPDATE
statements. Records reads and
updated. Selects on 1 table and selects with joins. Mean number of
tables in select. Number of ORDER BY
and GROUP BY
queries.
mysql
in the middle of a query, you should open
another connection and kill the old running query.
Alternatively, an attempt should be made to detect this in the server.
SHOW INFO FROM tbl_name
for basic table information
should be implemented.
NATURAL JOIN
and UNION JOIN
select a from crash_me left join crash_me2 using (a)
; In this
case a is assumed to come from the crash_me table.
ON
and USING
works with the JOIN
and
INNER JOIN
join types.
CONNECT BY PRIOR ...
to search hierarchy structures.
RENAME DATABASE
mysqladmin copy database new-database
. -- Requires COPY command to be
added to mysqld
SHOW HOSTS
for printing information about the hostname cache.
DELETE
and REPLACE
options to the UPDATE
statement
(this will delete rows when one gets a duplicate key error while updating).
DATETIME
to store fractions of seconds.
NULL
for calculated columns.
SELECT COUNT(*)*(id+0) FROM table_name GROUP BY id
ALTER TABLE
doesn't abort clients that executes
INSERT DELAYED
.
UPDATE
clause contains the old
values before the update started.
myisamchk
, REPAIR
and OPTIMIZE TABLE
should be able
to handle cases where the data and/or index files are symbolic links.
pread()
/pwrite()
on Windows to enable
concurrent inserts.
SUM(DISTINCT)
ANY()
,EVERY()
and SOME()
group functions. In
ANSI SQL these only works on boolean columns, but we can extend these to
work on any columns/expressions by applying: value == 0 -> FALSE and
value <> 0 -> TRUE.
MAX(column)
is the same as the column type.
create table t1 (a DATE); insert into t1 values (now()); create table t2 select max(a) from t1; show columns from t2;
UPDATE
the row
if it exists and INSERT
a new row if the row didn't exist.
(Like REPLACE
works with INSERT
/ DELETE
)
get_changed_tables(timeout,table1,table2,...)
update items,month set
items.price=month.price where items.id=month.id;
;
SHOW
commands.
SET TIMESTAMP=#;
UNION
, MINUS
, INTERSECT
and FULL OUTER JOIN
.
(Currently only LEFT OUTER JOIN
is supported)
UNIQUE
on fields that can be NULL
.
SQL_OPTION MAX_SELECT_TIME=#
to put a time limit on a query.
LIMIT
to retrieve data from the end.
safe_mysqld
: according to FSSTND (which
Debian tries to follow) PID files should go into `/var/run/<progname>.pid'
and log files into `/var/log'. It would be nice if you could put the
"DATADIR" in the first declaration of "pidfile" and "log", so the
placement of these files can be changed with a single statement.
zlib()
for gzip
-ed files to LOAD DATA INFILE
.
BLOB
columns (partly solved now).
AUTO_INCREMENT
value when one sets a column to 0.
Use NULL
instead.
JOIN
with parentheses.
GET_LOCK
. When doing this,
one must also handle the possible deadlocks this change will introduce.
Time is given according to amount of work, not real time.
This chapter describes how to obtain and install MySQL:
The recommended way to install MySQL on Linux is by using an RPM
file. The MySQL RPMs are currently being built on a RedHat Version
6.2 system but should work on other versions of Linux that support rpm
and use glibc
.
If you have problems with an RPM file, for example, if you receive the error
``Sorry, the host 'xxxx' could not be looked up
'', see
section 2.6.1.1 Linux Notes for Binary Distributions.
The RPM files you may want to use are:
MySQL-VERSION.i386.rpm
The MySQL server. You will need this unless you only want to
connect to a MySQL server running on another machine.
MySQL-client-VERSION.i386.rpm
The standard MySQL client programs. You probably always want to
install this package.
MySQL-bench-VERSION.i386.rpm
Tests and benchmarks. Requires Perl and msql-mysql-modules RPMs.
MySQL-devel-VERSION.i386.rpm
Libraries and include files needed if you want to compile other
MySQL clients, such as the Perl modules.
MySQL-VERSION.src.rpm
This contains the source code for all of the above packages. It can also
be used to try to build RPMs for other architectures (for example, Alpha
or SPARC).
To see all files in an RPM package, run:
shell> rpm -qpl MySQL-VERSION.i386.rpm
To perform a standard minimal installation, run:
shell> rpm -i MySQL-VERSION.i386.rpm MySQL-client-VERSION.i386.rpm
To install just the client package, run:
shell> rpm -i MySQL-client-VERSION.i386.rpm
The RPM places data in `/var/lib/mysql'. The RPM also creates the appropriate entries in `/etc/rc.d/' to start the server automatically at boot time. (This means that if you have performed a previous installation, you may want to make a copy of your previously installed MySQL startup file if you made any changes to it, so you don't lose your changes.)
After installing the RPM file(s), the mysqld
daemon should be running
and you should now be able to start using MySQL.
See section 2.4 Post-installation Setup and Testing.
If something goes wrong, you can find more information in the binary installation chapter. See section M.1 Installing a MySQL Binary Distribution.
The following instructions apply to precompiled binary distributions. If you download a source distribution, you will have to compile and install it yourself.
If you don't have a copy of the MySQL distribution, you should first download one from http://www.mysql.com/downloads/mysql-3.23.html.
If you plan to connect to MySQL from some other program, you will probably also need the MyODBC driver. You can find this at the MyODBC download page (http://www.mysql.com/downloads/api-myodbc.html).
To install either distribution, unzip it in some empty directory and run the
Setup.exe
program.
By default, MySQL-Windows is configured to be installed in
`C:\mysql'. If you want to install MySQL elsewhere,
install it in `C:\mysql' first, then move the installation to
where you want it. If you do move MySQL, you must indicate
where everything is located by supplying a --basedir
option when
you start the server. For example, if you have moved the MySQL
distribution to `D:\programs\mysql', you must start mysqld
like this:
C:\> D:\programs\mysql\bin\mysqld --basedir D:\programs\mysql
Use mysqld --help
to display all the options that mysqld
understands!
With all newer MySQL versions, you can also create a `C:\my.cnf' file that holds any default options for the MySQL server. Copy the file `\mysql\my-xxxxx.cnf' to `C:\my.cnf' and edit it to suit your setup. Note that you should specify all paths with `/' instead of `\'. If you use `\', you need to specify it twice, because `\' is the escape character in MySQL. See section 4.1.2 my.cnf Option Files.
Starting with MySQL 3.23.38, the Windows distribution includes
both the normal and the MySQL-Max binaries. The main benefit
of using the normal mysqld.exe
binary is that it's a little
faster and uses less resources.
Here is a list of the different MySQL servers you can use:
mysqld | Compiled with full debugging and automatic memory allocation checking, symbolic links, BDB and InnoDB tables. |
mysqld-opt | Optimized binary with no support for transactional tables. |
mysqld-nt | Optimized binary for NT with support for named pipes. You can run this version on Win98, but in this case no named pipes are created and you must have TCP/IP installed. |
mysqld-max | Optimized binary with support for symbolic links, BDB and InnoDB tables. |
mysqld-max-nt |
Like mysqld-max , but compiled with support for named pipes.
|
All of the above binaries are optimized for the Pentium Pro processor but should work on any Intel processor >= i386.
NOTE: If you want to use InnoDB tables, there are certain startup options that must be specified in your `my.ini' file! See section 7.6.2 InnoDB startup options.
Check the MySQL home page for information about the current version and for downloading instructions.
Our main download mirror is located at:
http://download.sourceforge.net/mirrors/mysql/
If you are interested in becoming a MySQL mirror site, you may
anonymously rsync with: rsync://download.sourceforge.net/mysql/
. Please
send e-mail to webmaster@mysql.com notifying us of your mirror to be
added to the list below.
If you have problems downloading from our main site, try using one of the mirrors listed below.
Please report bad or out-of-date mirrors to webmaster@mysql.com.
Europe:
North America:
South America:
Asia:
Africa:
We use GNU Autoconf, so it is possible to port MySQL to all modern systems with working Posix threads and a C++ compiler. (To compile only the client code, a C++ compiler is required but not threads.) We use and develop the software ourselves primarily on Sun Solaris (Versions 2.5 - 2.7) and SuSE Linux Version 7.x.
Note that for many operating systems, the native thread support works only in the latest versions. MySQL has been reported to compile successfully on the following operating system/thread package combinations:
glibc
2.0.7+. See section 2.6.1 Linux Notes (All Linux Versions).
Note that not all platforms are suited equally well for running MySQL. How well a certain platform is suited for a high-load mission critical MySQL server is determined by the following factors:
pthread_mutex_lock()
is too anxious to yield CPU, this will hurt
MySQL tremendously. If this issue
is not taken care of, adding extra CPUs will actually make MySQL
slower.
Based on the above criteria, the best platforms for running MySQL at this point are x86 with SuSE Linux 7.1, 2.4 kernel and ReiserFS (or any similar Linux distribution) and Sparc with Solaris 2.7 or 2.8. FreeBSD comes third, but we really hope it will join the top club once the thread library is improved. We also hope that at some point we will be able to include all other platforms on which MySQL compiles, runs ok, but not quite with the same level of stability and performance, into the top category. This will require some effort on our part in cooperation with the developers of the OS/library components MySQL depends upon. If you are interested in making one of those components better, are in a position to influence their development, and need more detailed instructions on what MySQL needs to run better, send an e-mail to internals@lists.mysql.com.
Please note that the comparison above is not to say that one OS is better or worse than the other in general. We are talking about choosing a particular OS for a dedicated purpose - running MySQL, and compare platforms in that regard only. With this in mind, the result of this comparison would be different if we included more issues into it. And in some cases, the reason one OS is better than the other could simply be that we have put forth more effort into testing on and optimizing for that particular platform. We are just stating our observations to help you make a decision on which platform to use MySQL on in your setup.
The first decision to make is whether you want to use the latest development release or the last stable release:
The second decision to make is whether you want to use a source distribution or a binary distribution. In most cases you should probably use a binary distribution, if one exists for your platform, as this generally will be easier to install than a source distribution.
In the following cases you probably will be better off with a source installation:
MySQL
clients can connect to both MySQL versions.
The extended MySQL binary distribution is marked with the
-max
suffix and is configured with the same options as
mysqld-max
. See section 4.7.5 mysqld-max, An extended mysqld server.
If you want to use the MySQL-Max
RPM, you must first
install the standard MySQL
RPM.
mysqld
with some extra features that are
NOT in the standard binary distributions. Here is a list of the most
common extra options that you may want to use:
--with-berkeley-db
--with-innodb
--with-raid
--with-libwrap
--with-named-z-lib (This is done for some of the binaries)
--with-debug[=full]
pgcc
) or use compiler options that are better optimized for your
processor.
The MySQL naming scheme uses release numbers that consist of three
numbers and a suffix. For example, a release name like
mysql-3.21.17-beta
is interpreted like this:
3
) describes the file format. All Version 3
releases have the same file format.
21
) is the release level. Normally there are two to
choose from. One is the release/stable branch (currently 23
) and the
other is the development branch (currently 4.0
). Normally both are
stable, but the development version may have quirks, missing documentation on
new features, or may fail to compile on some systems.
17
) is the version number within the
release level. This is incremented for each new distribution. Usually you
want the latest version for the release level you have chosen.
beta
) indicates the stability level of the release.
The possible suffixes are:
alpha
indicates that the release contains some large section of
new code that hasn't been 100% tested. Known bugs (usually there are none)
should be documented in the News section. See section G MySQL change history. There are also new
commands and extensions in most alpha releases. Active development that
may involve major code changes can occur on an alpha release, but everything
will be tested before doing a release. There should be no known bugs in any
MySQL release.
beta
means that all new code has been tested. No major new
features that could cause corruption on old code are added. There should
be no known bugs. A version changes from alpha to beta when there
haven't been any reported fatal bugs within an alpha version for at least
a month and we don't plan to add any features that could make any old command
more unreliable.
gamma
is a beta that has been around a while and seems to work fine.
Only minor fixes are added. This is what many other companies call a release.
All versions of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Because the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better.
Note that all releases have been tested at least with:
crash-me
test
Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100 gigabytes of data to work with.
This section describes the default layout of the directories created by installing binary and source distributions.
A binary distribution is installed by unpacking it at the installation location you choose (typically `/usr/local/mysql') and creates the following directories in that location:
Directory | Contents of directory |
`bin' | Client programs and the mysqld server
|
`data' | Log files, databases |
`include' | Include (header) files |
`lib' | Libraries |
`scripts' | mysql_install_db
|
`share/mysql' | Error message files |
`sql-bench' | Benchmarks |
A source distribution is installed after you configure and compile it. By default, the installation step installs files under `/usr/local', in the following subdirectories:
Directory | Contents of directory |
`bin' | Client programs and scripts |
`include/mysql' | Include (header) files |
`info' | Documentation in Info format |
`lib/mysql' | Libraries |
`libexec' | The mysqld server
|
`share/mysql' | Error message files |
`sql-bench' | Benchmarks and crash-me test
|
`var' | Databases and log files |
Within an installation directory, the layout of a source installation differs from that of a binary installation in the following ways:
mysqld
server is installed in the `libexec'
directory rather than in the `bin' directory.
mysql_install_db
is installed in the `/usr/local/bin' directory
rather than in `/usr/local/mysql/scripts'.
You can create your own binary installation from a compiled source distribution by executing the script `scripts/make_binary_distribution'.
MySQL is evolving quite rapidly here at MySQL AB and we want to share this with other MySQL users. We try to make a release when we have very useful features that others seem to have a need for.
We also try to help out users who request features that are easy to implement. We take note of what our licensed users want to have, and we especially take note of what our extended e-mail supported customers want and try to help them out.
No one has to download a new release. The News section will tell you if the new release has something you really want. See section G MySQL change history.
We use the following policy when updating MySQL:
The current stable release is Version 3.23; We have already moved active development to Version 4.0. Bugs will still be fixed in the stable version. We don't believe in a complete freeze, as this also leaves out bug fixes and things that ``must be done.'' ``Somewhat frozen'' means that we may add small things that ``almost surely will not affect anything that's already working.''
As a service, we at MySQL AB provide a set of binary distributions of MySQL that are compiled at our site or at sites where customers kindly have given us access to their machines.
These distributions are generated with scripts/make_binary_distribution
and are configured with the following compilers and options:
gcc
2.7.2.1
CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-assembler
egcs
1.0.3a or 2.90.27 or gcc 2.95.2 and newer
CC=gcc CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex --enable-assembler
gcc
2.8.1
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex
pgcc
2.90.29 (egcs
1.0.3a)
CFLAGS="-O3 -mpentium -mstack-align-double" CXX=gcc CXXFLAGS="-O3 -mpentium -mstack-align-double -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --with-extra-charsets=complex
gcc
2.95.2
CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-extra-charset=complex
gcc
2.7-95q4
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
gcc
2.7.2.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
gcc
2.8.1
CC=gcc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex
gcc
2.8.0
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
gcc
2.7.2.1
CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
gcc
2.7.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
Anyone who has more optimal options for any of the configurations listed above can always mail them to the developer's mailing list at internals@lists.mysql.com.
RPM distributions prior to MySQL Version 3.22 are user-contributed. Beginning with Version 3.22, the RPMs are generated by us at MySQL AB.
If you want to compile a debug version of MySQL, you should add
--with-debug
or --with-debug=full
to the above configure lines
and remove any -fomit-frame-pointer
options.
Before you proceed with the source installation, check first to see if our binary is available for your platform and if it will work for you. We put in a lot of effort into making sure that our binaries are built with the best possible options.
You need the following tools to build and install MySQL from source:
gunzip
to uncompress the distribution.
tar
to unpack the distribution. GNU tar
is
known to work. Sun tar
is known to have problems.
gcc
>= 2.95.2, egcs
>= 1.0.2
or egcs 2.91.66
, SGI C++, and SunPro C++ are some of the
compilers that are known to work. libg++
is not needed when
using gcc
. gcc
2.7.x has a bug that makes it impossible
to compile some perfectly legal C++ files, such as
`sql/sql_base.cc'. If you only have gcc
2.7.x, you must
upgrade your gcc
to be able to compile MySQL. gcc
2.8.1 is also known to have problems on some platforms so it should be
avoided if there exists a new compiler for the platform..
gcc
>= 2.95.2 is recommended when compiling MySQL
Version 3.23.x.
make
program. GNU make
is always recommended and is
sometimes required. If you have problems, we recommend trying GNU
make
3.75 or newer.
If you are using a recent version of gcc, recent enough to understand
-fno-exceptions
option, it is VERY IMPORTANT that you use
it. Otherwise, you may compile a binary that crashes randomly. We also
recommend that you use -felide-contructors
and -fno-rtti
along
with -fno-exceptions
. When in doubt, do the following:
CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static
On most systems this will give you a fast and stable binary.
If you run into problems, PLEASE ALWAYS USE mysqlbug
when
posting questions to mysql@lists.mysql.com. Even if the problem
isn't a bug, mysqlbug
gathers system information that will help others
solve your problem. By not using mysqlbug
, you lessen the likelihood
of getting a solution to your problem! You will find mysqlbug
in the
`scripts' directory after you unpack the distribution.
See section 1.2.22.3 How to Report Bugs or Problems.
The basic commands you must execute to install a MySQL source distribution are:
shell> groupadd mysql shell> useradd -g mysql mysql shell> gunzip < mysql-VERSION.tar.gz | tar -xvf - shell> cd mysql-VERSION shell> ./configure --prefix=/usr/local/mysql shell> make shell> make install shell> scripts/mysql_install_db shell> chown -R root /usr/local/mysql shell> chown -R mysql /usr/local/mysql/var shell> chgrp -R mysql /usr/local/mysql shell> cp support-files/my-medium.cnf /etc/my.cnf shell> /usr/local/mysql/bin/safe_mysqld --user=mysql &
If you want have support for InnoDB tables, you should edit the
/etc/my.cnf
file and remove the #
character before the
parameters that starts with innodb_...
. See section 4.1.2 my.cnf Option Files. See section 7.6.2 InnoDB startup options.
If you start from a source RPM, then do the following:
shell> rpm --rebuild MySQL-VERSION.src.rpm
This will make a binary RPM that you can install.
You can add new users using the bin/mysql_setpermission
script if
you install the DBI
and Msql-Mysql-modules
Perl modules.
A more detailed description follows.
To install a source distribution, follow the steps below, then proceed to section 2.4 Post-installation Setup and Testing, for post-installation initialization and testing:
tar
archives and have names like `mysql-VERSION.tar.gz', where
VERSION
is a number like 3.23.41.
mysqld
to run as:
shell> groupadd mysql shell> useradd -g mysql mysqlThese commands add the
mysql
group, and the mysql
user. The
syntax for useradd
and groupadd
may differ slightly on different
versions of Unix. They may also be called adduser
and addgroup
.
You may wish to call the user and group something else instead of mysql
.
shell> gunzip < /path/to/mysql-VERSION.tar.gz | tar xvf -This command creates a directory named `mysql-VERSION'.
shell> cd mysql-VERSIONNote that currently you must configure and build MySQL from this top-level directory. You can not build it in a different directory.
shell> ./configure --prefix=/usr/local/mysql shell> makeWhen you run
configure
, you might want to specify some options.
Run ./configure --help
for a list of options.
section 2.3.3 Typical configure
Options, discusses some of the
more useful options.
If configure
fails, and you are going to send mail to
mysql@lists.mysql.com to ask for assistance, please include any
lines from `config.log' that you think can help solve the problem. Also
include the last couple of lines of output from configure
if
configure
aborts. Post the bug report using the mysqlbug
script. See section 1.2.22.3 How to Report Bugs or Problems.
If the compile fails, see section 2.3.5 Problems Compiling?, for help with
a number of common problems.
shell> make installYou might need to run this command as
root
.
shell> scripts/mysql_install_dbNote that MySQL versions older than Version 3.22.10 started the MySQL server when you run
mysql_install_db
. This is no
longer true!
root
and ownership of the data
directory to the user that you will run mysqld
as:
shell> chown -R root /usr/local/mysql shell> chown -R mysql /usr/local/mysql/var shell> chgrp -R mysql /usr/local/mysqlThe first command changes the
owner
attribute of the files to the
root
user, the second one changes the owner
attribute of the
data directory to the mysql
user, and the third one changes the
group
attribute to the mysql
group.
DBI
/DBD
interface,
see section M.2 Perl Installation Comments.
support-files/mysql.server
to the location where
your system has its startup files. More information can be found in the
support-files/mysql.server
script itself and in
section 2.4.3 Starting and Stopping MySQL Automatically.
After everything has been installed, you should initialize and test your distribution:
shell> /usr/local/mysql/bin/safe_mysqld --user=mysql &
If that command fails immediately with mysqld daemon ended
then you can
find some information in the file `mysql-data-directory/'hostname'.err'.
The likely reason is that you already have another mysqld
server
running. See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.
See section 2.4 Post-installation Setup and Testing.
Sometimes patches appear on the mailing list or are placed in the patches area of the MySQL Web site.
To apply a patch from the mailing list, save the message in which the patch appears in a file, change into the top-level directory of your MySQL source tree, and run these commands:
shell> patch -p1 < patch-file-name shell> rm config.cache shell> make clean
Patches from the FTP site are distributed as plain text files or as files
compressed with gzip
. Apply a plain patch as shown above for
mailing list patches. To apply a compressed patch, change into the
top-level directory of your MySQL source tree and run these
commands:
shell> gunzip < patch-file-name.gz | patch -p1 shell> rm config.cache shell> make clean
After applying a patch, follow the instructions for a normal source install,
beginning with the ./configure
step. After running the make
install
step, restart your MySQL server.
You may need to bring down any currently running server before you run
make install
. (Use mysqladmin shutdown
to do this.) Some
systems do not allow you to install a new version of a program if it replaces
the version that is currently executing.
configure
Options
The configure
script gives you a great deal of control over how
you configure your MySQL distribution. Typically you do this
using options on the configure
command line. You can also affect
configure
using certain environment variables. See section B Environment Variables. For a list of options supported by configure
, run
this command:
shell> ./configure --help
Some of the more commonly-used configure
options are described below:
--without-server
option:
shell> ./configure --without-serverIf you don't have a C++ compiler,
mysql
will not compile (it is the
one client program that requires C++). In this case,
you can remove the code in configure
that tests for the C++ compiler
and then run ./configure
with the --without-server
option. The
compile step will still try to build mysql
, but you can ignore any
warnings about `mysql.cc'. (If make
stops, try make -k
to tell it to continue with the rest of the build even if errors occur.)
configure
command, something like one
of these:
shell> ./configure --prefix=/usr/local/mysql shell> ./configure --prefix=/usr/local \ --localstatedir=/usr/local/mysql/dataThe first command changes the installation prefix so that everything is installed under `/usr/local/mysql' rather than the default of `/usr/local'. The second command preserves the default installation prefix, but overrides the default location for database directories (normally `/usr/local/var') and changes it to
/usr/local/mysql/data
.
configure
command like this:
shell> ./configure --with-unix-socket-path=/usr/local/mysql/tmp/mysql.sockNote that the given file must be an absolute pathname!
configure
like this:
shell> ./configure --with-client-ldflags=-all-static \ --with-mysqld-ldflags=-all-static
gcc
and don't have libg++
or libstdc++
installed, you can tell configure
to use gcc
as your C++
compiler:
shell> CC=gcc CXX=gcc ./configureWhen you use
gcc
as your C++ compiler, it will not attempt to link in
libg++
or libstdc++
.
Here is some common environment variables to set depending on
the compiler you are using:
gcc 2.7.2.1 | CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors" |
egcs 1.0.3a | CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" |
gcc 2.95.2 | CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" |
pgcc 2.90.29 or newer | CFLAGS="-O3 -mpentiumpro -mstack-align-double" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -mstack-align-double -felide-constructors -fno-exceptions -fno-rtti" |
--prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-staticThe full configure line would in other words be something like the following for all recent gcc versions:
CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-staticThe binaries we provide on the MySQL Web site at http://www.mysql.com are all compiled with full optimization and should be perfect for most users. See section 2.2.6 MySQL Binaries Compiled by MySQL AB. There are some things you can tweak to make an even faster binary, but this is only for advanced users. See section 5.5.3 How Compiling and Linking Affects the Speed of MySQL. If the build fails and produces errors about your compiler or linker not being able to create the shared library `libmysqlclient.so.#' (`#' is a version number), you can work around this problem by giving the
--disable-shared
option to configure
. In this case,
configure
will not build a shared libmysqlclient.so.#
library.
DEFAULT
column values for
non-NULL
columns (that is, columns that are not allowed to be
NULL
). This causes INSERT
statements to generate an error
unless you explicitly specify values for all columns that require a
non-NULL
value. To suppress use of default values, run
configure
like this:
shell> CXXFLAGS=-DDONT_USE_DEFAULT_FIELDS ./configure
--with-charset
option:
shell> ./configure --with-charset=CHARSET
CHARSET
may be one of big5
, cp1251
, cp1257
,
czech
, danish
, dec8
, dos
, euc_kr
,
gb2312
, gbk
, german1
, hebrew
, hp8
,
hungarian
, koi8_ru
, koi8_ukr
, latin1
,
latin2
, sjis
, swe7
, tis620
, ujis
,
usa7
, or win1251ukr
.
See section 4.6.1 The Character Set Used for Data and Sorting.
If you want to convert characters between the server and the client,
you should take a look at the SET OPTION CHARACTER SET
command.
See section 5.5.6 SET
Syntax.
Warning: If you change character sets after having created any
tables, you will have to run myisamchk -r -q
on every table. Your
indexes may be sorted incorrectly otherwise. (This can happen if you
install MySQL, create some tables, then reconfigure
MySQL to use a different character set and reinstall it.)
With the option --with-extra-charset=LIST
you can define
which additional character sets should be incompiled in the server.
Here LIST
is either a list of character set separated with space,
complex
to include all characters that can't be dynamically loaded
or all
to include all character sets into the binaries.
--with-debug
option:
shell> ./configure --with-debugThis causes a safe memory allocator to be included that can find some errors and that provides output about what is happening. See section H.1 Debugging a MySQL server.
--with-thread-safe-client
configure options. This will create a
libmysqlclient_r
library with which you should link your threaded
applications. See section 8.4.6 How to Make a Thread-safe Client.
CAUTION: You should read this section only if you are interested in helping us test our new code. If you just want to get MySQL up and running on your system, you should use a standard release distribution (either a source or binary distribution will do).
To obtain our most recent development source tree, use these instructions:
shell> bk clone bk://work.mysql.com:7000 mysqlTo clone the 4.0 branch, use this command instead:
shell> bk clone bk://work.mysql.com:7001 mysql-4.0The initial download of the source tree may take a while, depending on the speed of your connection; be patient.
autoconf
, automake
, libtool
, and
m4
to run the next set of commands.
If you get some strange error during this stage, check that you really
have libtool
installed!
shell> cd mysql shell> bk -r edit shell> aclocal; autoheader; autoconf; automake; shell> ./configure # Add your favorite options here shell> makeA collection of our standard configure scripts is located in the `BUILD/' subdirectory. If you are lazy, you can use `BUILD/compile-pentium-debug'. To compile on a different architecture, modify the script removing flags that are Pentium-specific.
make install
. Be careful with this
on a production machine; the command may overwrite your live release
installation. If you have another installation of MySQL, we
recommand that you run ./configure
with different values for the
prefix
, tcp-port
, and unix-socket-path
options than
those used for your production server.
make test
. See section 9.3.2 MySQL Test Suite.
make
stage and the distribution does
not compile, please report it to bugs@lists.mysql.com. If you
have installed the latest versions of the required GNU tools, and they
crash trying to process our configuration files, please report that also.
However, if you execute aclocal
and get a command not found
error or a similar problem, do not report it. Instead, make sure all
the necessary tools are installed and that your PATH
variable is
set correctly so your shell can find them.
bk clone
operation to get the source tree, you
should run bk pull
periodically to get the updates.
bk sccstool
. If you see some funny diffs or code that you have a
question about, do not hesitate to send e-mail to
internals@lists.mysql.com. Also, if you think you have a better idea
on how to do something, send an email to the same address with a patch.
bk diffs
will produce a patch for you after you have made changes
to the source. If you do not have the time to code your idea, just send
a description.
bk helptool
.
All MySQL programs compile cleanly for us with no warnings on
Solaris using gcc
. On other systems, warnings may occur due to
differences in system include files. See section 2.3.6 MIT-pthreads Notes for warnings
that may occur when using MIT-pthreads. For other problems, check the list
below.
The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following:
configure
is run after it already has been run, it may use
information that was gathered during its previous invocation. This
information is stored in `config.cache'. When configure
starts
up, it looks for that file and reads its contents if it exists, on the
assumption that the information is still correct. That assumption is invalid
when you reconfigure.
configure
, you must run make
again
to recompile. However, you may want to remove old object files from previous
builds first, because they were compiled using different configuration options.
To prevent old configuration information or object files from being used,
run these commands before rerunning configure
:
shell> rm config.cache shell> make clean
Alternatively, you can run make distclean
.
The list below describes some of the problems compiling MySQL that have been found to occur most often:
Internal compiler error: program cc1plus got fatal signal 11 or Out of virtual memory or Virtual memory exhaustedThe problem is that
gcc
requires huge amounts of memory to compile
`sql_yacc.cc' with inline functions. Try running configure
with
the --with-low-memory
option:
shell> ./configure --with-low-memoryThis option causes
-fno-inline
to be added to the compile line if you
are using gcc
and -O0
if you are using something else. You
should try the --with-low-memory
option even if you have so much
memory and swap space that you think you can't possibly have run out. This
problem has been observed to occur even on systems with generous hardware
configurations, and the --with-low-memory
option usually fixes it.
configure
picks c++
as the compiler name and
GNU c++
links with -lg++
. If you are using gcc
,
that behavior can cause problems during configuration such as this:
configure: error: installation or configuration problem: C++ compiler cannot create executables.You might also observe problems during compilation related to
g++
, libg++
, or libstdc++
.
One cause of these problems is that you may not have g++
, or you may
have g++
but not libg++
, or libstdc++
. Take a look at
the `config.log' file. It should contain the exact reason why your c++
compiler didn't work! To work around these problems, you can use gcc
as your C++ compiler. Try setting the environment variable CXX
to
"gcc -O3"
. For example:
shell> CXX="gcc -O3" ./configureThis works because
gcc
compiles C++ sources as well as g++
does, but does not link in libg++
or libstdc++
by default.
Another way to fix these problems, of course, is to install g++
,
libg++
and libstdc++
.
make
to GNU make
:
making all in mit-pthreads make: Fatal error in reader: Makefile, line 18: Badly formed macro assignment or make: file `Makefile' line 18: Must be a separator (: or pthread.h: No such file or directorySolaris and FreeBSD are known to have troublesome
make
programs.
GNU make
Version 3.75 is known to work.
CFLAGS
and CXXFLAGS
environment
variables. You can also specify the compiler names this way using CC
and CXX
. For example:
shell> CC=gcc shell> CFLAGS=-O3 shell> CXX=gcc shell> CXXFLAGS=-O3 shell> export CC CFLAGS CXX CXXFLAGSSee section 2.2.6 MySQL Binaries Compiled by MySQL AB, for a list of flag definitions that have been found to be useful on various systems.
gcc
compiler:
client/libmysql.c:273: parse error before `__attribute__'
gcc
2.8.1 is known to work, but we recommend using gcc
2.95.2 or
egcs
1.0.3a instead.
mysqld
,
configure
didn't correctly detect the type of the last argument to
accept()
, getsockname()
, or getpeername()
:
cxx: Error: mysqld.cc, line 645: In this statement, the referenced type of the pointer value "&length" is "unsigned long", which is not compatible with "int". new_sock = accept(sock, (struct sockaddr *)&cAddr, &length);To fix this, edit the `config.h' file (which is generated by
configure
). Look for these lines:
/* Define as the base type of the last arg to accept */ #define SOCKET_SIZE_TYPE XXXChange
XXX
to size_t
or int
, depending on your
operating system. (Note that you will have to do this each time you run
configure
, because configure
regenerates `config.h'.)
"sql_yacc.yy", line xxx fatal: default action causes potential...This is a sign that your version of
yacc
is deficient.
You probably need to install bison
(the GNU version of yacc
)
and use that instead.
mysqld
or a MySQL client, run
configure
with the --with-debug
option, then recompile and
link your clients with the new client library. See section H.2 Debugging a MySQL client.
This section describes some of the issues involved in using MIT-pthreads.
Note that on Linux you should NOT use MIT-pthreads but install LinuxThreads! See section 2.6.1 Linux Notes (All Linux Versions).
If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes older FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. See section 2.2.2 Operating Systems Supported by MySQL.
configure
with the --with-mit-threads
option:
shell> ./configure --with-mit-threadsBuilding in a non-source directory is not supported when using MIT-pthreads, because we want to minimize our changes to this code.
--without-server
to build only the client code, clients will not know whether or not
MIT-pthreads is being used and will use Unix socket connections by default.
Because Unix sockets do not work under MIT-pthreads, this means you will need
to use -h
or --host
when you run client programs.
--use-locking
option.
bind()
command fails to bind to a socket without
any error message (at least on Solaris). The result is that all connections
to the server fail. For example:
shell> mysqladmin version mysqladmin: connect to server at '' failed; error: 'Can't connect to mysql server on localhost (146)'The solution to this is to kill the
mysqld
server and restart it.
This has only happened to us when we have forced the server down and done
a restart immediately.
sleep()
system call isn't interruptible with
SIGINT
(break). This is only noticeable when you run
mysqladmin --sleep
. You must wait for the sleep()
call to
terminate before the interrupt is served and the process stops.
ld: warning: symbol `_iob' has differing sizes: (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4; file /usr/lib/libc.so value=0x140); /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken ld: warning: symbol `__iob' has differing sizes: (file /my/local/pthreads/lib/libpthread.a(findfp.o) value=0x4; file /usr/lib/libc.so value=0x140); /my/local/pthreads/lib/libpthread.a(findfp.o) definition taken
implicit declaration of function `int strtoll(...)' implicit declaration of function `int strtoul(...)'
readline
to work with MIT-pthreads. (This isn't
needed, but may be interesting for someone.)
Once you've installed MySQL (from either a binary or source distribution), you need to initialize the grant tables, start the server, and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts up and shuts down.
Normally you install the grant tables and start the server like this for installation from a source distribution:
shell> ./scripts/mysql_install_db shell> cd mysql_installation_directory shell> ./bin/safe_mysqld --user=mysql &
For a binary distribution (not RPM or pkg packages), do this:
shell> cd mysql_installation_directory shell> ./bin/mysql_install_db shell> ./bin/safe_mysqld --user=mysql &
This creates the mysql
database which will hold all database
privileges, the test
database which you can use to test
MySQL and also privilege entries for the user that run
mysql_install_db
and a root
user (without any passwords).
This also starts the mysqld
server.
mysql_install_db
will not overwrite any old privilege tables, so
it should be safe to run in any circumstances. If you don't want to
have the test
database you can remove it with mysqladmin -u
root drop test
.
Testing is most easily done from the top-level directory of the MySQL distribution. For a binary distribution, this is your installation directory (typically something like `/usr/local/mysql'). For a source distribution, this is the main directory of your MySQL source tree.
In the commands shown below in this section and in the following
subsections, BINDIR
is the path to the location in which programs
like mysqladmin
and safe_mysqld
are installed. For a
binary distribution, this is the `bin' directory within the
distribution. For a source distribution, BINDIR
is probably
`/usr/local/bin', unless you specified an installation directory
other than `/usr/local' when you ran configure
.
EXECDIR
is the location in which the mysqld
server is
installed. For a binary distribution, this is the same as
BINDIR
. For a source distribution, EXECDIR
is probably
`/usr/local/libexec'.
Testing is described in detail below:
mysqld
server and set up the initial
MySQL grant tables containing the privileges that determine how
users are allowed to connect to the server. This is normally done with the
mysql_install_db
script:
shell> scripts/mysql_install_dbTypically,
mysql_install_db
needs to be run only the first time you
install MySQL. Therefore, if you are upgrading an existing
installation, you can skip this step. (However, mysql_install_db
is
quite safe to use and will not update any tables that already exist, so if
you are unsure of what to do, you can always run mysql_install_db
.)
mysql_install_db
creates six tables (user
, db
,
host
, tables_priv
, columns_priv
, and func
) in the
mysql
database. A description of the initial privileges is given in
section 4.3.4 Setting Up the Initial MySQL Privileges. Briefly, these privileges allow the MySQL
root
user to do anything, and allow anybody to create or use databases
with a name of 'test'
or starting with 'test_'
.
If you don't set up the grant tables, the following error will appear in the
log file when you start the server:
mysqld: Can't find file: 'host.frm'The above may also happen with a binary MySQL distribution if you don't start MySQL by executing exactly
./bin/safe_mysqld
!
See section 4.7.2 safe_mysqld, the wrapper around mysqld.
You might need to run mysql_install_db
as root
. However,
if you prefer, you can run the MySQL server as an unprivileged
(non-root
) user, provided that user can read and write files in
the database directory. Instructions for running MySQL as an
unprivileged user are given in section A.8 How to Run MySQL As a Normal User.
If you have problems with mysql_install_db
, see
section 2.4.1 Problems Running mysql_install_db
.
There are some alternatives to running the mysql_install_db
script as it is provided in the MySQL distribution:
mysql_install_db
before running it, to change
the initial privileges that are installed into the grant tables. This is
useful if you want to install MySQL on a lot of machines with the
same privileges. In this case you probably should need only to add a few
extra INSERT
statements to the mysql.user
and mysql.db
tables!
mysql_install_db
, then use mysql -u root mysql
to
connect to the grant tables as the MySQL root
user and issue
SQL statements to modify the grant tables directly.
mysql_install_db
.
shell> cd mysql_installation_directory shell> bin/safe_mysqld &If you have problems starting the server, see section 2.4.2 Problems Starting the MySQL Server.
mysqladmin
to verify that the server is running. The following
commands provide a simple test to check that the server is up and responding
to connections:
shell> BINDIR/mysqladmin version shell> BINDIR/mysqladmin variablesThe output from
mysqladmin version
varies slightly depending on your
platform and version of MySQL, but should be similar to that shown
below:
shell> BINDIR/mysqladmin version mysqladmin Ver 8.14 Distrib 3.23.32, for linux on i586 Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB This software comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to modify and redistribute it under the GPL license Server version 3.23.32-debug Protocol version 10 Connection Localhost via Unix socket TCP port 3306 UNIX socket /tmp/mysql.sock Uptime: 16 sec Threads: 1 Questions: 9 Slow queries: 0 Opens: 7 Flush tables: 2 Open tables: 0 Queries per second avg: 0.000 Memory in use: 132K Max memory used: 16773KTo get a feeling for what else you can do with
BINDIR/mysqladmin
,
invoke it with the --help
option.
shell> BINDIR/mysqladmin -u root shutdown
safe_mysqld
or
by invoking mysqld
directly. For example:
shell> BINDIR/safe_mysqld --log &If
safe_mysqld
fails, try running it from the MySQL
installation directory (if you are not already there). If that doesn't work,
see section 2.4.2 Problems Starting the MySQL Server.
shell> BINDIR/mysqlshow +-----------+ | Databases | +-----------+ | mysql | +-----------+ shell> BINDIR/mysqlshow mysql Database: mysql +--------------+ | Tables | +--------------+ | columns_priv | | db | | func | | host | | tables_priv | | user | +--------------+ shell> BINDIR/mysql -e "select host,db,user from db" mysql +------+--------+------+ | host | db | user | +------+--------+------+ | % | test | | | % | test_% | | +------+--------+------+There is also a benchmark suite in the `sql-bench' directory (under the MySQL installation directory) that you can use to compare how MySQL performs on different platforms. The `sql-bench/Results' directory contains the results from many runs against different databases and platforms. To run all tests, execute these commands:
shell> cd sql-bench shell> run-all-testsIf you don't have the `sql-bench' directory, you are probably using an RPM for a binary distribution. (Source distribution RPMs include the benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL Version 3.22, there are benchmark RPM files named `mysql-bench-VERSION-i386.rpm' that contain benchmark code and data. If you have a source distribution, you can also run the tests in the `tests' subdirectory. For example, to run `auto_increment.tst', do this:
shell> BINDIR/mysql -vvf test < ./tests/auto_increment.tstThe expected results are shown in the `./tests/auto_increment.res' file.
mysql_install_db
The purpose of the mysql_install_db
script is to generate new
MySQL privilege tables. It will not affect any other data!
It will also not do anything if you already have MySQL privilege
tables installed!
If you want to re-create your privilege tables, you should take down
the mysqld
server, if it's running, and then do something like:
mv mysql-data-directory/mysql mysql-data-directory/mysql-old mysql_install_db
This section lists problems you might encounter when you run
mysql_install_db
:
mysql_install_db
doesn't install the grant tables
mysql_install_db
fails to install the grant
tables and terminates after displaying the following messages:
starting mysqld daemon with databases from XXXXXX mysql daemon endedIn this case, you should examine the log file very carefully! The log should be located in the directory `XXXXXX' named by the error message, and should indicate why
mysqld
didn't start. If you don't understand
what happened, include the log when you post a bug report using
mysqlbug
!
See section 1.2.22.3 How to Report Bugs or Problems.
mysqld
daemon running
mysql_install_db
at
all. You have to run mysql_install_db
only once, when you install
MySQL the first time.
mysqld
daemon doesn't work when one daemon is running
Can't start server: Bind on
TCP/IP port: Address already in use
or Can't start server : Bind on
unix socket...
. See section 4.1.3 Installing Many Servers on the Same Machine.
mysql_install_db
or when
starting or using mysqld
.
You can specify a different socket and temporary directory as follows:
shell> TMPDIR=/some_tmp_dir/ shell> MYSQL_UNIX_PORT=/some_tmp_dir/mysqld.sock shell> export TMPDIR MYSQL_UNIX_PORT`some_tmp_dir' should be the path to some directory for which you have write permission. See section B Environment Variables. After this you should be able to run
mysql_install_db
and start
the server with these commands:
shell> scripts/mysql_install_db shell> BINDIR/safe_mysqld &
mysqld
crashes immediately
glibc
older than
2.0.7-5, you should make sure you have installed all glibc
patches!
There is a lot of information about this in the MySQL mail
archives. Links to the mail archives are available online at
http://www.mysql.com/documentation/.
Also, see section 2.6.1 Linux Notes (All Linux Versions).
You can also start mysqld
manually using the --skip-grant-tables
option and add the privilege information yourself using mysql
:
shell> BINDIR/safe_mysqld --skip-grant-tables & shell> BINDIR/mysql -u root mysqlFrom
mysql
, manually execute the SQL commands in
mysql_install_db
. Make sure you run mysqladmin
flush-privileges
or mysqladmin reload
afterward to tell the server to
reload the grant tables.
If you are going to use tables that support transactions (BDB, InnoDB), you should first create a my.cnf file and set startup options for the table types you plan to use. See section 7 MySQL Table Types.
Generally, you start the mysqld
server in one of three ways:
mysql.server
. This script is used primarily at
system startup and shutdown, and is described more fully in
section 2.4.3 Starting and Stopping MySQL Automatically.
safe_mysqld
, which tries to determine the proper options
for mysqld
and then runs it with those options. See section 4.7.2 safe_mysqld, the wrapper around mysqld.
mysqld
as a service as follows:
bin\mysqld-nt --install # Install MySQL as a serviceYou can now start/stop
mysqld
as follows:
NET START mysql NET STOP mysqlNote that in this case you can't use any other options for
mysqld
!
You can remove the service as follows:
bin\mysqld-nt --remove # remove MySQL as a service
mysqld
directly.
When the mysqld
daemon starts up, it changes directory to the
data directory. This is where it expects to write log files and the pid
(process ID) file, and where it expects to find databases.
The data directory location is hardwired in when the distribution is
compiled. However, if mysqld
expects to find the data directory
somewhere other than where it really is on your system, it will not work
properly. If you have problems with incorrect paths, you can find out
what options mysqld
allows and what the default path settings are by
invoking mysqld
with the --help
option. You can override the
defaults by specifying the correct pathnames as command-line arguments to
mysqld
. (These options can be used with safe_mysqld
as well.)
Normally you should need to tell mysqld
only the base directory under
which MySQL is installed. You can do this with the --basedir
option. You can also use --help
to check the effect of changing path
options (note that --help
must be the final option of the
mysqld
command). For example:
shell> EXECDIR/mysqld --basedir=/usr/local --help
Once you determine the path settings you want, start the server without
the --help
option.
Whichever method you use to start the server, if it fails to start up
correctly, check the log file to see if you can find out why. Log files
are located in the data directory (typically
`/usr/local/mysql/data' for a binary distribution,
`/usr/local/var' for a source distribution,
`\mysql\data\mysql.err' on Windows.) Look in the data directory for
files with names of the form `host_name.err' and
`host_name.log' where host_name
is the name of your server
host. Then check the last few lines of these files:
shell> tail host_name.err shell> tail host_name.log
If you find something like the following in the log file:
000729 14:50:10 bdb: Recovery function for LSN 1 27595 failed 000729 14:50:10 bdb: warning: ./test/t1.db: No such file or directory 000729 14:50:10 Can't init databases
This means that you didn't start mysqld
with --bdb-no-recover
and Berkeley DB found something wrong with its log files when it
tried to recover your databases. To be able to continue, you should
move away the old Berkeley DB log file from the database directory to
some other place, where you can later examine these. The log files are
named `log.0000000001', where the number will increase over time.
If you are running mysqld
with BDB table support and mysqld
core
dumps at start this could be because of some problems with the BDB
recover log. In this case you can try starting mysqld
with
--bdb-no-recover
. If this helps, then you should remove all
`log.*' files from the data directory and try starting mysqld
again.
If you get the following error, it means that some other program (or another
mysqld
server) is already using the TCP/IP port or socket
mysqld
is trying to use:
Can't start server: Bind on TCP/IP port: Address already in use or Can't start server : Bind on unix socket...
Use ps
to make sure that you don't have another mysqld
server
running. If you can't find another server running, you can try to execute
the command telnet your-host-name tcp-ip-port-number
and press
RETURN
a couple of times. If you don't get an error message like
telnet: Unable to connect to remote host: Connection refused
,
something is using the TCP/IP port mysqld
is trying to use.
See section 2.4.1 Problems Running mysql_install_db
and section 4.1.4 Running Multiple MySQL Servers on the Same Machine.
If mysqld
is currently running, you can find out what path settings
it is using by executing this command:
shell> mysqladmin variables
or
shell> mysqladmin -h 'your-host-name' variables
If safe_mysqld
starts the server but you can't connect to it,
you should make sure you have an entry in `/etc/hosts' that looks like
this:
127.0.0.1 localhost
This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads.
If you can't get mysqld
to start you can try to make a trace file
to find the problem. See section H.1.2 Creating trace files.
If you are using InnoDB tables, refer to the InnoDB-specific startup options. See section 7.6.2 InnoDB startup options.
If you are using BDB (Berkeley DB) tables, you should familiarize yourself with the different BDB specific startup options. See section 7.5.3 BDB startup options.
The mysql.server
and safe_mysqld
scripts can be used to start
the server automatically at system startup time. mysql.server
can also
be used to stop the server.
The mysql.server
script can be used to start or stop the server
by invoking it with start
or stop
arguments:
shell> mysql.server start shell> mysql.server stop
mysql.server
can be found in the `share/mysql' directory
under the MySQL installation directory or in the `support-files'
directory of the MySQL source tree.
Before mysql.server
starts the server, it changes directory to
the MySQL installation directory, then invokes safe_mysqld
.
You might need to edit mysql.server
if you have a binary distribution
that you've installed in a non-standard location. Modify it to cd
into the proper directory before it runs safe_mysqld
. If you want the
server to run as some specific user, add an appropriate user
line
to the `/etc/my.cnf' file, as shown later in this section.
mysql.server stop
brings down the server by sending a signal to it.
You can take down the server manually by executing mysqladmin shutdown
.
You might want to add these start and stop commands to the appropriate places
in your `/etc/rc*' files when you start using MySQL for
production applications. Note that if you modify mysql.server
, then
upgrade MySQL sometime, your modified version will be overwritten,
so you should make a copy of your edited version that you can reinstall.
If your system uses `/etc/rc.local' to start external scripts, you should append the following to it:
/bin/sh -c 'cd /usr/local/mysql ; ./bin/safe_mysqld --user=mysql &'
You can also add options for mysql.server
in a global
`/etc/my.cnf' file. A typical `/etc/my.cnf' file might look like
this:
[mysqld] datadir=/usr/local/mysql/var socket=/tmp/mysqld.sock port=3306 user=mysql [mysql.server] basedir=/usr/local/mysql
The mysql.server
script understands the following options:
datadir
, basedir
, and pid-file
.
The following table shows which option groups each of the startup scripts read from option files:
Script | Option groups |
mysqld | mysqld and server
|
mysql.server | mysql.server , mysqld , and server
|
safe_mysqld | mysql.server , mysqld , and server
|
See section 4.1.2 my.cnf Option Files.
You can always move the MySQL form and data files between
different versions on the same architecture as long as you have the same
base version of MySQL. The current base version is
3. If you change the character set when running MySQL (which may
also change the sort order), you must run myisamchk -r -q
on all
tables. Otherwise your indexes may not be ordered correctly.
If you are afraid of new versions, you can always rename your old
mysqld
to something like mysqld
-'old-version-number'. If
your new mysqld
then does something unexpected, you can simply shut it
down and restart with your old mysqld
!
When you do an upgrade you should also back up your old databases, of course.
If after an upgrade, you experience problems with recompiled client programs,
like Commands out of sync
or unexpected core dumps, you probably have
used an old header or library file when compiling your programs. In this
case you should check the date for your `mysql.h' file and
`libmysqlclient.a' library to verify that they are from the new
MySQL distribution. If not, please recompile your programs!
If you get some problems that the new mysqld
server doesn't want to
start or that you can't connect without a password, check that you don't
have some old `my.cnf' file from your old installation! You can
check this with: program-name --print-defaults
. If this outputs
anything other than the program name, you have an active my.cnf
file that will affect things!
It is a good idea to rebuild and reinstall the Msql-Mysql-modules
distribution whenever you install a new release of MySQL,
particularly if you notice symptoms such as all your DBI
scripts
dumping core after you upgrade MySQL.
MySQL Version 3.23 supports tables of the new MyISAM
type and
the old ISAM
type. You don't have to convert your old tables to
use these with Version 3.23. By default, all new tables will be created with
type MyISAM
(unless you start mysqld
with the
--default-table-type=isam
option). You can change an ISAM
table to a MyISAM
table with ALTER TABLE table_name TYPE=MyISAM
or the Perl script mysql_convert_table_format
.
Version 3.22 and 3.21 clients will work without any problems with a Version 3.23 server.
The following lists tell what you have to watch out for when upgrading to Version 3.23:
tis620
character set must be fixed
with myisamchk -r
or REPAIR TABLE
.
DROP DATABASE
on a symbolic linked database, both the
link and the original database is deleted. (This didn't happen in 3.22
because configure didn't detect the readlink
system call).
OPTIMIZE TABLE
now only works for MyISAM tables.
For other table types, you can use ALTER TABLE
to optimize the table.
During OPTIMIZE TABLE
the table is now locked from other threads.
mysql
is now by default started with the
option --no-named-commands (-g)
. This option can be disabled with
--enable-named-commands (-G)
. This may cause incompatibility problems in
some cases, for example in SQL scripts that use named commands without a
semicolon! Long format commands still work from the first line.
german
character sort order, you must repair
all your tables with isamchk -r
, as we have made some changes in
the sort order!
IF
will now depend on both arguments
and not only the first argument.
AUTO_INCREMENT
will not work with negative numbers. The reason
for this is that negative numbers caused problems when wrapping from -1 to 0.
AUTO_INCREMENT
is now for MyISAM tables handled at a lower level and
is much faster than before. For MyISAM tables old numbers are also not reused
anymore, even if you delete some rows from the table.
CASE
, DELAYED
, ELSE
, END
, FULLTEXT
,
INNER
, RIGHT
, THEN
and WHEN
are now reserved words.
FLOAT(X)
is now a true floating-point type and not a value with a
fixed number of decimals.
DECIMAL(length,dec)
the length argument no longer
includes a place for the sign or the decimal point.
TIME
string must now be of one of the following formats:
[[[DAYS] [H]H:]MM:]SS[.fraction]
or
[[[[[H]H]H]H]MM]SS[.fraction]
LIKE
now compares strings using the same character comparison rules
as '='
. If you require the old behavior, you can compile
MySQL with the CXXFLAGS=-DLIKE_CMP_TOUPPER
flag.
REGEXP
is now case insensitive for normal (not binary) strings.
CHECK TABLE
or myisamchk
for MyISAM
tables (.MYI
) and
isamchk
for ISAM (.ISM
) tables.
mysqldump
files to be compatible between
MySQL Version 3.22 and Version 3.23, you should not use the
--opt
or --full
option to mysqldump
.
DATE_FORMAT()
to make sure there is a
`%' before each format character. (Later MySQL Version 3.22
did allow this syntax.)
mysql_fetch_fields_direct
is now a function (it was a macro) and
it returns a pointer to a MYSQL_FIELD
instead of a
MYSQL_FIELD
.
mysql_num_fields()
can no longer be used on a MYSQL*
object (it's
now a function that takes MYSQL_RES*
as an argument. You should now
use mysql_field_count()
instead.
SELECT DISTINCT ...
was
almost always sorted. In Version 3.23, you must use GROUP BY
or
ORDER BY
to obtain sorted output.
SUM()
now returns NULL
, instead of 0, if there is no matching
rows. This is according to ANSI SQL.
AND
or OR
with NULL
values will now return
NULL
instead of 0. This mostly affects queries that use NOT
on an AND/OR
expression as NOT NULL
= NULL
.
LPAD()
and RPAD()
will shorten the result string if it's longer
than the length argument.
Nothing that affects compatibility has changed between Version 3.21 and 3.22.
The only pitfall is that new tables that are created with DATE
type
columns will use the new way to store the date. You can't access these new
fields from an old version of mysqld
.
After installing MySQL Version 3.22, you should start the new server
and then run the mysql_fix_privilege_tables
script. This will add the
new privileges that you need to use the GRANT
command. If you forget
this, you will get Access denied
when you try to use ALTER
TABLE
, CREATE INDEX
, or DROP INDEX
. If your MySQL root
user requires a password, you should give this as an argument to
mysql_fix_privilege_tables
.
The C API interface to mysql_real_connect()
has changed. If you have
an old client program that calls this function, you must place a 0
for
the new db
argument (or recode the client to send the db
element for faster connections). You must also call mysql_init()
before calling mysql_real_connect()
! This change was done to allow
the new mysql_options()
function to save options in the MYSQL
handler structure.
The mysqld
variable key_buffer
has changed names to
key_buffer_size
, but you can still use the old name in your
startup files.
If you are running a version older than Version 3.20.28 and want to switch to Version 3.21, you need to do the following:
You can start the mysqld
Version 3.21 server with safe_mysqld
--old-protocol
to use it with clients from a Version 3.20 distribution.
In this case, the new client function mysql_errno()
will not
return any server error, only CR_UNKNOWN_ERROR
(but it
works for client errors), and the server uses the old password()
checking rather than the new one.
If you are NOT using the --old-protocol
option to
mysqld
, you will need to make the following changes:
scripts/add_long_password
must be run to convert the
Password
field in the mysql.user
table to CHAR(16)
.
mysql.user
table (to get 62-bit
rather than 31-bit passwords).
MySQL Version 3.20.28 and above can handle the new user
table
format without affecting clients. If you have a MySQL version earlier
than Version 3.20.28, passwords will no longer work with it if you convert the
user
table. So to be safe, you should first upgrade to at least Version
3.20.28 and then upgrade to Version 3.21.
The new client code works with a 3.20.x mysqld
server, so
if you experience problems with 3.21.x, you can use the old 3.20.x server
without having to recompile the clients again.
If you are not using the --old-protocol
option to mysqld
,
old clients will issue the error message:
ERROR: Protocol mismatch. Server Version = 10 Client Version = 9
The new Perl DBI
/DBD
interface also supports the old
mysqlperl
interface. The only change you have to make if you use
mysqlperl
is to change the arguments to the connect()
function.
The new arguments are: host
, database
, user
,
password
(the user
and password
arguments have changed
places).
See section 8.2.2 The DBI
Interface.
The following changes may affect queries in old applications:
HAVING
must now be specified before any ORDER BY
clause.
LOCATE()
have been swapped.
DATE
,
TIME
, and TIMESTAMP
.
If you are using MySQL Version 3.23, you can copy the .frm
,
.MYI
, and .MYD
files between different architectures that
support the same floating-point format. (MySQL takes care of any
byte swapping issues.)
The MySQL ISAM
data and index files (`.ISD' and
`*.ISM', respectively) are architecture-dependent and in some cases
OS-dependent. If you want to move your applications to another machine
that has a different architecture or OS than your current machine, you
should not try to move a database by simply copying the files to the
other machine. Use mysqldump
instead.
By default, mysqldump
will create a file full of SQL statements.
You can then transfer the file to the other machine and feed it as input
to the mysql
client.
Try mysqldump --help
to see what options are available.
If you are moving the data to a newer version of MySQL, you should use
mysqldump --opt
with the newer version to get a fast, compact dump.
The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located:
shell> mysqladmin -h 'other hostname' create db_name shell> mysqldump --opt db_name \ | mysql -h 'other hostname' db_name
If you want to copy a database from a remote machine over a slow network, you can use:
shell> mysqladmin create db_name shell> mysqldump -h 'other hostname' --opt --compress db_name \ | mysql db_name
You can also store the result in a file, then transfer the file to the target machine and load the file into the database there. For example, you can dump a database to a file on the source machine like this:
shell> mysqldump --quick db_name | gzip > db_name.contents.gz
(The file created in this example is compressed.) Transfer the file containing the database contents to the target machine and run these commands there:
shell> mysqladmin create db_name shell> gunzip < db_name.contents.gz | mysql db_name
You can also use mysqldump
and mysqlimport
to accomplish
the database transfer.
For big tables, this is much faster than simply using mysqldump
.
In the commands shown below, DUMPDIR
represents the full pathname
of the directory you use to store the output from mysqldump
.
First, create the directory for the output files and dump the database:
shell> mkdir DUMPDIR shell> mysqldump --tab=DUMPDIR db_name
Then transfer the files in the DUMPDIR
directory to some corresponding
directory on the target machine and load the files into MySQL
there:
shell> mysqladmin create db_name # create database shell> cat DUMPDIR/*.sql | mysql db_name # create tables in database shell> mysqlimport db_name DUMPDIR/*.txt # load data into tables
Also, don't forget to copy the mysql
database, because that's where the
grant tables (user
, db
, host
) are stored. You may have
to run commands as the MySQL root
user on the new machine
until you have the mysql
database in place.
After you import the mysql
database on the new machine, execute
mysqladmin flush-privileges
so that the server reloads the grant table
information.
The notes below regarding glibc apply only to the situation when you build MySQL yourself. If you are running Linux on an x86 machine, in most cases it is much better for you to just use our binary. We link our binaries against the best patched version of glibc we can come up with and with the best compiler options, in an attempt to make it suitable for a high-load server. So if you read the text below, and are in doubt about what you should do, try our binary first to see if it meets your needs, and worry about your own build only after you have discovered that our binary is not good enough. In that case, we would appreciate a note about it, so we can build a better binary next time. For a typical user, even for setups with a lot of concurrent connections and/or tables exceeding 2GB limit, our binary in most cases is the best choice.
MySQL uses LinuxThreads on Linux. If you are using an old
Linux version that doesn't have glibc2
, you must install
LinuxThreads before trying to compile MySQL. You can get
LinuxThreads at http://www.mysql.com/Downloads/Linux.
NOTE: We have seen some strange problems with Linux 2.2.14 and MySQL on SMP systems; If you have a SMP system, we recommend you to upgrade to Linux 2.4 ASAP! Your system will be faster and more stable by doing this!
Note that glibc
versions before and including Version 2.1.1 have
a fatal bug in pthread_mutex_timedwait
handling, which is used
when you do INSERT DELAYED
. We recommend you to not use
INSERT DELAYED
before upgrading glibc.
If you plan to have 1000+ concurrent connections, you will need to make
some changes to LinuxThreads, recompile it, and relink MySQL against
the new `libpthread.a'. Increase PTHREAD_THREADS_MAX
in
`sysdeps/unix/sysv/linux/bits/local_lim.h' to 4096 and decrease
STACK_SIZE
in `linuxthreads/internals.h' to 256 KB. The paths are
relative to the root of glibc
Note that MySQL will not be
stable with around 600-1000 connections if STACK_SIZE
is the default
of 2 MB.
If you have a problem with that MySQL can't open enough files, or connections, it may be that you haven't configured Linux to handle enough files.
In Linux 2.2 and forwards, you can check the number of allocated file handlers by doing:
cat /proc/sys/fs/file-max cat /proc/sys/fs/dquot-max cat /proc/sys/fs/super-max
If you have more than 16M of memory, you should add something like the following in your boot script (`/etc/rc/boot.local' on SuSE):
echo 65536 > /proc/sys/fs/file-max echo 8192 > /proc/sys/fs/dquot-max echo 1024 > /proc/sys/fs/super-max
You can also run the above from the command line as root, but in this case your old limits will be used next time your computer reboots.
You should also add /etc/my.cnf:
[safe_mysqld] open-files-limit=8192
The above should allow MySQL to create up to 8192 connections + files.
The STACK_SIZE
constant in LinuxThreads controls the spacing of thread
stacks in the address space. It needs to be large enough so that there will
be plenty of room for the stack of each individual thread, but small enough
to keep the stack of some thread from running into the global mysqld
data. Unfortunately, the Linux implementation of mmap()
, as we have
experimentally discovered, will successfully unmap an already mapped region
if you ask it to map out an address already in use, zeroing out the data
on the entire page, instead of returning an error. So, the safety of
mysqld
or any other threaded application depends on the "gentleman"
behavior of the code that creates threads. The user must take measures to
make sure the number of running threads at any time is sufficiently low for
thread stacks to stay away from the global heap. With mysqld
, you
should enforce this "gentleman" behavior by setting a reasonable value for
the max_connections
variable.
If you build MySQL yourself and do not what to mess with patching
LinuxThreads, you should set max_connections
to a value no higher
than 500. It should be even less if you have a large key buffer, large
heap tables, or some other things that make mysqld allocate a lot
of memory or if you are running a 2.2 kernel with a 2GB patch. If you are
using our binary or RPM version 3.23.25 or later, you can safely set
max_connections
at 1500, assuming no large key buffer or heap tables
with lots of data. The more you reduce STACK_SIZE
in LinuxThreads
the more threads you can safely create. We recommend the values between
128K and 256K.
If you use a lot of concurrent connections, you may suffer from a "feature"
in the 2.2 kernel that penalizes a process for forking or cloning a child
in an attempt to prevent a fork bomb attack. This will cause MySQL
not to scale well as you increase the number of concurrent clients. On
single CPU systems, we have seen this manifested in a very slow thread
creation, which means it may take a long time to connect to MySQL
(as long as 1 minute), and it may take just as long to shut it down. On
multiple CPU systems, we have observed a gradual drop in query speed as
the number of clients increases. In the process of trying to find a
solution, we have received a kernel patch from one of our users, who
claimed it made a lot of difference for his site. The patch is available here
(http://www.mysql.com/Downloads/Patches/linux-fork.patch). We have
now done rather extensive testing of this patch on both development and
production systems. It has significantly
improved MySQL
performance without causing any problems and we now
recommend it to our users who are still running high-load servers on
2.2 kernels. This issue has been fixed in the 2.4 kernel, so if you are not
satisfied with
the current performance of your system, rather than patching your 2.2 kernel,
it might be easier to just upgrade to 2.4, which will also give you a nice
SMP boost in addition to fixing this fairness bug.
We have tested MySQL on the 2.4 kernel on a 2 CPU machine and
found MySQL scales MUCH better - there was virtually no slowdown
on query throughput all the way up
to 1000 clients, and MySQL scaling factor ( computed as the ratio of
maximum throughput to the throughput with one client) was 180%.
We have observed similar results on a 4-CPU system - virtually no
slowdown as the number of
clients was increased up to 1000, and 300% scaling factor. So for a high-load
SMP server we would definitely recommend the 2.4 kernel at this point. We
have discovered that it is essential to run mysqld
process with the
highest possible priority on the 2.4 kernel to achieve maximum performance.
This can be done by adding
renice -20 $$
command to safe_mysqld
. In our testing on a
4-CPU machine, increasing the priority gave 60% increase in throughput with
400 clients.
We are currently also trying to collect
more info on how well MySQL
performs on 2.4 kernel on 4-way and 8-way
systems. If you have access such a system and have done some benchmarks,
please send a mail to docs@mysql.com with the results - we will
include them in the manual.
There is another issue that greatly hurts MySQL performance, especially on SMP systems. The implementation of mutex in LinuxThreads in glibc-2.1 is very bad for programs with many threads that only hold the mutex for a short time. On an SMP system, ironic as it is, if you link MySQL against unmodified LinuxThreads, removing processors from the machine improves MySQL performance in many cases. We have made a patch available for glibc 2.1.3, linuxthreads-2.1-patch to correct this behavior.
With glibc-2.2.2
MySQL version 3.23.36 will use the adaptive mutex, which is much
better than even the patched one in glibc-2.1.3. Be warned, however,
that under some conditions, the current mutex code in glibc-2.2.2
overspins, which hurts MySQL performance. The chance of this
condition can be reduced by renicing mysqld
process to the highest
priority. We have also been able to correct the overspin behavior with
a patch, available here. It combines the correction of overspin, maximum number of
threads, and stack spacing all in one. You will need to apply it in the
linuxthreads
directory with
patch -p0 </tmp/linuxthreads-2.2.2.patch
.
We hope it will be included in
some form in to the future releases of glibc-2.2
. In any case, if
you link against glibc-2.2.2
you still need to correct
STACK_SIZE
and PTHREAD_THREADS_MAX
. We hope that the defaults
will be corrected to some more acceptable values for high-load
MySQL setup in the future, so that your own build can be reduced
to ./configure; make; make install
.
We recommend that you use the above patches to build a special static
version of libpthread.a
and use it only for statically linking
against MySQL
. We know that the patches are safe for MySQL
and significantly improve its performance, but we cannot say anything
about other applications. If you link other applications against the
patched version of the library, or build a patched shared version and
install it on your system, you are doing it at your own risk with regard
to other applications that depend on LinuxThreads
.
If you experience any strange problems during the installation of MySQL, or with some common utilties hanging, it is very likely that they are either library or compiler related. If this is the case, using our binary will resolve them.
One known problem with the binary distribution is that with older Linux
systems that use libc
(like RedHat 4.x or Slackware), you will get
some non-fatal problems with hostname resolution.
See section 2.6.1.1 Linux Notes for Binary Distributions.
When using LinuxThreads you will see a minimum of three processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.
Note that the Linux kernel and the LinuxThread library can by default only have 1024 threads. This means that you can only have up to 1021 connections to MySQL on an unpatched system. The page http://www.volano.com/linuxnotes.html contains information how to go around this limit.
If you see a dead mysqld
daemon process with ps
, this usually
means that you have found a bug in MySQL or you have a corrupted
table. See section A.2 What to Do if MySQL Keeps Crashing.
To get a core dump on Linux if mysqld
dies with a SIGSEGV
signal, you can start mysqld
with the --core-file
option. Note
that you also probably need to raise the core file size
by adding
ulimit -c 1000000
to safe_mysqld
or starting safe_mysqld
with --core-file-sizes=1000000
. See section 4.7.2 safe_mysqld, the wrapper around mysqld.
To get a core dump on Linux if mysqld
dies with a SIGSEGV signal, you can
start mysqld
with the --core-file
option. Note that you also probably
need to raise the core file size
by adding ulimit -c 1000000
to
safe_mysqld
or starting safe_mysqld
with
--core-file-sizes=1000000
. See section 4.7.2 safe_mysqld, the wrapper around mysqld.
If you are linking your own MySQL client and get the error:
ld.so.1: ./my: fatal: libmysqlclient.so.4: open failed: No such file or directory
When executing them, the problem can be avoided by one of the following methods:
-Lpath
):
-Wl,r/path-libmysqlclient.so
.
libmysqclient.so
to `/usr/lib'.
libmysqlclient.so
is located
to the LD_RUN_PATH
environment variable before running your client.
If you are using the Fujitsu compiler (fcc / FCC)
you will have
some problems compiling MySQL because the Linux header files are very
gcc
oriented.
The following configure
line should work with fcc/FCC
:
CC=fcc CFLAGS="-O -K fast -K lib -K omitfp -Kpreex -D_GNU_SOURCE -DCONST=const -DNO_STRTOLL_PROTO" CXX=FCC CXXFLAGS="-O -K fast -K lib -K omitfp -K preex --no_exceptions --no_rtti -D_GNU_SOURCE -DCONST=const -Dalloca=__builtin_alloca -DNO_STRTOLL_PROTO '-D_EXTERN_INLINE=static __inline'" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-low-memory
MySQL needs at least Linux Version 2.0.
The binary release is linked with -static
, which means you do not
normally need to worry about which version of the system libraries you
have. You need not install LinuxThreads, either. A program linked with
-static
is slightly bigger than a dynamically linked program but
also slightly faster (3-5%). One problem, however, is that you can't use
user-definable functions (UDFs) with a statically linked program. If
you are going to write or use UDF functions (this is something only for
C or C++ programmers), you must compile MySQL yourself, using
dynamic linking.
If you are using a libc
-based system (instead of a glibc2
system), you will probably get some problems with hostname resolving and
getpwnam()
with the binary release. (This is because glibc
unfortunately depends on some external libraries to resolve hostnames
and getpwent()
, even when compiled with -static
). In this
case you probably get the following error message when you run
mysql_install_db
:
Sorry, the host 'xxxx' could not be looked up
or the following error when you try to run mysqld
with the --user
option:
getpwnam: No such file or directory
You can solve this problem in one of the following ways:
tar.gz
distribution) and install this instead.
mysql_install_db --force
; This will not execute the
resolveip
test in mysql_install_db
. The downside is that
you can't use host names in the grant tables; you must use IP numbers
instead (except for localhost
). If you are using an old MySQL
release that doesn't support --force
, you have to remove the
resolveip
test in mysql_install
with an editor.
mysqld
with su
instead of using --user
.
The Linux-Intel binary and RPM releases of MySQL are configured for the highest possible speed. We are always trying to use the fastest stable compiler available.
MySQL Perl support requires Version Perl 5.004_03 or newer.
On some Linux 2.2 versions, you may get the error Resource
temporarily unavailable
when you do a lot of new connections to a
mysqld
server over TCP/IP.
The problem is that Linux has a delay between when you close a TCP/IP socket and until this is actually freed by the system. As there is only room for a finite number of TCP/IP slots, you will get the above error if you try to do too many new TCP/IP connections during a small time, like when you run the MySQL `test-connect' benchmark over TCP/IP.
We have mailed about this problem a couple of times to different Linux mailing lists but have never been able to resolve this properly.
The only known 'fix' to this problem is to use persistent connections in
your clients or use sockets, if you are running the database server
and clients on the same machine. We hope that the Linux 2.4
kernel will fix this problem in the future.
MySQL requires libc
Version 5.4.12 or newer. It's known to
work with libc
5.4.46. glibc
Version 2.0.6 and later should
also work. There have been some problems with the glibc
RPMs from
RedHat, so if you have problems, check whether or not there are any updates!
The glibc
2.0.7-19 and 2.0.7-29 RPMs are known to work.
On some older Linux distributions, configure
may produce an error
like this:
Syntax error in sched.h. Change _P to __P in the /usr/include/sched.h file. See the Installation chapter in the Reference Manual.
Just do what the error message says and add an extra underscore to the
_P
macro that has only one underscore, then try again.
You may get some warnings when compiling; those shown below can be ignored:
mysqld.cc -o objs-thread/mysqld.o mysqld.cc: In function `void init_signals()': mysqld.cc:315: warning: assignment of negative value `-1' to `long unsigned int' mysqld.cc: In function `void * signal_hand(void *)': mysqld.cc:346: warning: assignment of negative value `-1' to `long unsigned int'
In Debian GNU/Linux, if you want MySQL to start automatically when the system boots, do the following:
shell> cp support-files/mysql.server /etc/init.d/mysql.server shell> /usr/sbin/update-rc.d mysql.server defaults 99
mysql.server
can be found in the `share/mysql' directory
under the MySQL installation directory or in the
`support-files' directory of the MySQL source tree.
If mysqld
always core dumps when it starts up, the problem may be that
you have an old `/lib/libc.a'. Try renaming it, then remove
`sql/mysqld' and do a new make install
and try again. This
problem has been reported on some Slackware installations.
If you get the following error when linking mysqld
,
it means that your `libg++.a' is not installed correctly:
/usr/lib/libc.a(putc.o): In function `_IO_putc': putc.o(.text+0x0): multiple definition of `_IO_putc'
You can avoid using `libg++.a' by running configure
like this:
shell> CXX=gcc ./configure
In some implementations, readdir_r()
is broken. The symptom is that
SHOW DATABASES
always returns an empty set. This can be fixed by
removing HAVE_READDIR_R
from `config.h' after configuring and
before compiling.
Some problems will require patching your Linux installation. The patch can
be found at
http://www.mysql.com/Downloads/patches/Linux-sparc-2.0.30.diff.
This patch is against the Linux distribution `sparclinux-2.0.30.tar.gz'
that is available at vger.rutgers.edu
(a version of Linux that was
never merged with the official 2.0.30). You must also install LinuxThreads
Version 0.6 or newer.
MySQL Version 3.23.12 is the first MySQL version that is tested on Linux-Alpha. If you plan to use MySQL on Linux-Alpha, you should ensure that you have this version or newer.
We have tested MySQL on Alpha with our benchmarks and test suite, and it appears to work nicely. The main thing we haven't yet had time to test is how things works with many concurrent users.
When we compiled the standard MySQL binary we are using SuSE 6.4, kernel 2.2.13-SMP, Compaq C compiler (V6.2-504) and Compaq C++ compiler (V6.3-005) on a Comaq DS20 machine with an Alpha EV6 processor.
You can find the above compilers at http://www.support.compaq.com/alpha-tools). By using these compilers, instead of gcc, we get about 9-14 % better performance with MySQL.
Note that the configure line optimized the binary for the current CPU; This means you can only use our binary if you have an Alpha EV6 processor. We also compile statically to avoid library problems.
CC=ccc CFLAGS="-fast" CXX=cxx CXXFLAGS="-fast -noexceptions -nortti" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-thread-safe-client --with-mysqld-ldflags=-non_shared --with-client-ldflags=-non_shared
If you want to use egcs the following configure line worked for us:
CFLAGS="-O3 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --disable-shared
Some known problems when running MySQL on Linux-Alpha:
gdb 4.18
. You should download and use gdb 5.0 instead!
mysqld
statically when using gcc
, the
resulting image will core dump at start. In other words, DON'T
use --with-mysqld-ldflags=-all-static
with gcc
.
MySQL should work on MkLinux with the newest glibc
package
(tested with glibc
2.0.7).
To get MySQL to work on Qube2, (Linux Mips), you need the
newest glibc
libraries (glibc-2.0.7-29C2
is known to
work). You must also use the egcs
C++ compiler
(egcs-1.0.2-9
, gcc 2.95.2
or newer).
To get MySQL to compile on Linux Ia64, we had to do the following (we assume that this will be easier when next gcc version for ia64 is released).
Using gcc-2.9-final
:
CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-extra-charsets=complex
After make
you will get an error that sql/opt_range.cc
will not compile (internal compiler error). To fix this, go to the sql
directory and type make
again. Copy the compile line, but change
-O2 to -O0. The file should now compile.
Now you can do:
cd .. make make_install
and mysqld should be ready to run.
This section describes installation and use of MySQL on Windows. This information is also provided in the `README' file that comes with the MySQL Windows distribution.
MySQL uses TCP/IP to connect a client to a server. (This will allow any machine on your network to connect to your MySQL server.) Because of this, you must install TCP/IP on your machine before starting MySQL. You can find TCP/IP on your Windows CD-ROM.
Note that if you are using an old Win95 release (for example OSR2), it's likely that you have an old Winsock package! MySQL requires Winsock 2! You can get the newest Winsock from http://www.microsoft.com/. Win98 has the new Winsock 2 library, so the above doesn't apply for Win98.
To start the mysqld
server, you should start an MS-DOS window and type:
C:\> C:\mysql\bin\mysqld
This will start mysqld
in the background without a window.
You can kill the MySQL server by executing:
C:\> C:\mysql\bin\mysqladmin -u root shutdown
Note that Win95 and Win98 don't support creation of named pipes.
On Win95 and Win98, you can only use named pipes to connect to a
remote MySQL server running on a Windows NT server host.
(The MySQL server must also support named pipes, of
course. For example, using mysqld-opt
under NT will not allow
named pipe connections. You should use either mysqld-nt
or
mysqld-max-nt
.)
If mysqld
doesn't start, please check the
`\mysql\data\mysql.err' file to see if the server wrote any message
there to indicate the cause of the problem. You can also try to start
the server with mysqld --standalone
; In this case, you may get
some useful information on the screen that may help solve the problem.
The last option is to start mysqld
with --standalone
--debug
. In this case mysqld
will write a log file
`C:\mysqld.trace' that should contain the reason why mysqld
doesn't start. See section H.1.2 Creating trace files.
The Win95/Win98 section also applies to MySQL on NT/Win2000, with the following differences:
To get MySQL to work with TCP/IP on NT, you must install service pack 3 (or newer)!
Note that everything in the following that applies for NT also applies for Win2000!
For NT/Win2000, the server name is mysqld-nt
. Normally you
should install MySQL as a service on NT/Win2000:
C:\> C:\mysql\bin\mysqld-nt --install
or
C:\> C:\mysql\bin\mysqld-max-nt --install
(Under Windows NT, you can actually install any of the server binaries
as a service, but only those having names that end with -nt.exe
provide support for named pipes.)
You can start and stop the MySQL service with these commands:
C:\> NET START mysql C:\> NET STOP mysql
Note that in this case you can't use any other options for mysqld-nt
!
You can also run mysqld-nt
as a stand-alone program on NT if you need
to start mysqld-nt
with any options! If you start mysqld-nt
without options on NT, mysqld-nt
tries to start itself as a service
with the default service options. If you have stopped mysqld-nt
, you
have to start it with NET START mysql
.
The service is installed with the name MySQL
. Once installed, it must
be started using the Services Control Manager (SCM) Utility found in the
Control Panel, or by using the NET START MySQL
command. If any options
are desired, they must be specified as ``Startup parameters'' in the SCM utility
before you start the MySQL service. Once running, mysqld-nt
can be stopped using mysqladmin
, or from the SCM utility or by using
the command NET STOP MySQL
. If you use SCM to stop mysqld-nt
,
there is a strange message from SCM about mysqld shutdown normally
.
When run as a service, mysqld-nt
has no access to a console and so no
messages can be seen.
On NT you can get the following service error messages:
Permission Denied | Means that it cannot find mysqld-nt.exe .
|
Cannot Register | Means that the path is incorrect. |
Failed to install service. | Means that the service is already installed or that the Service Control Manager is in bad state. |
If you have problems installing mysqld-nt
as a service, try starting
it with the full path:
C:\> C:\mysql\bin\mysqld-nt --install
If this doesn't work, you can get mysqld-nt
to start properly by fixing
the path in the registry!
If you don't want to start mysqld-nt
as a service, you can start it as
follows:
C:\> C:\mysql\bin\mysqld-nt --standalone
or
C:\> C:\mysql\bin\mysqld --standalone --debug
The last version gives you a debug trace in `C:\mysqld.trace'. See section H.1.2 Creating trace files.
MySQL supports TCP/IP on all Windows platforms and named pipes on NT. The default is to use named pipes for local connections on NT and TCP/IP for all other cases if the client has TCP/IP installed. The host name specifies which protocol is used:
Protocol | |
NULL (none) | On NT, try named pipes first; if that doesn't work, use TCP/IP. On Win95/Win98, TCP/IP is used. |
. | Named pipes |
localhost | TCP/IP to current host |
hostname | TCP/IP |
You can force a MySQL client to use named pipes by specifying the
--pipe
option or by specifying .
as the host name. Use the
--socket
option to specify the name of the pipe.
You can test whether or not MySQL is working by executing the following commands:
C:\> C:\mysql\bin\mysqlshow C:\> C:\mysql\bin\mysqlshow -u root mysql C:\> C:\mysql\bin\mysqladmin version status proc C:\> C:\mysql\bin\mysql test
If mysqld
is slow to answer to connections on Win95/Win98, there is
probably a problem with your DNS. In this case, start mysqld
with
--skip-name-resolve
and use only localhost
and IP numbers in
the MySQL grant tables. You can also avoid DNS when connecting to a
mysqld-nt
MySQL server running on NT by using the
--pipe
argument to specify use of named pipes. This works for most
MySQL clients.
There are two versions of the MySQL command-line tool:
mysql | Compiled on native Windows, which offers very limited text editing capabilities. |
mysqlc | Compiled with the Cygnus GNU compiler and libraries, which offers readline editing.
|
If you want to use mysqlc.exe
, you must copy
`C:\mysql\lib\cygwinb19.dll' to your Windows system directory
(`\windows\system' or similar place).
The default privileges on Windows give all local users full privileges
to all databases without specifying a password. To make MySQL
more secure, you should set a password for all users and remove the row in
the mysql.user
table that has Host='localhost'
and
User=''
.
You should also add a password for the root
user. The following
example starts by removing the anonymous user that can be used by anyone
to access the test
database, then sets a root
user password:
C:\> C:\mysql\bin\mysql mysql mysql> DELETE FROM user WHERE Host='localhost' AND User=''; mysql> QUIT C:\> C:\mysql\bin\mysqladmin reload C:\> C:\mysql\bin\mysqladmin -u root password your_password
After you've set the password, if you want to take down the mysqld
server, you can do so using this command:
C:\> mysqladmin --user=root --password=your_password shutdown
If you are using the old shareware version of MySQL Version
3.21 under Windows, the above command will fail with an error:
parse error near 'SET OPTION password'
. The fix is in to upgrade
to the current MySQL version, which is freely available.
With the current MySQL versions you can easily add new users
and change privileges with GRANT
and REVOKE
commands.
See section 4.3.1 GRANT
and REVOKE
Syntax.
Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson dcarlson@mplcomm.com):
SecureCRT
from http://www.vandyke.com/.
Another option is f-secure
from http://www.f-secure.com/. You
can also find some free ones on Google at
http://directory.google.com/Top/Computers/Security/Products_and_Tools/Cryptography/SSH/Clients/Windows/.
Host_Name = yourmysqlserver_URL_or_IP
.
Set userid=your_userid
to log in to your server (probably not the same
as your MySQL login/password.
local_port: 3306
, remote_host: yourmysqlservername_or_ip
, remote_port: 3306
)
or a local forward (Set port: 3306
, host: localhost
, remote port: 3306
).
localhost
for the MySQL host server -- not yourmysqlservername
.
You should now have an ODBC connection to MySQL, encrypted using SSH.
Beginning with MySQL Version 3.23.16, the mysqld-max
and mysql-max-nt
servers in the MySQL distribution are
compiled with the -DUSE_SYMDIR
option. This allows you to put a
database on different disk by adding a symbolic link to it
(in a manner similar to the way that symbolic links work on Unix).
On Windows, you make a symbolic link to a database by creating a file that contains the path to the destination directory and saving this in the `mysql_data' directory under the filename `database.sym'. Note that the symbolic link will be used only if the directory `mysql_data_dir\database' doesn't exist.
For example, if the MySQL data directory is `C:\mysql\data'
and you want to have database foo
located at `D:\data\foo', you
should create the file `C:\mysql\data\foo.sym' that contains the
text D:\data\foo\
. After that, all tables created in the database
foo
will be created in `D:\data\foo'.
Note that because of the speed penalty you get when opening every table,
we have not enabled this by default even if you have compiled
MySQL with support for this. To enable symlinks you should put
in your my.cnf
or my.ini
file the following entry:
[mysqld] use-symbolic-links
In MySQL 4.0 we will enable symlinks by default. Then you
should instead use the skip-symlink
option if you want to
disable this.
In your source files, you should include `windows.h' before you include `mysql.h':
#if defined(_WIN32) || defined(_WIN64) #include <windows.h> #endif #include <mysql.h>
You can either link your code with the dynamic `libmysql.lib' library, which is just a wrapper to load in `libmysql.dll' on demand, or link with the static `mysqlclient.lib' library.
Note that as the mysqlclient libraries are compiled as threaded libraries, you should also compile your code to be multi-threaded!
MySQL-Windows has by now proven itself to be very stable. This version of MySQL has the same features as the corresponding Unix version with the following exceptions:
mysqld
for an extended time on Win95 if your server handles
many connections! WinNT and Win98 don't suffer from this bug.
pread()
and pwrite()
calls to be
able to mix INSERT
and SELECT
. Currently we use mutexes
to emulate pread()
/pwrite()
. We will, in the long run,
replace the file level interface with a virtual interface so that we can
use the readfile()
/writefile()
interface on NT to get more speed.
The current implementation limits the number of open files MySQL
can use to 1024, which means that you will not be able to run as many
concurrent threads on NT as on Unix.
mysqladmin kill
will not work on a sleeping connection.
mysqladmin shutdown
can't abort as long as there are sleeping
connections.
DROP DATABASE
mysqladmin shutdown
.
LOAD
DATA INFILE
or SELECT ... INTO OUTFILE
, you must double the `\'
character:
mysql> LOAD DATA INFILE "C:\\tmp\\skr.txt" INTO TABLE skr; mysql> SELECT * INTO OUTFILE 'C:\\tmp\\skr.txt' FROM skr;Alternatively, use Unix style filenames with `/' characters:
mysql> LOAD DATA INFILE "C:/tmp/skr.txt" INTO TABLE skr; mysql> SELECT * INTO OUTFILE 'C:/tmp/skr.txt' FROM skr;
Can't open named pipe
error
error 2017: can't open named pipe to host: . pipe...This is because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the
--host=localhost
option to the new MySQL clients or create an option file
`C:\my.cnf' that contains the following information:
[client] host = localhost
Access denied for user
error
Access denied for user: 'some-user@unknown'
to database 'mysql'
when accessing a MySQL server on the same
machine, this means that MySQL can't resolve your host name
properly.
To fix this, you should create a file `\windows\hosts' with the
following information:
127.0.0.1 localhost
ALTER TABLE
ALTER TABLE
statement, the table is locked
from usage by other threads. This has to do with the fact that on Windows,
you can't delete a file that is in use by another threads. (In the future,
we may find some way to work around this problem.)
DROP TABLE
on a table that is in use by a MERGE
table will not work
MERGE
handler does its table mapping hidden from MySQL.
Because Windows doesn't allow you to drop files that are open, you first
must flush all MERGE
tables (with FLUSH TABLES
) or drop the
MERGE
table before dropping the table. We will fix this at the same
time we introduce VIEW
s.
Here are some open issues for anyone who might want to help us with the Windows release:
MYSQL.DLL
server. This should include everything in
a standard MySQL server, except thread creation. This will make
MySQL much easier to use in applications that don't need a true
client/server and don't need to access the server from other hosts.
mysqld
as a service with --install
(on NT)
it would be nice if you could also add default options on the command line.
For the moment, the workaround is to list the parameters in the
`C:\my.cnf' file instead.
mysqld
from the task manager.
For the moment, you must use mysqladmin shutdown
.
readline
to Windows for use in the mysql
command line tool.
mysql
,
mysqlshow
, mysqladmin
, and mysqldump
) would be nice.
mysqladmin kill
on Windows.
mysqld
always starts in the "C" locale and not in the default locale.
We would like to have mysqld
use the current locale for the sort order.
.DLL
s.
Other Windows-specific issues are described in the `README' file that comes with the MySQL-Windows distribution.
On Solaris, you may run into trouble even before you get the MySQL
distribution unpacked! Solaris tar
can't handle long file names, so
you may see an error like this when you unpack MySQL:
x mysql-3.22.12-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks tar: directory checksum error
In this case, you must use GNU tar
(gtar
) to unpack the
distribution. You can find a precompiled copy for Solaris at
http://www.mysql.com/Downloads/.
Sun native threads work only on Solaris 2.5 and higher. For Version 2.4 and earlier, MySQL will automatically use MIT-pthreads. See section 2.3.6 MIT-pthreads Notes.
If you get the following error from configure:
checking for restartable system calls... configure: error can not run test programs while cross compiling
This means that you have something wrong with your compiler installation! In this case you should upgrade your compiler to a newer version. You may also be able to solve this problem by inserting the following row into the `config.cache' file:
ac_cv_sys_restartable_syscalls=${ac_cv_sys_restartable_syscalls='no'}
If you are using Solaris on a SPARC, the recommended compiler is
gcc
2.95.2. You can find this at http://gcc.gnu.org/.
Note that egcs
1.1.1 and gcc
2.8.1 don't work reliably on
SPARC!
The recommended configure
line when using gcc
2.95.2 is:
CC=gcc CFLAGS="-O3" \ CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" \ ./configure --prefix=/usr/local/mysql --with-low-memory --enable-assembler
If you have a ultra sparc, you can get 4 % more performance by adding "-mcpu=v8 -Wa,-xarch=v8plusa" to CFLAGS and CXXFLAGS.
If you have the Sun Workshop (SunPro) 4.2 (or newer) compiler, you can
run configure
like this:
CC=cc CFLAGS="-Xa -fast -xO4 -native -xstrconst -mt" \ CXX=CC CXXFLAGS="-noex -xO4 -mt" \ ./configure --prefix=/usr/local/mysql --enable-assembler
You may also have to edit the configure
script to change this line:
#if !defined(__STDC__) || __STDC__ != 1
to this:
#if !defined(__STDC__)
If you turn on __STDC__
with the -Xc
option, the Sun compiler
can't compile with the Solaris `pthread.h' header file. This is a Sun
bug (broken compiler or broken include file).
If mysqld
issues the error message shown below when you run it, you have
tried to compile MySQL with the Sun compiler without enabling the
multi-thread option (-mt
):
libc internal error: _rmutex_unlock: rmutex not held
Add -mt
to CFLAGS
and CXXFLAGS
and try again.
If you get the following error when compiling MySQL with gcc
,
it means that your gcc
is not configured for your version of Solaris:
shell> gcc -O3 -g -O2 -DDBUG_OFF -o thr_alarm ... ./thr_alarm.c: In function `signal_hand': ./thr_alarm.c:556: too many arguments to function `sigwait'
The proper thing to do in this case is to get the newest version of
gcc
and compile it with your current gcc
compiler! At
least for Solaris 2.5, almost all binary versions of gcc
have
old, unusable include files that will break all programs that use
threads (and possibly other programs)!
Solaris doesn't provide static versions of all system libraries
(libpthreads
and libdl
), so you can't compile MySQL
with --static
. If you try to do so, you will get the error:
ld: fatal: library -ldl: not found or undefined reference to `dlopen'
If too many processes try to connect very rapidly to mysqld
, you will
see this error in the MySQL log:
Error in accept: Protocol error
You might try starting the server with the --set-variable back_log=50
option as a workaround for this. See section 4.1.1 mysqld Command-line Options.
If you are linking your own MySQL client, you might get the following error when you try to execute it:
ld.so.1: ./my: fatal: libmysqlclient.so.#: open failed: No such file or directory
The problem can be avoided by one of the following methods:
-Lpath
):
-Wl,r/full-path-to-libmysqlclient.so
.
LD_RUN_PATH
environment variable before running your client.
When using the --with-libwrap
configure option, you must also
include the libraries that `libwrap.a' needs:
--with-libwrap="/opt/NUtcpwrapper-7.6/lib/libwrap.a -lnsl -lsocket
If you have problems with configure trying to link with -lz
and
you don't have zlib
installed, you have two options:
--with-named-z-libs=no
.
If you are using gcc and have problems with loading UDF
functions
into MySQL, try adding -lgcc
to the link line for the
UDF
function.
If you would like MySQL to start automatically, you can copy `support-files/mysql.server' to `/etc/init.d' and create a symbolic link to it named `/etc/rc3.d/S99mysql.server'.
You can normally use a Solaris 2.6 binary on Solaris 2.7 and 2.8. Most of the Solaris 2.6 issues also apply for Solaris 2.7 and 2.8.
Note that MySQL Version 3.23.4 and above should be able to autodetect new versions of Solaris and enable workarounds for the following problems!
Solaris 2.7 / 2.8 has some bugs in the include files. You may see the
following error when you use gcc
:
/usr/include/widec.h:42: warning: `getwc' redefined /usr/include/wchar.h:326: warning: this is the location of the previous definition
If this occurs, you can do the following to fix the problem:
Copy /usr/include/widec.h
to
.../lib/gcc-lib/os/gcc-version/include
and change line 41 from:
#if !defined(lint) && !defined(__lint) to #if !defined(lint) && !defined(__lint) && !defined(getwc)
Alternatively, you can edit `/usr/include/widec.h' directly. Either
way, after you make the fix, you should remove `config.cache' and run
configure
again!
If you get errors like this when you run make
, it's because
configure
didn't detect the `curses.h' file (probably
because of the error in `/usr/include/widec.h'):
In file included from mysql.cc:50: /usr/include/term.h:1060: syntax error before `,' /usr/include/term.h:1081: syntax error before `;'
The solution to this is to do one of the following:
CFLAGS=-DHAVE_CURSES_H CXXFLAGS=-DHAVE_CURSES_H ./configure
.
#define HAVE_TERM
line from `config.h' file and
run make
again.
If you get a problem that your linker can't find -lz
when linking
your client program, the problem is probably that your `libz.so' file is
installed in `/usr/local/lib'. You can fix this by one of the
following methods:
LD_LIBRARY_PATH
.
--with-named-z-libs=no
option.
On Solaris 2.8 on x86, mysqld will core dump if you run 'strip' in.
If you are using gcc
or egcs
on Solaris x86 and you
experience problems with core dumps under load, you should use the
following configure
command:
CC=gcc CFLAGS="-O3 -fomit-frame-pointer -DHAVE_CURSES_H" \ CXX=gcc \ CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti -DHAVE_CURSES_H" \ ./configure --prefix=/usr/local/mysql
This will avoid problems with the libstdc++
library and with C++
exceptions.
If this doesn't help, you should compile a debug version and run
it with a trace file or under gdb
. See section H.1.3 Debugging mysqld under gdb.
FreeBSD 3.x is recommended for running MySQL since the thread package is much more integrated.
The easiest and therefor the preferred way to install is to use the mysql-server and mysql-client ports available on http://www.freebsd.org.
Using these gives you:
It is recommended you use MIT-pthreads on FreeBSD 2.x and native threads on
Versions 3 and up. It is possible to run with native threads on some late
2.2.x versions but you may encounter problems shutting down mysqld
.
The MYSQL Makefiles require GNU make (gmake
) to work.
If you want to compile MYSQL you need to install GNU make
first.
Be sure to have your name resolver setup correct. Otherwise you may
experience resolver delays or failures when connecting to mysqld
.
Make sure that the localhost
entry in the `/etc/hosts' file is
correct (otherwise you will have problems connecting to the database). The
`/etc/hosts' file should start with a line:
127.0.0.1 localhost localhost.your.domain
If you notice that configure
will use MIT-pthreads, you should read
the MIT-pthreads notes. See section 2.3.6 MIT-pthreads Notes.
If you get an error from make install
that it can't find
`/usr/include/pthreads', configure
didn't detect that you need
MIT-pthreads. This is fixed by executing these commands:
shell> rm config.cache shell> ./configure --with-mit-threads
FreeBSD is also known to have a very low default file handle limit.
See section A.11 File Not Found. Uncomment the ulimit -n section in
safe_mysqld or raise the limits for the mysqld
user in /etc/login.conf
(and rebuild it with cap_mkdb /etc/login.conf). Also be sure you set the
appropriate class for this user in the password file if you are not
using the default (use: chpass mysqld-user-name). See section 4.7.2 safe_mysqld, the wrapper around mysqld.
If you get problems with the current date in MySQL, setting the
TZ
variable will probably help. See section B Environment Variables.
To get a secure and stable system you should only use FreeBSD kernels
that are marked -STABLE
.
To compile on NetBSD you need GNU make
. Otherwise the compile will
crash when make
tries to run lint
on C++ files.
On OpenBSD Version 2.5, you can compile MySQL with native threads with the following options:
CFLAGS=-pthread CXXFLAGS=-pthread ./configure --with-mit-threads=no
Our users have reported that OpenBSD 2.8 has a threading bug which causes problems with MySQL. The OpenBSD Developers have fixed the problem, but as of January 25th, 2001, it's only available in the ``-current'' branch. The symptoms of this threading bug are: slow response, high load, high CPU usage, and crashes.
If you get the following error when compiling MySQL, your
ulimit
value for virtual memory is too low:
item_func.h: In method `Item_func_ge::Item_func_ge(const Item_func_ge &)': item_func.h:28: virtual memory exhausted make[2]: *** [item_func.o] Error 1
Try using ulimit -v 80000
and run make
again. If this
doesn't work and you are using bash
, try switching to csh
or sh
; some BSDI users have reported problems with bash
and ulimit
.
If you are using gcc
, you may also use have to use the
--with-low-memory
flag for configure
to be able to compile
`sql_yacc.cc'.
If you get problems with the current date in MySQL, setting the
TZ
variable will probably help. See section B Environment Variables.
Upgrade to BSD/OS Version 3.1. If that is not possible, install BSDIpatch M300-038.
Use the following command when configuring MySQL:
shell> env CXX=shlicc++ CC=shlicc2 \ ./configure \ --prefix=/usr/local/mysql \ --localstatedir=/var/mysql \ --without-perl \ --with-unix-socket-path=/var/mysql/mysql.sock
The following is also known to work:
shell> env CC=gcc CXX=gcc CXXFLAGS=-O3 \ ./configure \ --prefix=/usr/local/mysql \ --with-unix-socket-path=/var/mysql/mysql.sock
You can change the directory locations if you wish, or just use the defaults by not specifying any locations.
If you have problems with performance under heavy load, try using the
--skip-thread-priority
option to mysqld
! This will run
all threads with the same priority; on BSDI Version 3.1, this gives better
performance (at least until BSDI fixes their thread scheduler).
If you get the error virtual memory exhausted
while compiling,
you should try using ulimit -v 80000
and run make
again.
If this doesn't work and you are using bash
, try switching to
csh
or sh
; some BSDI users have reported problems with
bash
and ulimit
.
BSDI Version 4.x has some thread-related bugs. If you want to use MySQL on this, you should install all thread-related patches. At least M400-023 should be installed.
On some BSDI Version 4.x systems, you may get problems with shared libraries.
The symptom is that you can't execute any client programs, for example,
mysqladmin
. In this case you need to reconfigure not to use
shared libraries with the --disable-shared
option to configure.
Some customers have had problems on BSDI 4.0.1 that the mysqld
binary after a while can't open tables. This is because some
library/system related bug causes mysqld
to change current
directory without asking for this!
The fix is to either upgrade to 3.23.34 or after running configure
remove the line #define HAVE_REALPATH
from config.h
before running make.
Note that the above means that you can't symbolic link a database directories to another database directory or symbolic link a table to another database on BSDI! (Making a symbolic link to another disk is ok).
MySQL should work without any problems on Mac OS X Public Beta (Darwin). You don't need the pthread patches for this OS!
Before trying to configure MySQL on Mac OS X server you must first install the pthread package from http://www.prnet.de/RegEx/mysql.html.
Our binary for Mac OS X is compiled on Rhapsody 5.5 with the following configure line:
CC=gcc CFLAGS="-O2 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O2 -fomit-frame-pointer" ./configure --prefix=/usr/local/mysql "--with-comment=Official MySQL binary" --with-extra-charsets=complex --disable-shared
You might want to also add aliases to your shell's resource file to
access mysql
and mysqladmin
from the command line:
alias mysql '/usr/local/mysql/bin/mysql' alias mysqladmin '/usr/local/mysql/bin/mysqladmin'
Some of the binary distributions of MySQL for HP-UX is distributed as an HP depot file and as a tar file. To use the depot file you must be running at least HP-UX 10.x to have access to HP's software depot tools.
The HP version of MySQL was compiled on an HP 9000/8xx server under HP-UX 10.20, and uses MIT-pthreads. It is known to work well under this configuration. MySQL Version 3.22.26 and newer can also be built with HP's native thread package.
Other configurations that may work:
The following configurations almost definitely won't work:
To install the distribution, use one of the commands below, where
/path/to/depot
is the full pathname of the depot file:
shell> /usr/sbin/swinstall -s /path/to/depot mysql.full
shell> /usr/sbin/swinstall -s /path/to/depot mysql.server
shell> /usr/sbin/swinstall -s /path/to/depot mysql.client
shell> /usr/sbin/swinstall -s /path/to/depot mysql.developer
The depot places binaries and libraries in `/opt/mysql' and data in
`/var/opt/mysql'. The depot also creates the appropriate entries in
`/etc/init.d' and `/etc/rc2.d' to start the server automatically
at boot time. Obviously, this entails being root
to install.
To install the HP-UX tar.gz distribution, you must have a copy of GNU
tar
.
There are a couple of small problems when compiling MySQL on
HP-UX. We recommend that you use gcc
instead of the HP-UX native
compiler, because gcc
produces better code!
We recommend using gcc 2.95 on HP-UX. Don't use high optimization flags (like -O6) as this may not be safe on HP-UX.
Note that MIT-pthreads can't be compiled with the HP-UX compiler
because it can't compile .S
(assembler) files.
The following configure line should work:
CFLAGS="-DHPUX -I/opt/dce/include" CXXFLAGS="-DHPUX -I/opt/dce/include -felide-constructors -fno-exceptions -fno-rtti" CXX=gcc ./configure --with-pthread --with-named-thread-libs='-ldce' --prefix=/usr/local/mysql --disable-shared
If you are compiling gcc
2.95 yourself, you should NOT link it with
the DCE libraries (libdce.a
or libcma.a
) if you want to compile
MySQL with MIT-pthreads. If you mix the DCE and MIT-pthreads
packages you will get a mysqld
to which you cannot connect. Remove
the DCE libraries while you compile gcc
2.95!
For HP-UX Version 11.x we recommend MySQL Version 3.23.15 or later.
Because of some critical bugs in the standard HP-UX libraries, you should install the following patches before trying to run MySQL on HP-UX 11.0:
PHKL_22840 Streams cumulative PHNE_22397 ARPA cumulative
This will solve a problem that one gets EWOULDBLOCK
from recv()
and EBADF
from accept()
in threaded applications.
If you are using gcc
2.95.1 on an unpatched HP-UX 11.x system,
you will get the error:
In file included from /usr/include/unistd.h:11, from ../include/global.h:125, from mysql_priv.h:15, from item.cc:19: /usr/include/sys/unistd.h:184: declaration of C function ... /usr/include/sys/pthread.h:440: previous declaration ... In file included from item.h:306, from mysql_priv.h:158, from item.cc:19:
The problem is that HP-UX doesn't define pthreads_atfork()
consistently.
It has conflicting prototypes in
`/usr/include/sys/unistd.h':184 and
`/usr/include/sys/pthread.h':440 (details below).
One solution is to copy `/usr/include/sys/unistd.h' into `mysql/include' and edit `unistd.h' and change it to match the definition in `pthread.h'. Here's the diff:
183,184c183,184 < extern int pthread_atfork(void (*prepare)(), void (*parent)(), < void (*child)()); --- > extern int pthread_atfork(void (*prepare)(void), void (*parent)(void), > void (*child)(void));
After this, the following configure line should work:
CFLAGS="-fomit-frame-pointer -O3 -fpic" CXX=gcc CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -O3" ./configure --prefix=/usr/local/mysql --disable-shared
Here is some information that a HP-UX Version 11.x user sent us about compiling MySQL with HP-UX:x compiler:
Environment: proper compilers. setenv CC cc setenv CXX aCC flags setenv CFLAGS -D_REENTRANT setenv CXXFLAGS -D_REENTRANT setenv CPPFLAGS -D_REENTRANT % aCC -V aCC: HP ANSI C++ B3910B X.03.14.06 % cc -V /tmp/empty.c cpp.ansi: HP92453-01 A.11.02.00 HP C Preprocessor (ANSI) ccom: HP92453-01 A.11.01.00 HP C Compiler cc: "/tmp/empty.c", line 1: warning 501: Empty source file. configuration: ./configure --with-pthread \ --prefix=/source-control/mysql \ --with-named-thread-libs=-lpthread \ --with-low-memory added '#define _CTYPE_INCLUDED' to include/m_ctype.h. This symbol is the one defined in HP's /usr/include/ctype.h: /* Don't include std ctype.h when this is included */ #define _CTYPE_H #define __CTYPE_INCLUDED #define _CTYPE_INCLUDED #define _CTYPE_USING /* Don't put names in global namespace. */
-D_REENTRANT
to get the compiler
to recognize the prototype for localtime_r
. Alternatively I could have
supplied the prototype for localtime_r
. But I wanted to catch other
bugs without needing to run into them. I wasn't sure where I needed it, so I
added it to all flags.
If you get the following error from configure
checking for cc option to accept ANSI C... no configure: error: MySQL requires a ANSI C compiler (and a C++ compiler). Try gcc. See the Installation chapter in the Reference Manual.
Check that you don't have the path to the K&R compiler before the path to the HP-UX C and C++ compiler.
Automatic detection of xlC
is missing from Autoconf, so a
configure
command something like this is needed when compiling
MySQL (This example uses the IBM compiler):
export CC="xlc_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192 " export CXX="xlC_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192" export CFLAGS="-I /usr/local/include" export LDLFAGS="-L /usr/local/lib" export CPPFLAGS=$CFLAGS export CXXFLAGS=$CFLAGS ./configure --prefix=/usr/local \ --localstatedir=/var/mysql \ --sysconfdir=/etc/mysql \ --sbindir='/usr/local/bin' \ --libexecdir='/usr/local/bin' \ --enable-thread-safe-client \ --enable-large-files
Above are the options used to compile the MySQL distribution that can be found at http://www-frec.bull.com/.
If you change the -O3
to -O2
in the above configure line,
you must also remove the -qstrict
option (this is a limitation in
the IBM C compiler).
If you are using gcc
or egcs
to compile MySQL, you
MUST use the -fno-exceptions
flag, as the exception
handling in gcc
/egcs
is not thread safe! (This is tested with
egcs
1.1.). There are also some known problems with IBM's assembler,
which may cause it to generate bad code when used with gcc.
We recommend the following configure
line with egcs
and
gcc 2.95
on AIX:
CC="gcc -pipe -mcpu=power -Wa,-many" \ CXX="gcc -pipe -mcpu=power -Wa,-many" \ CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \ ./configure --prefix=/usr/local/mysql --with-low-memory
The -Wa,-many
is necessary for the compile to be successful. IBM is
aware of this problem but is in to hurry to fix it because of the workaround
available. We don't know if the -fno-exceptions
is required with
gcc 2.95
, but as MySQL doesn't use exceptions and the above
option generates faster code, we recommend that you should always use this
option with egcs / gcc
.
If you get a problem with assembler code try changing the -mcpu=xxx to match your cpu. Typically power2, power, or powerpc may need to be used, alternatively you might need to use 604 or 604e. I'm not positive but I would think using "power" would likely be safe most of the time, even on a power2 machine.
If you don't know what your cpu is then do a "uname -m", this will give you back a string that looks like "000514676700", with a format of xxyyyyyymmss where xx and ss are always 0's, yyyyyy is a unique system id and mm is the id of the CPU Planar. A chart of these values can be found at http://www.rs6000.ibm.com/doc_link/en_US/a_doc_lib/cmds/aixcmds5/uname.htm. This will give you a machine type and a machine model you can use to determine what type of cpu you have.
If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:
shell> CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \ CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -DDONT_USE_THR_ALARM" \ ./configure --prefix=/usr/local/mysql --with-debug --with-low-memory
This doesn't affect the performance of MySQL, but has the side
effect that you can't kill clients that are ``sleeping'' on a connection with
mysqladmin kill
or mysqladmin shutdown
. Instead, the client
will die when it issues its next command.
On some versions of AIX, linking with libbind.a
makes
getservbyname
core dump. This is an AIX bug and should be reported
to IBM.
For AIX 4.2.1 and gcc you have to do the following changes.
After configuring, edit `config.h' and `include/my_config.h' and change the line that says
#define HAVE_SNPRINTF 1
to
#undef HAVE_SNPRINTF
And finally, in `mysqld.cc' you need to add a prototype for initgoups.
#ifdef _AIX41 extern "C" int initgroups(const char *,int); #endif
On SunOS 4, MIT-pthreads is needed to compile MySQL, which in turn
means you will need GNU make
.
Some SunOS 4 systems have problems with dynamic libraries and libtool
.
You can use the following configure
line to avoid this problem:
shell> ./configure --disable-shared --with-mysqld-ldflags=-all-static
When compiling readline
, you may get warnings about duplicate defines.
These may be ignored.
When compiling mysqld
, there will be some implicit declaration
of function
warnings. These may be ignored.
If you are using egcs 1.1.2 on Digital Unix, you should upgrade to gcc 2.95.2, as egcs on DEC has some serious bugs!
When compiling threaded programs under Digital Unix, the documentation
recommends using the -pthread
option for cc
and cxx
and
the libraries -lmach -lexc
(in addition to -lpthread
). You
should run configure
something like this:
CC="cc -pthread" CXX="cxx -pthread -O" \ ./configure --with-named-thread-libs="-lpthread -lmach -lexc -lc"
When compiling mysqld
, you may see a couple of warnings like this:
mysqld.cc: In function void handle_connections()': mysqld.cc:626: passing long unsigned int *' as argument 3 of accept(int,sockadddr *, int *)'
You can safely ignore these warnings. They occur because configure
can detect only errors, not warnings.
If you start the server directly from the command line, you may have problems
with it dying when you log out. (When you log out, your outstanding processes
receive a SIGHUP
signal.) If so, try starting the server like this:
shell> nohup mysqld [options] &
nohup
causes the command following it to ignore any SIGHUP
signal sent from the terminal. Alternatively, start the server by running
safe_mysqld
, which invokes mysqld
using nohup
for you.
See section 4.7.2 safe_mysqld, the wrapper around mysqld.
If you get a problem when compiling mysys/get_opt.c, just remove the line #define _NO_PROTO from the start of that file!
If you are using Compac's CC compiler, the following configure line should work:
CC="cc -pthread" CFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host" CXX="cxx -pthread" CXXFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host" export CC CFLAGS CXX CXXFLAGS ./configure \ --prefix=/usr/local/mysql \ --with-low-memory \ --enable-large-files \ --enable-shared=yes \ --with-named-thread-libs="-lpthread -lmach -lexc -lc" gnumake
If you get a problem with libtool, when compiling with shared libraries
as above, when linking mysql
, you should be able to get around
this by issuing:
cd mysql /bin/sh ../libtool --mode=link cxx -pthread -O3 -DDBUG_OFF \ -O4 -ansi_alias -ansi_args -fast -inline speed \ -speculate all \ -arch host -DUNDEF_HAVE_GETHOSTBYNAME_R \ -o mysql mysql.o readline.o sql_string.o completion_hash.o \ ../readline/libreadline.a -lcurses \ ../libmysql/.libs/libmysqlclient.so -lm cd .. gnumake gnumake install scripts/mysql_install_db
If you have problems compiling and have DEC CC
and gcc
installed, try running configure
like this:
CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql
If you get problems with the `c_asm.h' file, you can create and use a 'dummy' `c_asm.h' file with:
touch include/c_asm.h CC=gcc CFLAGS=-I./include \ CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql
Note that the following problems with the ld
program can be fixed
by downloading the latest DEC (Compaq) patch kit from:
http://ftp.support.compaq.com/public/unix/.
On OSF1 V4.0D and compiler "DEC C V5.6-071 on Digital Unix V4.0 (Rev. 878)"
the compiler had some strange behavior (undefined asm
symbols).
/bin/ld
also appears to be broken (problems with _exit
undefined
errors occuring while linking mysqld
). On this system, we
have managed to compile MySQL with the following configure
line, after replacing /bin/ld
with the version from OSF 4.0C:
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
With the Digital compiler "C++ V6.1-029", the following should work:
CC=cc -pthread CFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host CXX=cxx -pthread CXXFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host -noexceptions -nortti export CC CFLAGS CXX CXXFLAGS ./configure --prefix=/usr/mysql/mysql --with-mysqld-ldflags=-all-static --disable-shared --with-named-thread-libs="-lmach -lexc -lc"
In some versions of OSF1, the alloca()
function is broken. Fix
this by removing the line in `config.h' that defines 'HAVE_ALLOCA'
.
The alloca()
function also may have an incorrect prototype in
/usr/include/alloca.h
. This warning resulting from this can be ignored.
configure
will use the following thread libraries automatically:
--with-named-thread-libs="-lpthread -lmach -lexc -lc"
.
When using gcc
, you can also try running configure
like this:
shell> CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure ....
If you have problems with signals (MySQL dies unexpectedly under high load), you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:
shell> CFLAGS=-DDONT_USE_THR_ALARM \ CXXFLAGS=-DDONT_USE_THR_ALARM \ ./configure ...
This doesn't affect the performance of MySQL, but has the side
effect that you can't kill clients that are ``sleeping'' on a connection with
mysqladmin kill
or mysqladmin shutdown
. Instead, the client
will die when it issues its next command.
With gcc
2.95.2, you will probably run into the following compile error:
sql_acl.cc:1456: Internal compiler error in `scan_region', at except.c:2566 Please submit a full bug report.
To fix this you should change to the sql
directory and do a ``cut
and paste'' of the last gcc
line, but change -O3
to
-O0
(or add -O0
immediately after gcc
if you don't
have any -O
option on your compile line.) After this is done you
can just change back to the top-level directly and run make
again.
If you are using Irix Version 6.5.3 or newer mysqld
will only be able to
create threads if you run it as a user with CAP_SCHED_MGT
privileges (like root
) or give the mysqld
server this privilege
with the following shell command:
shell> chcap "CAP_SCHED_MGT+epi" /opt/mysql/libexec/mysqld
You may have to undefine some things in `config.h' after running
configure
and before compiling.
In some Irix implementations, the alloca()
function is broken. If the
mysqld
server dies on some SELECT
statements, remove the lines
from `config.h' that define HAVE_ALLOC
and HAVE_ALLOCA_H
.
If mysqladmin create
doesn't work, remove the line from `config.h'
that defines HAVE_READDIR_R
. You may have to remove the
HAVE_TERM_H
line as well.
SGI recommends that you install all of the patches on this page as a set: http://support.sgi.com/surfzone/patches/patchset/6.2_indigo.rps.html
At the very minimum, you should install the latest kernel rollup, the
latest rld
rollup, and the latest libc
rollup.
You definitely need all the POSIX patches on this page, for pthreads support:
http://support.sgi.com/surfzone/patches/patchset/6.2_posix.rps.html
If you get the something like the following error when compiling `mysql.cc':
"/usr/include/curses.h", line 82: error(1084): invalid combination of type
Type the following in the top-level directory of your MySQL source tree:
shell> extra/replace bool curses_bool < /usr/include/curses.h > include/curses.h shell> make
There have also been reports of scheduling problems. If only one thread is running, things go slow. Avoid this by starting another client. This may lead to a 2-to-10-fold increase in execution speed thereafter for the other thread. This is a poorly understood problem with Irix threads; you may have to improvise to find solutions until this can be fixed.
If you are compiling with gcc
, you can use the following
configure
command:
CC=gcc CXX=gcc CXXFLAGS=-O3 \ ./configure --prefix=/usr/local/mysql --with-thread-safe-client --with-named-thread-libs=-lpthread
On Irix 6.5.11 with native Irix C and C++ compilers ver. 7.3.1.2, the following is reported to work
CC=cc CXX=CC CFLAGS='-O3 -n32 -TARG:platform=IP22 -I/usr/local/include \ -L/usr/local/lib' CXXFLAGS='-O3 -n32 -TARG:platform=IP22 \ -I/usr/local/include -L/usr/local/lib' ./configure --prefix=/usr/local/mysql \ --with-berkeley-db --with-innodb \ --with-libwrap=/usr/local --with-named-curses-libs=/usr/local/lib/libncurses.a
The current port is tested only on a ``sco3.2v5.0.4'' and ``sco3.2v5.0.5'' system. There has also been a lot of progress on a port to ``sco 3.2v4.2''.
For the moment the recommended compiler on OpenServer is gcc 2.95.2. With this you should be able to compile MySQL with just:
CC=gcc CXX=gcc ./configure ... (options)
gcc
2.7.2 in Skunkware 97 does not have
GNU as
. You can also use egcs
1.1.2 or newer
http://www.egcs.com/. If you are using egcs
1.1.2 you have
to execute the following command:
shell> cp -p /usr/include/pthread/stdtypes.h /usr/local/lib/gcc-lib/i386-pc-sco3.2v5.0.5/egcs-2.91.66/include/pthread/
./configure
in the `threads/src' directory and select
the SCO OpenServer option. This command copies `Makefile.SCO5' to
`Makefile'.
make
.
cd
to the `thread/src' directory, and run make
install
.
make
when making MySQL.
safe_mysqld
as root, you probably will get only the
default 110 open files per process. mysqld
will write a note about this
in the log file.
configure
command should work:
shell> ./configure --prefix=/usr/local/mysql --disable-shared
configure
command should work:
shell> CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \ ./configure \ --prefix=/usr/local/mysql \ --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \ --with-named-curses-libs="-lcurses"You may get some problems with some include files. In this case, you can find new SCO-specific include files at http://www.mysql.com/Downloads/SCO/SCO-3.2v4.2-includes.tar.gz. You should unpack this file in the `include' directory of your MySQL source tree.
SCO development notes:
mysqld
with -lgthreads -lsocket -lgthreads
.
malloc
. If you encounter problems with memory usage,
make sure that `gmalloc.o' is included in `libgthreads.a' and
`libgthreads.so'.
read()
,
write()
, getmsg()
, connect()
, accept()
,
select()
, and wait()
.
If you want to install DBI on SCO, you have to edit the `Makefile' in DBI-xxx and each subdirectory.
Note that the following assumes gcc 2.95.2 or newer:
OLD: NEW: CC = cc CC = gcc CCCDLFLAGS = -KPIC -W1,-Bexport CCCDLFLAGS = -fpic CCDLFLAGS = -wl,-Bexport CCDLFLAGS = LD = ld LD = gcc -G -fpic LDDLFLAGS = -G -L/usr/local/lib LDDLFLAGS = -L/usr/local/lib LDFLAGS = -belf -L/usr/local/lib LDFLAGS = -L/usr/local/lib LD = ld LD = gcc -G -fpic OPTIMISE = -Od OPTIMISE = -O1 OLD: CCCFLAGS = -belf -dy -w0 -U M_XENIX -DPERL_SCO5 -I/usr/local/include NEW: CCFLAGS = -U M_XENIX -DPERL_SCO5 -I/usr/local/include
This is because the Perl dynaloader will not load the DBI
modules
if they were compiled with icc
or cc
.
Perl works best when compiled with cc
.
You must use a version of MySQL at least as recent as Version 3.22.13 because that version fixes some portability problems under Unixware.
We have been able to compile MySQL with the following configure
command on Unixware Version 7.0.1:
CC=cc CXX=CC ./configure --prefix=/usr/local/mysql
If you want to use gcc
, you must use gcc
2.95.2 or newer.
MySQL uses quite a few open files. Because of this, you should add something like the following to your `CONFIG.SYS' file:
SET EMXOPT=-c -n -h1024
If you don't do this, you will probably run into the following error:
File 'xxxx' not found (Errcode: 24)
When using MySQL with OS/2 Warp 3, FixPack 29 or above is required. With OS/2 Warp 4, FixPack 4 or above is required. This is a requirement of the Pthreads library. MySQL must be installed in a partition that supports long filenames such as HPFS, FAT32, etc.
The `INSTALL.CMD' script must be run from OS/2's own `CMD.EXE' and may not work with replacement shells such as `4OS2.EXE'.
The `scripts/mysql-install-db' script has been renamed. It is now called `install.cmd' and is a REXX script, which will set up the default MySQL security settings and create the WorkPlace Shell icons for MySQL.
Dynamic module support is compiled in but not fully tested. Dynamic modules should be compiled using the Pthreads run-time library.
gcc -Zdll -Zmt -Zcrtdll=pthrdrtl -I../include -I../regex -I.. \ -o example udf_example.cc -L../lib -lmysqlclient udf_example.def mv example.dll example.udf
Note: Due to limitations in OS/2, UDF module name stems must not
exceed 8 characters. Modules are stored in the `/mysql2/udf'
directory; the safe-mysqld.cmd
script will put this directory in
the BEGINLIBPATH
environment variable. When using UDF modules,
specified extensions are ignored -- it is assumed to be `.udf'.
For example, in Unix, the shared module might be named `example.so'
and you would load a function from it like this:
mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example.so";
Is OS/2, the module would be named `example.udf', but you would not specify the module extension:
mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example";
We are really interested in getting MySQL to work on BeOS, but unfortunately we don't have any person who knows BeOS or has time to do a port.
We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port.
We have previously talked with some BeOS developers that have said that MySQL is 80% ported to BeOS, but we haven't heard from them in a while.
We are really interested in getting MySQL to work on Netware, but unfortunately we don't have any person who knows Netware or has time to do a port.
We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port.
This chapter provides a tutorial introduction to MySQL by showing
how to use the mysql
client program to create and use a simple
database. mysql
(sometimes referred to as the ``terminal monitor'' or
just ``monitor'') is an interactive program that allows you to connect to a
MySQL server, run queries, and view the results. mysql
may
also be used in batch mode: you place your queries in a file beforehand, then
tell mysql
to execute the contents of the file. Both ways of using
mysql
are covered here.
To see a list of options provided by mysql
, invoke it with
the --help
option:
shell> mysql --help
This chapter assumes that mysql
is installed on your machine and that
a MySQL server is available to which you can connect. If this is
not true, contact your MySQL administrator. (If you are the
administrator, you will need to consult other sections of this manual.)
This chapter describes the entire process of setting up and using a database. If you are interested only in accessing an already-existing database, you may want to skip over the sections that describe how to create the database and the tables it contains.
Because this chapter is tutorial in nature, many details are necessarily left out. Consult the relevant sections of the manual for more information on the topics covered here.
To connect to the server, you'll usually need to provide a MySQL
user name when you invoke mysql
and, most likely, a password. If the
server runs on a machine other than the one where you log in, you'll also
need to specify a hostname. Contact your administrator to find out what
connection parameters you should use to connect (that is, what host, user name,
and password to use). Once you know the proper parameters, you should be
able to connect like this:
shell> mysql -h host -u user -p Enter password: ********
The ********
represents your password; enter it when mysql
displays the Enter password:
prompt.
If that works, you should see some introductory information followed by a
mysql>
prompt:
shell> mysql -h host -u user -p Enter password: ******** Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 459 to server version: 3.22.20a-log Type 'help' for help. mysql>
The prompt tells you that mysql
is ready for you to enter commands.
Some MySQL installations allow users to connect as the anonymous
(unnamed) user to the server running on the local host. If this is the case
on your machine, you should be able to connect to that server by invoking
mysql
without any options:
shell> mysql
After you have connected successfully, you can disconnect any time by typing
QUIT
at the mysql>
prompt:
mysql> QUIT Bye
You can also disconnect by pressing Control-D.
Most examples in the following sections assume you are connected to the
server. They indicate this by the mysql>
prompt.
Make sure you are connected to the server, as discussed in the previous
section. Doing so will not in itself select any database to work with, but
that's okay. At this point, it's more important to find out a little about
how to issue queries than to jump right in creating tables, loading data
into them, and retrieving data from them. This section describes the basic
principles of entering commands, using several queries you can try out to
familiarize yourself with how mysql
works.
Here's a simple command that asks the server to tell you its version number
and the current date. Type it in as shown below following the mysql>
prompt and hit the RETURN key:
mysql> SELECT VERSION(), CURRENT_DATE; +--------------+--------------+ | version() | CURRENT_DATE | +--------------+--------------+ | 3.22.20a-log | 1999-03-19 | +--------------+--------------+ 1 row in set (0.01 sec) mysql>
This query illustrates several things about mysql
:
QUIT
,
mentioned earlier, is one of them. We'll get to others later.)
mysql
sends it to the server for execution
and displays the results, then prints another mysql>
to indicate
that it is ready for another command.
mysql
displays query output as a table (rows and columns). The first
row contains labels for the columns. The rows following are the query
results. Normally, column labels are the names of the columns you fetch from
database tables. If you're retrieving the value of an expression rather than
a table column (as in the example just shown), mysql
labels the column
using the expression itself.
mysql
shows how many rows were returned and how long the query took
to execute, which gives you a rough idea of server performance. These values
are imprecise because they represent wall clock time (not CPU or machine
time), and because they are affected by factors such as server load and
network latency. (For brevity, the ``rows in set'' line is not shown in
the remaining examples in this chapter.)
Keywords may be entered in any lettercase. The following queries are equivalent:
mysql> SELECT VERSION(), CURRENT_DATE; mysql> select version(), current_date; mysql> SeLeCt vErSiOn(), current_DATE;
Here's another query. It demonstrates that you can use mysql
as a
simple calculator:
mysql> SELECT SIN(PI()/4), (4+1)*5; +-------------+---------+ | SIN(PI()/4) | (4+1)*5 | +-------------+---------+ | 0.707107 | 25 | +-------------+---------+
The commands shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon:
mysql> SELECT VERSION(); SELECT NOW(); +--------------+ | version() | +--------------+ | 3.22.20a-log | +--------------+ +---------------------+ | NOW() | +---------------------+ | 1999-03-19 00:15:33 | +---------------------+
A command need not be given all on a single line, so lengthy commands that
require several lines are not a problem. mysql
determines where your
statement ends by looking for the terminating semicolon, not by looking for
the end of the input line. (In other words, mysql
accepts free-format input: it collects input lines but does not execute them
until it sees the semicolon.)
Here's a simple multiple-line statement:
mysql> SELECT -> USER() -> , -> CURRENT_DATE; +--------------------+--------------+ | USER() | CURRENT_DATE | +--------------------+--------------+ | joesmith@localhost | 1999-03-18 | +--------------------+--------------+
In this example, notice how the prompt changes from mysql>
to
->
after you enter the first line of a multiple-line query. This is
how mysql
indicates that it hasn't seen a complete statement and is
waiting for the rest. The prompt is your friend, because it provides
valuable feedback. If you use that feedback, you will always be aware of
what mysql
is waiting for.
If you decide you don't want to execute a command that you are in the
process of entering, cancel it by typing \c
:
mysql> SELECT -> USER() -> \c mysql>
Here, too, notice the prompt. It switches back to mysql>
after you
type \c
, providing feedback to indicate that mysql
is ready
for a new command.
The following table shows each of the prompts you may see and summarizes what
they mean about the state that mysql
is in:
Prompt | Meaning |
mysql> | Ready for new command. |
-> | Waiting for next line of multiple-line command. |
'> | Waiting for next line, collecting a string that begins with a single quote (`''). |
"> | Waiting for next line, collecting a string that begins with a double quote (`"'). |
Multiple-line statements commonly occur by accident when you intend to
issue a command on a single line, but forget the terminating semicolon. In
this case, mysql
waits for more input:
mysql> SELECT USER() ->
If this happens to you (you think you've entered a statement but the only
response is a ->
prompt), most likely mysql
is waiting for the
semicolon. If you don't notice what the prompt is telling you, you might sit
there for a while before realizing what you need to do. Enter a semicolon to
complete the statement, and mysql
will execute it:
mysql> SELECT USER() -> ; +--------------------+ | USER() | +--------------------+ | joesmith@localhost | +--------------------+
The '>
and ">
prompts occur during string collection.
In MySQL, you can write strings surrounded by either `''
or `"' characters (for example, 'hello'
or "goodbye"
),
and mysql
lets you enter strings that span multiple lines. When you
see a '>
or ">
prompt, it means that you've entered a line
containing a string that begins with a `'' or `"' quote character,
but have not yet entered the matching quote that terminates the string.
That's fine if you really are entering a multiple-line string, but how likely
is that? Not very. More often, the '>
and ">
prompts indicate
that you've inadvertantly left out a quote character. For example:
mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30; ">
If you enter this SELECT
statement, then hit RETURN and wait for the
result, nothing will happen. Instead of wondering why this
query takes so long, notice the clue provided by the ">
prompt. It
tells you that mysql
expects to see the rest of an unterminated
string. (Do you see the error in the statement? The string "Smith
is
missing the second quote.)
At this point, what do you do? The simplest thing is to cancel the command.
However, you cannot just type \c
in this case, because mysql
interprets it as part of the string that it is collecting! Instead, enter
the closing quote character (so mysql
knows you've finished the
string), then type \c
:
mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30; "> "\c mysql>
The prompt changes back to mysql>
, indicating that mysql
is ready for a new command.
It's important to know what the '>
and ">
prompts signify,
because if you mistakenly enter an unterminated string, any further lines you
type will appear to be ignored by mysql
-- including a line
containing QUIT
! This can be quite confusing, especially if you
don't know that you need to supply the terminating quote before you can
cancel the current command.
Now that you know how to enter commands, it's time to access a database.
Suppose you have several pets in your home (your menagerie) and you'd like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer different sorts of questions about your animals by retrieving data from the tables. This section shows you how to:
The menagerie database will be simple (deliberately), but it is not difficult
to think of real-world situations in which a similar type of database might
be used. For example, a database like this could be used by a farmer to keep
track of livestock, or by a veterinarian to keep track of patient records.
A menagerie distribution containing some of the queries and sample data used
in the following sections can be obtained from the MySQL Web site.
It's available in either
compressed tar
format
or
Zip format.
Use the SHOW
statement to find out what databases currently exist
on the server:
mysql> SHOW DATABASES; +----------+ | Database | +----------+ | mysql | | test | | tmp | +----------+
The list of databases is probably different on your machine, but the
mysql
and test
databases are likely to be among them. The
mysql
database is required because it describes user access
privileges. The test
database is often provided as a workspace for
users to try things out.
If the test
database exists, try to access it:
mysql> USE test Database changed
Note that USE
, like QUIT
, does not require a semicolon. (You
can terminate such statements with a semicolon if you like; it does no harm.)
The USE
statement is special in another way, too: it must be given on
a single line.
You can use the test
database (if you have access to it) for the
examples that follow, but anything you create in that database can be
removed by anyone else with access to it. For this reason, you should
probably ask your MySQL administrator for permission to use a
database of your own. Suppose you want to call yours menagerie
. The
administrator needs to execute a command like this:
mysql> GRANT ALL ON menagerie.* TO your_mysql_name;
where your_mysql_name
is the MySQL user name assigned to
you.
If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to create it yourself:
mysql> CREATE DATABASE menagerie;
Under Unix, database names are case sensitive (unlike SQL keywords), so you
must always refer to your database as menagerie
, not as
Menagerie
, MENAGERIE
, or some other variant. This is also true
for table names. (Under Windows, this restriction does not apply, although
you must refer to databases and tables using the same lettercase throughout a
given query.)
Creating a database does not select it for use; you must do that explicitly.
To make menagerie
the current database, use this command:
mysql> USE menagerie Database changed
Your database needs to be created only once, but you must select it for use
each time you begin a mysql
session. You can do this by issuing a
USE
statement as shown above. Alternatively, you can select the
database on the command line when you invoke mysql
. Just specify its
name after any connection parameters that you might need to provide. For
example:
shell> mysql -h host -u user -p menagerie Enter password: ********
Note that menagerie
is not your password on the command just shown.
If you want to supply your password on the command line after the -p
option, you must do so with no intervening space (for example, as
-pmypassword
, not as -p mypassword
). However, putting your
password on the command line is not recommended, because doing so exposes it
to snooping by other users logged in on your machine.
Creating the database is the easy part, but at this point it's empty, as
SHOW TABLES
will tell you:
mysql> SHOW TABLES; Empty set (0.00 sec)
The harder part is deciding what the structure of your database should be: what tables you will need and what columns will be in each of them.
You'll want a table that contains a record for each of your pets. This can
be called the pet
table, and it should contain, as a bare minimum,
each animal's name. Because the name by itself is not very interesting, the
table should contain other information. For example, if more than one person
in your family keeps pets, you might want to list each animal's owner. You
might also want to record some basic descriptive information such as species
and sex.
How about age? That might be of interest, but it's not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it's better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too:
You can probably think of other types of information that would be useful in
the pet
table, but the ones identified so far are sufficient for now:
name, owner, species, sex, birth, and death.
Use a CREATE TABLE
statement to specify the layout of your table:
mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20), -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE);
VARCHAR
is a good choice for the name
, owner
, and
species
columns because the column values will vary in length. The
lengths of those columns need not all be the same, and need not be
20
. You can pick any length from 1
to 255
, whatever
seems most reasonable to you. (If you make a poor choice and it turns
out later that you need a longer field, MySQL provides an
ALTER TABLE
statement.)
Animal sex can be represented in a variety of ways, for example, "m"
and "f"
, or perhaps "male"
and "female"
. It's simplest
to use the single characters "m"
and "f"
.
The use of the DATE
data type for the birth
and death
columns is a fairly obvious choice.
Now that you have created a table, SHOW TABLES
should produce some
output:
mysql> SHOW TABLES; +---------------------+ | Tables in menagerie | +---------------------+ | pet | +---------------------+
To verify that your table was created the way you expected, use
a DESCRIBE
statement:
mysql> DESCRIBE pet; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | owner | varchar(20) | YES | | NULL | | | species | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | birth | date | YES | | NULL | | | death | date | YES | | NULL | | +---------+-------------+------+-----+---------+-------+
You can use DESCRIBE
any time, for example, if you forget the names of
the columns in your table or what types they are.
After creating your table, you need to populate it. The LOAD DATA
and
INSERT
statements are useful for this.
Suppose your pet records can be described as shown below.
(Observe that MySQL expects dates in YYYY-MM-DD
format;
this may be different than what you are used to.)
name | owner | species | sex | birth | death |
Fluffy | Harold | cat | f | 1993-02-04 | |
Claws | Gwen | cat | m | 1994-03-17 | |
Buffy | Harold | dog | f | 1989-05-13 | |
Fang | Benny | dog | m | 1990-08-27 | |
Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 |
Chirpy | Gwen | bird | f | 1998-09-11 | |
Whistler | Gwen | bird | 1997-12-09 | ||
Slim | Benny | snake | m | 1996-04-29 |
Because you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your animals, then load the contents of the file into the table with a single statement.
You could create a text file `pet.txt' containing one record per line,
with values separated by tabs, and given in the order in which the columns
were listed in the CREATE TABLE
statement. For missing values (such
as unknown sexes or death dates for animals that are still living), you can
use NULL
values. To represent these in your text file, use
\N
. For example, the record for Whistler the bird would look like
this (where the whitespace between values is a single tab character):
Whistler | Gwen | bird | \N | 1997-12-09 | \N
|
To load the text file `pet.txt' into the pet
table, use this
command:
mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;
You can specify the column value separator and end of line marker explicitly
in the LOAD DATA
statement if you wish, but the defaults are tab and
linefeed. These are sufficient for the statement to read the file
`pet.txt' properly.
When you want to add new records one at a time, the INSERT
statement
is useful. In its simplest form, you supply values for each column, in the
order in which the columns were listed in the CREATE TABLE
statement.
Suppose Diane gets a new hamster named Puffball. You could add a new record
using an INSERT
statement like this:
mysql> INSERT INTO pet -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL);
Note that string and date values are specified as quoted strings here. Also,
with INSERT
, you can insert NULL
directly to represent a
missing value. You do not use \N
like you do with LOAD DATA
.
From this example, you should be able to see that there would be a lot more
typing involved to load
your records initially using several INSERT
statements rather
than a single LOAD DATA
statement.
The SELECT
statement is used to pull information from a table.
The general form of the statement is:
SELECT what_to_select FROM which_table WHERE conditions_to_satisfy
what_to_select
indicates what you want to see. This can be a list of
columns, or *
to indicate ``all columns.'' which_table
indicates the table from which you want to retrieve data. The WHERE
clause is optional. If it's present, conditions_to_satisfy
specifies
conditions that rows must satisfy to qualify for retrieval.
The simplest form of SELECT
retrieves everything from a table:
mysql> SELECT * FROM pet; +----------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+--------+---------+------+------------+------------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Fang | Benny | dog | m | 1990-08-27 | NULL | | Bowser | Diane | dog | m | 1998-08-31 | 1995-07-29 | | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | | Slim | Benny | snake | m | 1996-04-29 | NULL | | Puffball | Diane | hamster | f | 1999-03-30 | NULL | +----------+--------+---------+------+------------+------------+
This form of SELECT
is useful if you want to review your entire table,
for instance, after you've just loaded it with your initial dataset. As it
happens, the output just shown reveals an error in your data file: Bowser
appears to have been born after he died! Consulting your original pedigree
papers, you find that the correct birth year is 1989, not 1998.
There are are least a couple of ways to fix this:
DELETE
and LOAD DATA
:
mysql> SET AUTOCOMMIT=1; # Used for quick re-create of the table mysql> DELETE FROM pet; mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;However, if you do this, you must also re-enter the record for Puffball.
UPDATE
statement:
mysql> UPDATE pet SET birth = "1989-08-31" WHERE name = "Bowser";
As shown above, it is easy to retrieve an entire table. But typically you don't want to do that, particularly when the table becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer.
You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this:
mysql> SELECT * FROM pet WHERE name = "Bowser"; +--------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+-------+---------+------+------------+------------+ | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+-------+---------+------+------------+------------+
The output confirms that the year is correctly recorded now as 1989, not 1998.
String comparisons are normally case insensitive, so you can specify the
name as "bowser"
, "BOWSER"
, etc. The query result will be
the same.
You can specify conditions on any column, not just name
. For example,
if you want to know which animals were born after 1998, test the birth
column:
mysql> SELECT * FROM pet WHERE birth >= "1998-1-1"; +----------+-------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+-------+ | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Puffball | Diane | hamster | f | 1999-03-30 | NULL | +----------+-------+---------+------+------------+-------+
You can combine conditions, for example, to locate female dogs:
mysql> SELECT * FROM pet WHERE species = "dog" AND sex = "f"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
The preceding query uses the AND
logical operator. There is also an
OR
operator:
mysql> SELECT * FROM pet WHERE species = "snake" OR species = "bird"; +----------+-------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+-------+ | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | | Slim | Benny | snake | m | 1996-04-29 | NULL | +----------+-------+---------+------+------------+-------+
AND
and OR
may be intermixed. If you do that, it's a good idea
to use parentheses to indicate how conditions should be grouped:
mysql> SELECT * FROM pet WHERE (species = "cat" AND sex = "m") -> OR (species = "dog" AND sex = "f"); +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
If you don't want to see entire rows from your table, just name the columns
in which you're interested, separated by commas. For example, if you want to
know when your animals were born, select the name
and birth
columns:
mysql> SELECT name, birth FROM pet; +----------+------------+ | name | birth | +----------+------------+ | Fluffy | 1993-02-04 | | Claws | 1994-03-17 | | Buffy | 1989-05-13 | | Fang | 1990-08-27 | | Bowser | 1989-08-31 | | Chirpy | 1998-09-11 | | Whistler | 1997-12-09 | | Slim | 1996-04-29 | | Puffball | 1999-03-30 | +----------+------------+
To find out who owns pets, use this query:
mysql> SELECT owner FROM pet; +--------+ | owner | +--------+ | Harold | | Gwen | | Harold | | Benny | | Diane | | Gwen | | Gwen | | Benny | | Diane | +--------+
However, notice that the query simply retrieves the owner
field from
each record, and some of them appear more than once. To minimize the output,
retrieve each unique output record just once by adding the keyword
DISTINCT
:
mysql> SELECT DISTINCT owner FROM pet; +--------+ | owner | +--------+ | Benny | | Diane | | Gwen | | Harold | +--------+
You can use a WHERE
clause to combine row selection with column
selection. For example, to get birth dates for dogs and cats only,
use this query:
mysql> SELECT name, species, birth FROM pet -> WHERE species = "dog" OR species = "cat"; +--------+---------+------------+ | name | species | birth | +--------+---------+------------+ | Fluffy | cat | 1993-02-04 | | Claws | cat | 1994-03-17 | | Buffy | dog | 1989-05-13 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | +--------+---------+------------+
You may have noticed in the preceding examples that the result rows are
displayed in no particular order. However, it's often easier to examine
query output when the rows are sorted in some meaningful way. To sort a
result, use an ORDER BY
clause.
Here are animal birthdays, sorted by date:
mysql> SELECT name, birth FROM pet ORDER BY birth; +----------+------------+ | name | birth | +----------+------------+ | Buffy | 1989-05-13 | | Bowser | 1989-08-31 | | Fang | 1990-08-27 | | Fluffy | 1993-02-04 | | Claws | 1994-03-17 | | Slim | 1996-04-29 | | Whistler | 1997-12-09 | | Chirpy | 1998-09-11 | | Puffball | 1999-03-30 | +----------+------------+
To sort in reverse order, add the DESC
(descending) keyword to the
name of the column you are sorting by:
mysql> SELECT name, birth FROM pet ORDER BY birth DESC; +----------+------------+ | name | birth | +----------+------------+ | Puffball | 1999-03-30 | | Chirpy | 1998-09-11 | | Whistler | 1997-12-09 | | Slim | 1996-04-29 | | Claws | 1994-03-17 | | Fluffy | 1993-02-04 | | Fang | 1990-08-27 | | Bowser | 1989-08-31 | | Buffy | 1989-05-13 | +----------+------------+
You can sort on multiple columns. For example, to sort by type of animal, then by birth date within animal type with youngest animals first, use the following query:
mysql> SELECT name, species, birth FROM pet ORDER BY species, birth DESC; +----------+---------+------------+ | name | species | birth | +----------+---------+------------+ | Chirpy | bird | 1998-09-11 | | Whistler | bird | 1997-12-09 | | Claws | cat | 1994-03-17 | | Fluffy | cat | 1993-02-04 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | | Buffy | dog | 1989-05-13 | | Puffball | hamster | 1999-03-30 | | Slim | snake | 1996-04-29 | +----------+---------+------------+
Note that the DESC
keyword applies only to the column name immediately
preceding it (birth
); species
values are still sorted in
ascending order.
MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates.
To determine how many years old each of your pets is, compute age as the difference between the birth date and the current date. Do this by converting the two dates to days, take the difference, and divide by 365 (the number of days in a year):
mysql> SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 FROM pet; +----------+-------------------------------------+ | name | (TO_DAYS(NOW())-TO_DAYS(birth))/365 | +----------+-------------------------------------+ | Fluffy | 6.15 | | Claws | 5.04 | | Buffy | 9.88 | | Fang | 8.59 | | Bowser | 9.58 | | Chirpy | 0.55 | | Whistler | 1.30 | | Slim | 2.92 | | Puffball | 0.00 | +----------+-------------------------------------+
Although the query works, there are some things about it that could be improved. First, the result could be scanned more easily if the rows were presented in some order. Second, the heading for the age column isn't very meaningful.
The first problem can be handled by adding an ORDER BY name
clause to
sort the output by name. To deal with the column heading, provide a name for
the column so that a different label appears in the output (this is called a
column alias):
mysql> SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 AS age -> FROM pet ORDER BY name; +----------+------+ | name | age | +----------+------+ | Bowser | 9.58 | | Buffy | 9.88 | | Chirpy | 0.55 | | Claws | 5.04 | | Fang | 8.59 | | Fluffy | 6.15 | | Puffball | 0.00 | | Slim | 2.92 | | Whistler | 1.30 | +----------+------+
To sort the output by age
rather than name
, just use a
different ORDER BY
clause:
mysql> SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 AS age -> FROM pet ORDER BY age; +----------+------+ | name | age | +----------+------+ | Puffball | 0.00 | | Chirpy | 0.55 | | Whistler | 1.30 | | Slim | 2.92 | | Claws | 5.04 | | Fluffy | 6.15 | | Fang | 8.59 | | Bowser | 9.58 | | Buffy | 9.88 | +----------+------+
A similar query can be used to determine age at death for animals that have
died. You determine which animals these are by checking whether or not the
death
value is NULL
. Then, for those with non-NULL
values, compute the difference between the death
and birth
values:
mysql> SELECT name, birth, death, (TO_DAYS(death)-TO_DAYS(birth))/365 AS age -> FROM pet WHERE death IS NOT NULL ORDER BY age; +--------+------------+------------+------+ | name | birth | death | age | +--------+------------+------------+------+ | Bowser | 1989-08-31 | 1995-07-29 | 5.91 | +--------+------------+------------+------+
The query uses death IS NOT NULL
rather than death != NULL
because NULL
is a special value. This is explained later.
See section 3.3.4.6 Working with NULL
Values.
What if you want to know which animals have birthdays next month? For this
type of calculation, year and day are irrelevant; you simply want to extract
the month part of the birth
column. MySQL provides several
date-part extraction functions, such as YEAR()
, MONTH()
, and
DAYOFMONTH()
. MONTH()
is the appropriate function here. To
see how it works, run a simple query that displays the value of both
birth
and MONTH(birth)
:
mysql> SELECT name, birth, MONTH(birth) FROM pet; +----------+------------+--------------+ | name | birth | MONTH(birth) | +----------+------------+--------------+ | Fluffy | 1993-02-04 | 2 | | Claws | 1994-03-17 | 3 | | Buffy | 1989-05-13 | 5 | | Fang | 1990-08-27 | 8 | | Bowser | 1989-08-31 | 8 | | Chirpy | 1998-09-11 | 9 | | Whistler | 1997-12-09 | 12 | | Slim | 1996-04-29 | 4 | | Puffball | 1999-03-30 | 3 | +----------+------------+--------------+
Finding animals with birthdays in the upcoming month is easy, too. Suppose
the current month is April. Then the month value is 4
and you look
for animals born in May (month 5) like this:
mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5; +-------+------------+ | name | birth | +-------+------------+ | Buffy | 1989-05-13 | +-------+------------+
There is a small complication if the current month is December, of course.
You don't just add one to the month number (12
) and look for animals
born in month 13, because there is no such month. Instead, you look for
animals born in January (month 1).
You can even write the query so that it works no matter what the current
month is. That way you don't have to use a particular month number
in the query. DATE_ADD()
allows you to add a time interval to a
given date. If you add a month to the value of NOW()
, then extract
the month part with MONTH()
, the result produces the month in which to
look for birthdays:
mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MONTH(DATE_ADD(NOW(), INTERVAL 1 MONTH));
A different way to accomplish the same task is to add 1
to get the
next month after the current one (after using the modulo function (MOD
)
to wrap around the month value to 0
if it is currently
12
):
mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MOD(MONTH(NOW()), 12) + 1;
Note that MONTH
returns a number between 1 and 12. And
MOD(something,12)
returns a number between 0 and 11. So the
addition has to be after the MOD()
otherwise we would go from
November (11) to January (1).
NULL
Values
The NULL
value can be surprising until you get used to it.
Conceptually, NULL
means missing value or unknown value and it
is treated somewhat differently than other values. To test for NULL
,
you cannot use the arithmetic comparison operators such as =
, <
,
or !=
. To demonstrate this for yourself, try the following query:
mysql> SELECT 1 = NULL, 1 != NULL, 1 < NULL, 1 > NULL; +----------+-----------+----------+----------+ | 1 = NULL | 1 != NULL | 1 < NULL | 1 > NULL | +----------+-----------+----------+----------+ | NULL | NULL | NULL | NULL | +----------+-----------+----------+----------+
Clearly you get no meaningful results from these comparisons. Use
the IS NULL
and IS NOT NULL
operators instead:
mysql> SELECT 1 IS NULL, 1 IS NOT NULL; +-----------+---------------+ | 1 IS NULL | 1 IS NOT NULL | +-----------+---------------+ | 0 | 1 | +-----------+---------------+
In MySQL, 0 or NULL
means false and anything else means true.
The default truth value from a boolean operation is 1.
This special treatment of NULL
is why, in the previous section, it
was necessary to determine which animals are no longer alive using
death IS NOT NULL
instead of death != NULL
.
MySQL provides standard SQL pattern matching as well as a form of
pattern matching based on extended regular expressions similar to those used
by Unix utilities such as vi
, grep
, and sed
.
SQL pattern matching allows you to use `_' to match any single
character and `%' to match an arbitrary number of characters (including
zero characters). In MySQL, SQL patterns are case insensitive by
default. Some examples are shown below. Note that you do not use =
or !=
when you use SQL patterns; use the LIKE
or NOT
LIKE
comparison operators instead.
To find names beginning with `b':
mysql> SELECT * FROM pet WHERE name LIKE "b%"; +--------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+------------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+--------+---------+------+------------+------------+
To find names ending with `fy':
mysql> SELECT * FROM pet WHERE name LIKE "%fy"; +--------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+-------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +--------+--------+---------+------+------------+-------+
To find names containing a `w':
mysql> SELECT * FROM pet WHERE name LIKE "%w%"; +----------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+------------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | +----------+-------+---------+------+------------+------------+
To find names containing exactly five characters, use the `_' pattern character:
mysql> SELECT * FROM pet WHERE name LIKE "_____"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
The other type of pattern matching provided by MySQL uses extended
regular expressions. When you test for a match for this type of pattern, use
the REGEXP
and NOT REGEXP
operators (or RLIKE
and
NOT RLIKE
, which are synonyms).
Some characteristics of extended regular expressions are:
To demonstrate how extended regular expressions work, the LIKE
queries
shown above are rewritten below to use REGEXP
.
To find names beginning with `b', use `^' to match the beginning of the name:
mysql> SELECT * FROM pet WHERE name REGEXP "^b"; +--------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+------------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+--------+---------+------+------------+------------+
Prior to MySQL Version 3.23.4, REGEXP
is case sensitive,
and the previous query will return no rows. To match either lowercase or
uppercase `b', use this query instead:
mysql> SELECT * FROM pet WHERE name REGEXP "^[bB]";
From MySQL 3.23.4 on, to force a REGEXP
comparison to
be case sensitive, use the BINARY
keyword to make one of the
strings a binary string. This query will match only lowercase `b'
at the beginning of a name:
mysql> SELECT * FROM pet WHERE name REGEXP BINARY "^b";
To find names ending with `fy', use `$' to match the end of the name:
mysql> SELECT * FROM pet WHERE name REGEXP "fy$"; +--------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+-------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +--------+--------+---------+------+------------+-------+
To find names containing a lowercase or uppercase `w', use this query:
mysql> SELECT * FROM pet WHERE name REGEXP "w"; +----------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+------------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | +----------+-------+---------+------+------------+------------+
Because a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wild card on either side of the pattern to get it to match the entire value like it would be if you used a SQL pattern.
To find names containing exactly five characters, use `^' and `$' to match the beginning and end of the name, and five instances of `.' in between:
mysql> SELECT * FROM pet WHERE name REGEXP "^.....$"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
You could also write the previous query using the `{n}'
``repeat-n
-times'' operator:
mysql> SELECT * FROM pet WHERE name REGEXP "^.{5}$"; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+
Databases are often used to answer the question, ``How often does a certain type of data occur in a table?'' For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of censuses on your animals.
Counting the total number of animals you have is the same question as ``How
many rows are in the pet
table?'' because there is one record per pet.
The COUNT()
function counts the number of non-NULL
results, so
the query to count your animals looks like this:
mysql> SELECT COUNT(*) FROM pet; +----------+ | COUNT(*) | +----------+ | 9 | +----------+
Earlier, you retrieved the names of the people who owned pets. You can
use COUNT()
if you want to find out how many pets each owner has:
mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner; +--------+----------+ | owner | COUNT(*) | +--------+----------+ | Benny | 2 | | Diane | 2 | | Gwen | 3 | | Harold | 2 | +--------+----------+
Note the use of GROUP BY
to group together all records for each
owner
. Without it, all you get is an error message:
mysql> SELECT owner, COUNT(owner) FROM pet; ERROR 1140 at line 1: Mixing of GROUP columns (MIN(),MAX(),COUNT()...) with no GROUP columns is illegal if there is no GROUP BY clause
COUNT()
and GROUP BY
are useful for characterizing your
data in various ways. The following examples show different ways to
perform animal census operations.
Number of animals per species:
mysql> SELECT species, COUNT(*) FROM pet GROUP BY species; +---------+----------+ | species | COUNT(*) | +---------+----------+ | bird | 2 | | cat | 2 | | dog | 3 | | hamster | 1 | | snake | 1 | +---------+----------+
Number of animals per sex:
mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex; +------+----------+ | sex | COUNT(*) | +------+----------+ | NULL | 1 | | f | 4 | | m | 4 | +------+----------+
(In this output, NULL
indicates sex unknown.)
Number of animals per combination of species and sex:
mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | bird | NULL | 1 | | bird | f | 1 | | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | | hamster | f | 1 | | snake | m | 1 | +---------+------+----------+
You need not retrieve an entire table when you use COUNT()
. For
example, the previous query, when performed just on dogs and cats, looks like
this:
mysql> SELECT species, sex, COUNT(*) FROM pet -> WHERE species = "dog" OR species = "cat" -> GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | +---------+------+----------+
Or, if you wanted the number of animals per sex only for known-sex animals:
mysql> SELECT species, sex, COUNT(*) FROM pet -> WHERE sex IS NOT NULL -> GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | bird | f | 1 | | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | | hamster | f | 1 | | snake | m | 1 | +---------+------+----------+
The pet
table keeps track of which pets you have. If you want to
record other information about them, such as events in their lives like
visits to the vet or when litters are born, you need another table. What
should this table look like? It needs:
Given these considerations, the CREATE TABLE
statement for the
event
table might look like this:
mysql> CREATE TABLE event (name VARCHAR(20), date DATE, -> type VARCHAR(15), remark VARCHAR(255));
As with the pet
table, it's easiest to load the initial records
by creating a tab-delimited text file containing the information:
Fluffy | 1995-05-15 | litter | 4 kittens, 3 female, 1 male |
Buffy | 1993-06-23 | litter | 5 puppies, 2 female, 3 male |
Buffy | 1994-06-19 | litter | 3 puppies, 3 female |
Chirpy | 1999-03-21 | vet | needed beak straightened |
Slim | 1997-08-03 | vet | broken rib |
Bowser | 1991-10-12 | kennel | |
Fang | 1991-10-12 | kennel | |
Fang | 1998-08-28 | birthday | Gave him a new chew toy |
Claws | 1998-03-17 | birthday | Gave him a new flea collar |
Whistler | 1998-12-09 | birthday | First birthday |
Load the records like this:
mysql> LOAD DATA LOCAL INFILE "event.txt" INTO TABLE event;
Based on what you've learned from the queries you've run on the pet
table, you should be able to perform retrievals on the records in the
event
table; the principles are the same. But when is the
event
table by itself insufficient to answer questions you might ask?
Suppose you want to find out the ages of each pet when they had their
litters. The event
table indicates when this occurred, but to
calculate the age of the mother, you need her birth date. Because that is
stored in the pet
table, you need both tables for the query:
mysql> SELECT pet.name, (TO_DAYS(date) - TO_DAYS(birth))/365 AS age, remark -> FROM pet, event -> WHERE pet.name = event.name AND type = "litter"; +--------+------+-----------------------------+ | name | age | remark | +--------+------+-----------------------------+ | Fluffy | 2.27 | 4 kittens, 3 female, 1 male | | Buffy | 4.12 | 5 puppies, 2 female, 3 male | | Buffy | 5.10 | 3 puppies, 3 female | +--------+------+-----------------------------+
There are several things to note about this query:
FROM
clause lists two tables because the query needs to pull
information from both of them.
name
column. The query uses
WHERE
clause to match up records in the two tables based on the
name
values.
name
column occurs in both tables, you must be specific
about which table you mean when referring to the column. This is done
by prepending the table name to the column name.
You need not have two different tables to perform a join. Sometimes it is
useful to join a table to itself, if you want to compare records in a table
to other records in that same table. For example, to find breeding pairs
among your pets, you can join the pet
table with itself to pair up
males and females of like species:
mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species -> FROM pet AS p1, pet AS p2 -> WHERE p1.species = p2.species AND p1.sex = "f" AND p2.sex = "m"; +--------+------+--------+------+---------+ | name | sex | name | sex | species | +--------+------+--------+------+---------+ | Fluffy | f | Claws | m | cat | | Buffy | f | Fang | m | dog | | Buffy | f | Bowser | m | dog | +--------+------+--------+------+---------+
In this query, we specify aliases for the table name in order to refer to the columns and keep straight which instance of the table each column reference is associated with.
What if you forget the name of a database or table, or what the structure of a given table is (for example, what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports.
You have already seen SHOW DATABASES
, which lists the databases
managed by the server. To find out which database is currently selected,
use the DATABASE()
function:
mysql> SELECT DATABASE(); +------------+ | DATABASE() | +------------+ | menagerie | +------------+
If you haven't selected any database yet, the result is blank.
To find out what tables the current database contains (for example, when you're not sure about the name of a table), use this command:
mysql> SHOW TABLES; +---------------------+ | Tables in menagerie | +---------------------+ | event | | pet | +---------------------+
If you want to find out about the structure of a table, the DESCRIBE
command is useful; it displays information about each of a table's columns:
mysql> DESCRIBE pet; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | owner | varchar(20) | YES | | NULL | | | species | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | birth | date | YES | | NULL | | | death | date | YES | | NULL | | +---------+-------------+------+-----+---------+-------+
Field
indicates the column name, Type
is the data type for
the column, Null
indicates whether or not the column can contain
NULL
values, Key
indicates whether or not the column is
indexed, and Default
specifies the column's default value.
If you have indexes on a table,
SHOW INDEX FROM tbl_name
produces information about them.
Here are examples of how to solve some common problems with MySQL.
Some of the examples use the table shop
to hold the price of each
article (item number) for certain traders (dealers). Supposing that each
trader has a single fixed price per article, then (item
,
trader
) is a primary key for the records.
Start the command line tool mysql
and select a database:
mysql your-database-name
(In most MySQL installations, you can use the database-name 'test').
You can create the example table as:
CREATE TABLE shop ( article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL, dealer CHAR(20) DEFAULT '' NOT NULL, price DOUBLE(16,2) DEFAULT '0.00' NOT NULL, PRIMARY KEY(article, dealer)); INSERT INTO shop VALUES (1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),(3,'C',1.69), (3,'D',1.25),(4,'D',19.95);
Okay, so the example data is:
mysql> SELECT * FROM shop; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0001 | A | 3.45 | | 0001 | B | 3.99 | | 0002 | A | 10.99 | | 0003 | B | 1.45 | | 0003 | C | 1.69 | | 0003 | D | 1.25 | | 0004 | D | 19.95 | +---------+--------+-------+
``What's the highest item number?''
SELECT MAX(article) AS article FROM shop +---------+ | article | +---------+ | 4 | +---------+
``Find number, dealer, and price of the most expensive article.''
In ANSI SQL this is easily done with a sub-query:
SELECT article, dealer, price FROM shop WHERE price=(SELECT MAX(price) FROM shop)
In MySQL (which does not yet have sub-selects), just do it in two steps:
SELECT
statement.
SELECT article, dealer, price FROM shop WHERE price=19.95
Another solution is to sort all rows descending by price and only
get the first row using the MySQL specific LIMIT
clause:
SELECT article, dealer, price FROM shop ORDER BY price DESC LIMIT 1
NOTE: If there are several most expensive articles (for example, each 19.95)
the LIMIT
solution shows only one of them!
``What's the highest price per article?''
SELECT article, MAX(price) AS price FROM shop GROUP BY article +---------+-------+ | article | price | +---------+-------+ | 0001 | 3.99 | | 0002 | 10.99 | | 0003 | 1.69 | | 0004 | 19.95 | +---------+-------+
``For each article, find the dealer(s) with the most expensive price.''
In ANSI SQL, I'd do it with a sub-query like this:
SELECT article, dealer, price FROM shop s1 WHERE price=(SELECT MAX(s2.price) FROM shop s2 WHERE s1.article = s2.article);
In MySQL it's best do it in several steps:
This can easily be done with a temporary table:
CREATE TEMPORARY TABLE tmp ( article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL, price DOUBLE(16,2) DEFAULT '0.00' NOT NULL); LOCK TABLES shop read; INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article; SELECT shop.article, dealer, shop.price FROM shop, tmp WHERE shop.article=tmp.article AND shop.price=tmp.price; UNLOCK TABLES; DROP TABLE tmp;
If you don't use a TEMPORARY
table, you must also lock the 'tmp' table.
``Can it be done with a single query?''
Yes, but only by using a quite inefficient trick that I call the ``MAX-CONCAT trick'':
SELECT article, SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer, 0.00+LEFT( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price FROM shop GROUP BY article; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0001 | B | 3.99 | | 0002 | A | 10.99 | | 0003 | C | 1.69 | | 0004 | D | 19.95 | +---------+--------+-------+
The last example can, of course, be made a bit more efficient by doing the splitting of the concatenated column in the client.
You can use MySQL user variables to remember results without having to store them in a temporary variables in the client. See section 6.1.4 User Variables.
For example, to find the articles with the highest and lowest price you can do:
select @min_price:=min(price),@max_price:=max(price) from shop; select * from shop where price=@min_price or price=@max_price; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0003 | D | 1.25 | | 0004 | D | 19.95 | +---------+--------+-------+
You don't need foreign keys to join 2 tables.
The only thing MySQL doesn't do is CHECK
to make sure that
the keys you use really exist in the table(s) you're referencing and it
doesn't automatically delete rows from table with a foreign key
definition. If you use your keys like normal, it'll work just fine:
CREATE TABLE persons ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, name CHAR(60) NOT NULL, PRIMARY KEY (id) ); CREATE TABLE shirts ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, style ENUM('t-shirt', 'polo', 'dress') NOT NULL, color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL, owner SMALLINT UNSIGNED NOT NULL REFERENCES persons, PRIMARY KEY (id) ); INSERT INTO persons VALUES (NULL, 'Antonio Paz'); INSERT INTO shirts VALUES (NULL, 'polo', 'blue', LAST_INSERT_ID()), (NULL, 'dress', 'white', LAST_INSERT_ID()), (NULL, 't-shirt', 'blue', LAST_INSERT_ID()); INSERT INTO persons VALUES (NULL, 'Lilliana Angelovska'); INSERT INTO shirts VALUES (NULL, 'dress', 'orange', LAST_INSERT_ID()), (NULL, 'polo', 'red', LAST_INSERT_ID()), (NULL, 'dress', 'blue', LAST_INSERT_ID()), (NULL, 't-shirt', 'white', LAST_INSERT_ID()); SELECT * FROM persons; +----+---------------------+ | id | name | +----+---------------------+ | 1 | Antonio Paz | | 2 | Lilliana Angelovska | +----+---------------------+ SELECT * FROM shirts; +----+---------+--------+-------+ | id | style | color | owner | +----+---------+--------+-------+ | 1 | polo | blue | 1 | | 2 | dress | white | 1 | | 3 | t-shirt | blue | 1 | | 4 | dress | orange | 2 | | 5 | polo | red | 2 | | 6 | dress | blue | 2 | | 7 | t-shirt | white | 2 | +----+---------+--------+-------+ SELECT s.* FROM persons p, shirts s WHERE p.name LIKE 'Lilliana%' AND s.owner = p.id AND s.color <> 'white'; +----+-------+--------+-------+ | id | style | color | owner | +----+-------+--------+-------+ | 4 | dress | orange | 2 | | 5 | polo | red | 2 | | 6 | dress | blue | 2 | +----+-------+--------+-------+
MySQL doesn't yet optimize when you search on two different
keys combined with OR
(Searching on one key with different OR
parts is optimized quite good):
SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1' OR field2_index = '1'
The reason is that we haven't yet had time to come up with an efficient
way to handle this in the general case. (The AND
handling is,
in comparison, now completely general and works very well).
For the moment you can solve this very efficiently by using a
TEMPORARY
table. This type of optimization is also very good if
you are using very complicated queries where the SQL server does the
optimizations in the wrong order.
CREATE TEMPORARY TABLE tmp SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1'; INSERT INTO tmp SELECT field1_index, field2_index FROM test_table WHERE field2_index = '1'; SELECT * from tmp; DROP TABLE tmp;
The above way to solve this query is in effect an UNION
of two queries.
The following shows an idea of how you can use the bit group functions to calculate the number of days per month a user has visited a web page.
CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL, day INT(2) UNSIGNED ZEROFILL); INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),(2000,2,23),(2000,2,23); SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1 GROUP BY year,month; Which returns: +------+-------+------+ | year | month | days | +------+-------+------+ | 2000 | 01 | 3 | | 2000 | 02 | 2 | +------+-------+------+
The above calculates how many different days was used for a given year/month combination, with automatic removal of duplicate entries.
mysql
in Batch Mode
In the previous sections, you used mysql
interactively to enter
queries and view the results. You can also run mysql
in batch
mode. To do this, put the commands you want to run in a file, then
tell mysql
to read its input from the file:
shell> mysql < batch-file
If you need to specify connection parameters on the command line, the command might look like this:
shell> mysql -h host -u user -p < batch-file Enter password: ********
When you use mysql
this way, you are creating a script file, then
executing the script.
Why use a script? Here are a few reasons:
mysql
to execute it again.
shell> mysql < batch-file | more
shell> mysql < batch-file > mysql.out
cron
job. In this case, you must use batch mode.
The default output format is different (more concise) when you run
mysql
in batch mode than when you use it interactively. For
example, the output of SELECT DISTINCT species FROM pet
looks like
this when run interactively:
+---------+ | species | +---------+ | bird | | cat | | dog | | hamster | | snake | +---------+
But like this when run in batch mode:
species bird cat dog hamster snake
If you want to get the interactive output format in batch mode, use
mysql -t
. To echo to the output the commands that are executed, use
mysql -vvv
.
At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California.
The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors.
More information about Twin studies can be found at:
http://www.imm.ki.se/TWIN/TWINUKW.HTM
The latter part of the project is administered with a Web interface written using Perl and MySQL.
Each night all data from the interviews are moved into a MySQL database.
The following query is used to determine who goes into the second part of the project:
select concat(p1.id, p1.tvab) + 0 as tvid, concat(p1.christian_name, " ", p1.surname) as Name, p1.postal_code as Code, p1.city as City, pg.abrev as Area, if(td.participation = "Aborted", "A", " ") as A, p1.dead as dead1, l.event as event1, td.suspect as tsuspect1, id.suspect as isuspect1, td.severe as tsevere1, id.severe as isevere1, p2.dead as dead2, l2.event as event2, h2.nurse as nurse2, h2.doctor as doctor2, td2.suspect as tsuspect2, id2.suspect as isuspect2, td2.severe as tsevere2, id2.severe as isevere2, l.finish_date from twin_project as tp /* For Twin 1 */ left join twin_data as td on tp.id = td.id and tp.tvab = td.tvab left join informant_data as id on tp.id = id.id and tp.tvab = id.tvab left join harmony as h on tp.id = h.id and tp.tvab = h.tvab left join lentus as l on tp.id = l.id and tp.tvab = l.tvab /* For Twin 2 */ left join twin_data as td2 on p2.id = td2.id and p2.tvab = td2.tvab left join informant_data as id2 on p2.id = id2.id and p2.tvab = id2.tvab left join harmony as h2 on p2.id = h2.id and p2.tvab = h2.tvab left join lentus as l2 on p2.id = l2.id and p2.tvab = l2.tvab, person_data as p1, person_data as p2, postal_groups as pg where /* p1 gets main twin and p2 gets his/her twin. */ /* ptvab is a field inverted from tvab */ p1.id = tp.id and p1.tvab = tp.tvab and p2.id = p1.id and p2.ptvab = p1.tvab and /* Just the sceening survey */ tp.survey_no = 5 and /* Skip if partner died before 65 but allow emigration (dead=9) */ (p2.dead = 0 or p2.dead = 9 or (p2.dead = 1 and (p2.death_date = 0 or (((to_days(p2.death_date) - to_days(p2.birthday)) / 365) >= 65)))) and ( /* Twin is suspect */ (td.future_contact = 'Yes' and td.suspect = 2) or /* Twin is suspect - Informant is Blessed */ (td.future_contact = 'Yes' and td.suspect = 1 and id.suspect = 1) or /* No twin - Informant is Blessed */ (ISNULL(td.suspect) and id.suspect = 1 and id.future_contact = 'Yes') or /* Twin broken off - Informant is Blessed */ (td.participation = 'Aborted' and id.suspect = 1 and id.future_contact = 'Yes') or /* Twin broken off - No inform - Have partner */ (td.participation = 'Aborted' and ISNULL(id.suspect) and p2.dead = 0)) and l.event = 'Finished' /* Get at area code */ and substring(p1.postal_code, 1, 2) = pg.code /* Not already distributed */ and (h.nurse is NULL or h.nurse=00 or h.doctor=00) /* Has not refused or been aborted */ and not (h.status = 'Refused' or h.status = 'Aborted' or h.status = 'Died' or h.status = 'Other') order by tvid;
Some explanations:
concat(p1.id, p1.tvab) + 0 as tvid
id
and tvab
in
numerical order. Adding 0
to the result causes MySQL to
treat the result as a number.
id
tvab
1
or 2
.
ptvab
tvab
. When tvab
is 1
this is
2
, and vice versa. It exists to save typing and to make it easier for
MySQL to optimize the query.
This query demonstrates, among other things, how to do lookups on a
table from the same table with a join (p1
and p2
). In the example, this
is used to check whether a twin's partner died before the age of 65. If so,
the row is not returned.
All of the above exist in all tables with twin-related information. We
have a key on both id,tvab
(all tables), and id,ptvab
(person_data
) to make queries faster.
On our production machine (A 200MHz UltraSPARC), this query returns about 150-200 rows and takes less than one second.
The current number of records in the tables used above:
Table | Rows |
person_data | 71074 |
lentus | 5291 |
twin_project | 5286 |
twin_data | 2012 |
informant_data | 663 |
harmony | 381 |
postal_groups | 100 |
Each interview ends with a status code called event
. The query
shown below is used to display a table over all twin pairs combined by
event. This indicates in how many pairs both twins are finished, in how many
pairs one twin is finished and the other refused, and so on.
select t1.event, t2.event, count(*) from lentus as t1, lentus as t2, twin_project as tp where /* We are looking at one pair at a time */ t1.id = tp.id and t1.tvab=tp.tvab and t1.id = t2.id /* Just the sceening survey */ and tp.survey_no = 5 /* This makes each pair only appear once */ and t1.tvab='1' and t2.tvab='2' group by t1.event, t2.event;
The Contrib section includes programs that let you authenticate your users from a MySQL database and also let you log your log files into a MySQL table. See section E Contributed Programs.
You can change the Apache logging format to be easily readable by MySQL by putting the following into the Apache configuration file:
LogFormat \ "\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\", \ \"%U\",\"%{Referer}i\",\"%{User-Agent}i\""
In MySQL you can do something like this:
LOAD DATA INFILE '/local/access_log' INTO TABLE table_name FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\'
mysqld
accepts the following command-line options:
--ansi
-b, --basedir=path
--big-tables
--bind-address=IP
--character-sets-dir=path
--chroot=path
mysqld
daemon during startup. Recommended security measure. It will
somewhat limit LOAD DATA INFILE
and SELECT ... INTO OUTFILE
though.
--core-file
mysqld
dies. For some systems you must also
specify --core-file-size
to safe_mysqld
. See section 4.7.2 safe_mysqld, the wrapper around mysqld.
-h, --datadir=path
--default-character-set=charset
--default-table-type=type
--debug[...]=
--with-debug
, you can use this
option to get a trace file of what mysqld
is doing.
See section H.1.2 Creating trace files.
--delay-key-write-for-all-tables
MyISAM
table.
See section 5.5.2 Tuning Server Parameters.
--enable-locking
-T, --exit-info
--flush
-?, --help
--init-file=file
-L, --language=...
-l, --log[=file]
--log-isam[=file]
--log-slow-queries[=file]
long_query_time
seconds to
execute to file. See section 4.9.5 The Slow Query Log.
--log-update[=file]
file.#
where #
is a unique number if not given.
See section 4.9.3 The Update Log.
--log-long-format
--log-slow-queries
then queries that are not using indexes are logged
to the slow query log.
--low-priority-updates
INSERT
/DELETE
/UPDATE
)
will have lower priority than selects. It can also be done via
{INSERT | REPLACE | UPDATE | DELETE} LOW_PRIORITY ...
to lower
the priority of only one query, or by
SET OPTION SQL_LOW_PRIORITY_UPDATES=1
to change the priority in one
thread. See section 5.3.2 Table Locking Issues.
--memlock
mysqld
process in memory. This works only if your
system supports the mlockall()
system call (like Solaris). This
may help if you have a problem where the operating system is causing
mysqld
to swap on disk.
--myisam-recover [=option[,option...]]] where option is any combination
DEFAULT
, BACKUP
, FORCE
or QUICK
. You can
also set this explicitely to ""
if you want to disable this
option. If this option is used, mysqld
will on open check if the
table is marked as crashed or if if the table wasn't closed properly.
(The last option only works if you are running with
--skip-locking
). If this is the case mysqld
will run
check on the table. If the table was corrupted, mysqld
will
attempt to repair it.
The following options affects how the repair works.
DEFAULT | The same as not giving any option to
--myisam-recover .
|
BACKUP | If the data table was changed during recover, save a backup of the `table_name.MYD' data file as `table_name-datetime.BAK'. |
FORCE | Run recover even if we will loose more than one row from the .MYD file. |
QUICK | Don't check the rows in the table if there isn't any delete blocks. |
BACKUP,FORCE
. This will force a repair of a table even if some rows
would be deleted, but it will keep the old data file as a backup so that
you can later examine what happened.
--pid-file=path
safe_mysqld
.
-P, --port=...
-o, --old-protocol
--one-thread
-O, --set-variable var=option
--help
lists variables. You can find a full
description for all variables in the SHOW VARIABLES
section in this
manual. See section 4.5.5.4 SHOW VARIABLES
. The tuning server parameters section includes
information of how to optimize these. See section 5.5.2 Tuning Server Parameters.
--safe-mode
--skip-delay-key-write
.
--safe-show-database
--safe-user-create
INSERT
privilege to the
mysql.user
table or any column in this table.
--skip-concurrent-insert
MyISAM
tables. (This is only to be used if you think you have found a bug in this
feature).
--skip-delay-key-write
delay_key_write
option for all tables.
See section 5.5.2 Tuning Server Parameters.
--skip-grant-tables
mysqladmin
flush-privileges
or mysqladmin reload
.)
--skip-host-cache
--skip-locking
isamchk
or myisamchk
you must
shut down the server. See section 1.1.7 How Stable Is MySQL?. Note that in MySQL Version
3.23 you can use REPAIR
and CHECK
to repair/check MyISAM
tables.
--skip-name-resolve
Host
column values in the grant tables
must be IP numbers or localhost
. See section 5.5.5 How MySQL uses DNS.
--skip-networking
mysqld
must be made via Unix sockets. This option is highly
recommended for systems where only local requests are allowed. See section 5.5.5 How MySQL uses DNS.
--skip-new
--skip-delay-key-write
.
This will also set default table type to ISAM
. See section 7.3 ISAM Tables.
--skip-symlink
--skip-safemalloc
--with-debug=full
, all programs
will check the memory for overruns for every memory allocation and memory
freeing. As this checking is very slow, you can avoid this, when you don't
need memory checking, by using this option.
--skip-show-database
--skip-stack-trace
mysqld
under a debugger. See section H.1 Debugging a MySQL server.
--skip-thread-priority
--socket=path
/tmp/mysql.sock
.
--sql-mode=option[,option[,option...]]
REAL_AS_FLOAT
,
PIPES_AS_CONCAT
, ANSI_QUOTES
, IGNORE_SPACE
,
SERIALIZE
, ONLY_FULL_GROUP_BY
. It can also be empty
(""
) if you want to reset this.
By specifying all of the above options is same as using --ansi.
With this option one can turn on only needed SQL modes. See section 1.4.3 Running MySQL in ANSI Mode.
transaction-isolation= { READ-UNCOMMITTED | READ-COMMITTED | REPEATABLE-READ | SERIALIZABLE }
SET TRANSACTION
Syntax.
-t, --tmpdir=path
/tmp
directory resides on a partition too small to hold temporary tables.
-u, --user=user_name
mysqld
daemon as user user_name
. This option is
mandatory when starting mysqld
as root.
-V, --version
-W, --warnings
Aborted connection...
to the .err
file.
See section A.4.9 Communication Errors / Aborted Connection.
MySQL can, since Version 3.22, read default startup options for the server and for clients from option files.
MySQL reads default options from the following files on Unix:
Filename | Purpose |
/etc/my.cnf | Global options |
DATADIR/my.cnf | Server-specific options |
defaults-extra-file | The file specified with --defaults-extra-file=# |
~/.my.cnf | User-specific options |
DATADIR
is the MySQL data directory (typically
`/usr/local/mysql/data' for a binary installation or
`/usr/local/var' for a source installation). Note that this is the
directory that was specified at configuration time, not the one specified
with --datadir
when mysqld
starts up! (--datadir
has no
effect on where the server looks for option files, because it looks for them
before it processes any command-line arguments.)
MySQL reads default options from the following files on Windows:
Filename | Purpose |
windows-system-directory\my.ini | Global options |
C:\my.cnf | Global options |
C:\mysql\data\my.cnf | Server-specific options |
Note that on Windows, you should specify all paths with /
instead of
\
. If you use \
, you need to specify this twice, as
\
is the escape character in MySQL.
MySQL tries to read option files in the order listed above. If multiple option files exist, an option specified in a file read later takes precedence over the same option specified in a file read earlier. Options specified on the command line take precedence over options specified in any option file. Some options can be specified using environment variables. Options specified on the command line or in option files take precedence over environment variable values. See section B Environment Variables.
The following programs support option files: mysql
,
mysqladmin
, mysqld
, mysqldump
, mysqlimport
,
mysql.server
, myisamchk
, and myisampack
.
You can use option files to specify any long option that a program supports!
Run the program with --help
to get a list of available options.
An option file can contain lines of the following forms:
#comment
[group]
group
is the name of the program or group for which you want to set
options. After a group line, any option
or set-variable
lines
apply to the named group until the end of the option file or another group
line is given.
option
--option
on the command line.
option=value
--option=value
on the command line.
set-variable = variable=value
--set-variable variable=value
on the command line.
This syntax must be used to set a mysqld
variable.
The client
group allows you to specify options that apply to all
MySQL clients (not mysqld
). This is the perfect group to use
to specify the password you use to connect to the server. (But make
sure the option file is readable and writable only by yourself.)
Note that for options and values, all leading and trailing blanks are automatically deleted. You may use the escape sequences `\b', `\t', `\n', `\r', `\\', and `\s' in your value string (`\s' == blank).
Here is a typical global option file:
[client] port=3306 socket=/tmp/mysql.sock [mysqld] port=3306 socket=/tmp/mysql.sock set-variable = key_buffer_size=16M set-variable = max_allowed_packet=1M [mysqldump] quick
Here is typical user option file:
[client] # The following password will be sent to all standard MySQL clients password=my_password [mysql] no-auto-rehash set-variable = connect_timeout=2 [mysqlhotcopy] interactive-timeout
If you have a source distribution, you will find sample configuration
files named `my-xxxx.cnf' in the `support-files' directory.
If you have a binary distribution, look in the `DIR/support-files'
directory, where DIR
is the pathname to the MySQL
installation directory (typically `/usr/local/mysql'). Currently
there are sample configuration files for small, medium, large, and very
large systems. You can copy `my-xxxx.cnf' to your home directory
(rename the copy to `.my.cnf') to experiment with this.
All MySQL clients that support option files support the following options:
--no-defaults | Don't read any option files. |
--print-defaults | Print the program name and all options that it will get. |
--defaults-file=full-path-to-default-file | Only use the given configuration file. |
--defaults-extra-file=full-path-to-default-file | Read this configuration file after the global configuration file but before the user configuration file. |
Note that the above options must be first on the command line to work!
--print-defaults
may however be used directly after the
--defaults-xxx-file
commands.
Note for developers: Option file handling is implemented simply by processing all matching options (that is, options in the appropriate group) before any command-line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have an old program that handles multiply-specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this.
In shell scripts you can use the `my_print_defaults' command to parse the config files:
shell> my_print_defaults client mysql --port=3306 --socket=/tmp/mysql.sock --no-auto-rehash
The above output contains all options for the groups 'client' and 'mysql'.
In some cases you may want to have many different mysqld
daemons
(servers) running on the same machine. You may for example want to run
a new version of MySQL for testing together with an old version
that is in production. Another case is when you want to give different
users access to different mysqld
servers that they manage themselves.
One way to get a new server running is by starting it with a different socket and port as follows:
shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock shell> MYSQL_TCP_PORT=3307 shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT shell> scripts/mysql_install_db shell> bin/safe_mysqld &
The environment variables appendix includes a list of other environment
variables you can use to affect mysqld
. See section B Environment Variables.
The above is the quick and dirty way that one commonly uses for testing. The nice thing with this is that all connections you do in the above shell will automatically be directed to the new running server!
If you need to do this more permanently, you should create an option file for each server. See section 4.1.2 my.cnf Option Files. In your startup script that is executed at boot time (mysql.server?) you should specify for both servers:
safe_mysqld --default-file=path-to-option-file
At least the following options should be different per server:
port=#
socket=path
pid-file=path
The following options should be different, if they are used:
log=path
log-bin=path
log-update=path
log-isam=path
bdb-logdir=path
If you want more performance, you can also specify the following differently:
tmpdir=path
bdb-tmpdir=path
See section 4.1.1 mysqld Command-line Options.
If you are installing binary MySQL versions (.tar files) and
start them with ./bin/safe_mysqld
then in most cases the only
option you need to add/change is the socket
and port
argument to safe_mysqld
.
There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers.
If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file. See section 4.7.3 mysqld_multi, program for managing multiple MySQL servers.
Assume an existing server is configured for the default port number and
socket file. Then configure the new server with a configure
command
something like this:
shell> ./configure --with-tcp-port=port_number \ --with-unix-socket-path=file_name \ --prefix=/usr/local/mysql-3.22.9
Here port_number
and file_name
should be different than the
default port number and socket file pathname, and the --prefix
value
should specify an installation directory different than the one under which
the existing MySQL installation is located.
You can check the socket used by any currently executing MySQL server with this command:
shell> mysqladmin -h hostname --port=port_number variables
Note that if you specify ``localhost
'' as a hostname, mysqladmin
will default to using Unix sockets instead of TCP/IP.
If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.
You don't have to recompile a new MySQL server just to start with
a different port and socket. You can change the port and socket to be used
by specifying them at run time as options to safe_mysqld
:
shell> /path/to/safe_mysqld --socket=file_name --port=port_number
mysqld_multi
can also take safe_mysqld
(or mysqld
)
as an argument and pass the options from a configuration file to
safe_mysqld
and further to mysqld
.
If you run the new server on the same database directory as another
server with logging enabled, you should also specify the name of the log
files to safe_mysqld
with --log
, --log-update
, or
--log-slow-queries
. Otherwise, both servers may be trying to
write to the same log file.
WARNING: Normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!
If you want to use another database directory for the second server, you
can use the --datadir=path
option to safe_mysqld
.
NOTE also that starting several MySQL servers
(mysqlds
) in different machines and letting them access one data
directory over NFS
is generally a BAD IDEA! The problem
is that the NFS
will become the bottleneck with the speed. It is
not meant for such use. And last but not least, you would still have to
come up with a solution how to make sure that two or more mysqlds
are not interfering with each other. At the moment there is no platform
that would 100% reliable do the file locking (lockd
daemon
usually) in every situation. Yet there would be one more possible risk
with NFS
; it would make the work even more complicated for
lockd
daemon to handle. So make it easy for your self and forget
about the idea. The working solution is to have one computer with an
operating system that efficiently handles threads and have several CPUs
in it.
When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:
--host 'hostname' --port=port_number
to connect
with TCP/IP, or [--host localhost] --socket=file_name
to connect via
a Unix socket.
DBD::mysql
module you can read the options
from the MySQL option files. See section 4.1.2 my.cnf Option Files.
$dsn = "DBI:mysql:test;mysql_read_default_group=client;mysql_read_default_file=/usr/local/mysql/data/my.cnf" $dbh = DBI->connect($dsn, $user, $password);
MYSQL_UNIX_PORT
and MYSQL_TCP_PORT
environment variables
to point to the Unix socket and TCP/IP port before you start your clients.
If you normally use a specific socket or port, you should place commands
to set these environment variables in your `.login' file.
See section B Environment Variables.
MySQL has an advanced but non-standard security/privilege system. This section describes how it works.
Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.
In discussing security, we emphasize the necessity of fully protecting the entire server host (not simply the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.
MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that a user may attempt to perform. There is also some support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.
When running MySQL, follow these guidelines whenever possible:
user
TABLE IN THE mysql
DATABASE! The encrypted password
is the real password in MySQL. If you know the password listed in
the user
table for a given user, you can easily log in as that
user if you have access to the host listed for that account.
GRANT
and
REVOKE
commands are used for controlling access to MySQL. Do
not grant any more privileges than necessary. Never grant privileges to all
hosts.
Checklist:
mysql -u root
. If you are able to connect successfully to the
server without being asked for a password, you have problems. Anyone
can connect to your MySQL server as the MySQL
root
user with full privileges!
Review the MySQL installation instructions, paying particular
attention to the item about setting a root
password.
SHOW GRANTS
and check to see who has access to
what. Remove those privileges that are not necessary using the REVOKE
command.
MD5()
or another one-way
hashing function.
nmap
. MySQL uses port 3306 by default. This port should
be inaccessible from untrusted hosts. Another simple way to check whether
or not your MySQL port is open is to try the following command
from some remote machine, where server_host
is the hostname of
your MySQL server:
shell> telnet server_host 3306If you get a connection and some garbage characters, the port is open, and should be closed on your firewall or router, unless you really have a good reason to keep it open. If
telnet
just hangs or the
connection is refused, everything is OK; the port is blocked.
; DROP
DATABASE mysql;
''. This is an extreme example, but large security leaks
and data loss may occur as a result of hackers using similar techniques,
if you do not prepare for them.
Also remember to check numeric data. A common mistake is to protect only
strings. Sometimes people think that if a database contains only publicly
available data that it need not be protected. This is incorrect. At least
denial-of-service type attacks can be performed on such
databases. The simplest way to protect from this type of attack is to use
apostrophes around the numeric constants: SELECT * FROM table
WHERE ID='234'
rather than SELECT * FROM table WHERE ID=234
.
MySQL automatically converts this string to a number and
strips all non-numeric symbols from it.
Checklist:
%22
(`"'), %23
(`#'), and %27
(`'') in the URL.
addslashes()
function.
As of PHP 4.0.3, a mysql_escape_string()
function is available
that is based on the function of the same name in the MySQL C API.
mysql_escape_string()
API call.
escape
and quote
modifiers for query streams.
quote()
method or use placeholders.
PreparedStatement
object and placeholders.
tcpdump
and strings
utilities. For most cases,
you can check whether or not MySQL data streams are unencrypted
by issuing a command like the following:
shell> tcpdump -l -i eth0 -w - src or dst port 3306 | strings(This works under Linux and should work with small modifications under other systems). Warning: If you do not see data this doesn't always actually mean that it is encrypted. If you need high security, you should consult with a security expert.
When you connect to a MySQL server, you normally should use a password. The password is not transmitted in clear text over the connection, however the encryption algorithm is not very strong, and with some effort a clever attacker can crack the password if he is able to sniff the traffic between the client and the server. If the connection between the client and the server goes through an untrusted network, you should use an SSH tunnel to encrypt the communication.
All other information is transferred as text that can be read by anyone
who is able to watch the connection. If you are concerned about this,
you can use the compressed protocol (in MySQL Version 3.22 and above)
to make things much harder. To make things even more secure you should use
ssh
. You can find an open source ssh
client at
http://www.openssh.org, and a commercial ssh
client at
http://www.ssh.com. With this, you can get an encrypted TCP/IP
connection between a MySQL server and a MySQL client.
To make a MySQL system secure, you should strongly consider the following suggestions:
mysql -u other_user db_name
if
other_user
has no password. It is common behavior with client/server
applications that the client may specify any user name. You can change the
password of all users by editing the mysql_install_db
script before
you run it, or only the password for the MySQL root
user like
this:
shell> mysql -u root mysql mysql> UPDATE user SET Password=PASSWORD('new_password') WHERE user='root'; mysql> FLUSH PRIVILEGES;
root
user. This is
very dangerous, because any user with FILE
privileges will be able
to create files as root
(for example, ~root/.bashrc
). To
prevent this, mysqld
will refuse to run as root
unless it
is specified directly using a --user=root
option.
mysqld
can be run as an ordinary unprivileged user instead.
You can also create a new Unix user mysql
to make everything
even more secure. If you run mysqld
as another Unix user,
you don't need to change the root
user name in the user
table, because MySQL user names have nothing to do with Unix
user names. To start mysqld
as another Unix user, add a user
line that specifies the user name to the [mysqld]
group of the
`/etc/my.cnf' option file or the `my.cnf' option file in the
server's data directory. For example:
[mysqld] user=mysqlThis will cause the server to start as the designated user whether you start it manually or by using
safe_mysqld
or mysql.server
.
For more details, see section A.8 How to Run MySQL As a Normal User.
--skip-symlink
option. This is especially important if you run
mysqld
as root as anyone that has write access to the mysqld data
directories could then delete any file in the system!
See section 5.6.1.2 Using Symbolic Links for Tables.
mysqld
runs as is the only user with
read/write privileges in the database directories.
mysqladmin processlist
shows the text of the currently executing
queries, so any user who is allowed to execute that command might be able to
see if another user issues an UPDATE user SET
password=PASSWORD('not_secure')
query.
mysqld
reserves an extra connection for users who have the
process privilege, so that a MySQL root
user can log
in and check things even if all normal connections are in use.
mysqld
daemon! To make this a bit safer, all files generated with
SELECT ... INTO OUTFILE
are readable to everyone, and you cannot
overwrite existing files.
The file privilege may also be used to read any file accessible
to the Unix user that the server runs as. This could be abused, for example,
by using LOAD DATA
to load `/etc/passwd' into a table, which
can then be read with SELECT
.
max_user_connections
variable in
mysqld
.
mysqld
Concerning Security
The following mysqld
options affect security:
--safe-show-database
SHOW DATABASES
returns only those databases for which the user has
some kind of privilege.
--safe-user-create
GRANT
command, if the user doesn't have INSERT
privilege to the
mysql.user
table. If you want to give a user access to just create
new users with those privileges that the user has right to grant, you should
give the user the following privilege:
GRANT INSERT(user) on mysql.user to 'user''hostname';This will ensure that the user can't change any privilege columns directly, but has to use the
GRANT
command to give privileges to other users.
--skip-grant-tables
mysqladmin
flush-privileges
or mysqladmin reload
.)
--skip-name-resolve
Host
column values in the grant
tables must be IP numbers or localhost
.
--skip-networking
mysqld
must be made via Unix sockets. This option is unsuitable for
systems that use MIT-pthreads, because the MIT-pthreads package doesn't
support Unix sockets.
--skip-show-database
SHOW DATABASES
statement doesn't return anything.
The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as select, insert, update and delete.
Additional functionality includes the ability to have an anonymous user and
to grant privileges for MySQL-specific functions such as LOAD
DATA INFILE
and administrative operations.
The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. The system grants privileges according to your identity and what you want to do.
MySQL considers both your hostname and user name in identifying you
because there is little reason to assume that a given user name belongs to
the same person everywhere on the Internet. For example, the user
bill
who connects from whitehouse.gov
need not be the same
person as the user bill
who connects from microsoft.com
.
MySQL handles this by allowing you to distinguish users on different
hosts that happen to have the same name: you can grant bill
one set
of privileges for connections from whitehouse.gov
, and a different set
of privileges for connections from microsoft.com
.
MySQL access control involves two stages:
The server uses the user
, db
, and host
tables in the
mysql
database at both stages of access control. The fields in these
grant tables are shown below:
Table name | user | db | host
|
Scope fields | Host | Host | Host
|
User | Db | Db
| |
Password | User | ||
Privilege fields | Select_priv | Select_priv | Select_priv
|
Insert_priv | Insert_priv | Insert_priv
| |
Update_priv | Update_priv | Update_priv
| |
Delete_priv | Delete_priv | Delete_priv
| |
Index_priv | Index_priv | Index_priv
| |
Alter_priv | Alter_priv | Alter_priv
| |
Create_priv | Create_priv | Create_priv
| |
Drop_priv | Drop_priv | Drop_priv
| |
Grant_priv | Grant_priv | Grant_priv
| |
References_priv | |||
Reload_priv | |||
Shutdown_priv | |||
Process_priv | |||
File_priv |
For the second stage of access control (request verification), the server
may, if the request involves tables, additionally consult the
tables_priv
and columns_priv
tables. The fields in these
tables are shown below:
Table name | tables_priv | columns_priv
|
Scope fields | Host | Host
|
Db | Db
| |
User | User
| |
Table_name | Table_name
| |
Column_name
| ||
Privilege fields | Table_priv | Column_priv
|
Column_priv | ||
Other fields | Timestamp | Timestamp
|
Grantor |
Each grant table contains scope fields and privilege fields.
Scope fields determine the scope of each entry in the tables, that is, the
context in which the entry applies. For example, a user
table entry
with Host
and User
values of 'thomas.loc.gov'
and
'bob'
would be used for authenticating connections made to the server
by bob
from the host thomas.loc.gov
. Similarly, a db
table entry with Host
, User
, and Db
fields of
'thomas.loc.gov'
, 'bob'
and 'reports'
would be used when
bob
connects from the host thomas.loc.gov
to access the
reports
database. The tables_priv
and columns_priv
tables contain scope fields indicating tables or table/column combinations
to which each entry applies.
For access-checking purposes, comparisons of Host
values are
case insensitive. User
, Password
, Db
, and
Table_name
values are case sensitive.
Column_name
values are case insensitive in MySQL Version
3.22.12 or later.
Privilege fields indicate the privileges granted by a table entry, that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. The rules used to do this are described in section 4.2.9 Access Control, Stage 2: Request Verification.
Scope fields are strings, declared as shown below; the default value for each is the empty string:
Field name | Type | |
Host | CHAR(60)
| |
User | CHAR(16)
| |
Password | CHAR(16)
| |
Db | CHAR(64) | (CHAR(60) for the
tables_priv and columns_priv tables)
|
Table_name | CHAR(60)
| |
Column_name | CHAR(60)
|
In the user
, db
and host
tables,
all privilege fields are declared as ENUM('N','Y')
-- each can have a
value of 'N'
or 'Y'
, and the default value is 'N'
.
In the tables_priv
and columns_priv
tables, the privilege
fields are declared as SET
fields:
Table name | Field name | Possible set elements |
tables_priv | Table_priv | 'Select', 'Insert',
'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'
|
tables_priv | Column_priv | 'Select', 'Insert',
'Update', 'References'
|
columns_priv | Column_priv | 'Select', 'Insert',
'Update', 'References'
|
Briefly, the server uses the grant tables like this:
user
table scope fields determine whether to allow or reject
incoming connections. For allowed connections, any privileges granted in
the user
table indicate the user's global (superuser) privileges.
These privileges apply to all databases on the server.
db
and host
tables are used together:
db
table scope fields determine which users can access which
databases from which hosts. The privilege fields determine which operations
are allowed.
host
table is used as an extension of the db
table when you
want a given db
table entry to apply to several hosts. For example,
if you want a user to be able to use a database from several hosts in
your network, leave the Host
value empty in the user's db
table
entry, then populate the host
table with an entry for each of those
hosts. This mechanism is described more detail in section 4.2.9 Access Control, Stage 2: Request Verification.
tables_priv
and columns_priv
tables are similar to
the db
table, but are more fine-grained: they apply at the
table and column levels rather than at the database level.
Note that administrative privileges (reload, shutdown,
etc.) are specified only in the user
table. This is because
administrative operations are operations on the server itself and are not
database-specific, so there is no reason to list such privileges in the
other grant tables. In fact, only the user
table need
be consulted to determine whether or not you can perform an administrative
operation.
The file privilege is specified only in the user
table, too.
It is not an administrative privilege as such, but your ability to read or
write files on the server host is independent of the database you are
accessing.
The mysqld
server reads the contents of the grant tables once, when it
starts up. Changes to the grant tables take effect as indicated in
section 4.3.3 When Privilege Changes Take Effect.
When you modify the contents of the grant tables, it is a good idea to make
sure that your changes set up privileges the way you want. For help in
diagnosing problems, see section 4.2.10 Causes of Access denied
Errors. For advice on security issues,
see section 4.2.2 How to Make MySQL Secure Against Crackers.
A useful
diagnostic tool is the mysqlaccess
script, which Yves Carlier has
provided for the MySQL distribution. Invoke mysqlaccess
with
the --help
option to find out how it works.
Note that mysqlaccess
checks access using only the user
,
db
and host
tables. It does not check table- or column-level
privileges.
Information about user privileges is stored in the user
, db
,
host
, tables_priv
, and columns_priv
tables in the
mysql
database (that is, in the database named mysql
). The
MySQL server reads the contents of these tables when it starts up
and under the circumstances indicated in section 4.3.3 When Privilege Changes Take Effect.
The names used in this manual to refer to the privileges provided by MySQL are shown below, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies:
Privilege | Column | Context |
select | Select_priv | tables |
insert | Insert_priv | tables |
update | Update_priv | tables |
delete | Delete_priv | tables |
index | Index_priv | tables |
alter | Alter_priv | tables |
create | Create_priv | databases, tables, or indexes |
drop | Drop_priv | databases or tables |
grant | Grant_priv | databases or tables |
references | References_priv | databases or tables |
reload | Reload_priv | server administration |
shutdown | Shutdown_priv | server administration |
process | Process_priv | server administration |
file | File_priv | file access on server |
The select, insert, update, and delete privileges allow you to perform operations on rows in existing tables in a database.
SELECT
statements require the select privilege only if they
actually retrieve rows from a table. You can execute certain SELECT
statements even without permission to access any of the databases on the
server. For example, you could use the mysql
client as a simple
calculator:
mysql> SELECT 1+1; mysql> SELECT PI()*2;
The index privilege allows you to create or drop (remove) indexes.
The alter privilege allows you to use ALTER TABLE
.
The create and drop privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables.
Note that if you grant the drop privilege for the mysql
database to a user, that user can drop the database in which the
MySQL access privileges are stored!
The grant privilege allows you to give to other users those privileges you yourself possess.
The file privilege gives you permission to read and write files on
the server using the LOAD DATA INFILE
and SELECT ... INTO
OUTFILE
statements. Any user to whom this privilege is granted can read or
write any file that the MySQL server can read or write.
The remaining privileges are used for administrative operations, which are
performed using the mysqladmin
program. The table below shows which
mysqladmin
commands each administrative privilege allows you to
execute:
Privilege | Commands permitted to privilege holders |
reload | reload , refresh ,
flush-privileges , flush-hosts , flush-logs , and
flush-tables
|
shutdown | shutdown
|
process | processlist , kill
|
The reload
command tells the server to re-read the grant tables. The
refresh
command flushes all tables and opens and closes the log
files. flush-privileges
is a synonym for reload
. The other
flush-*
commands perform functions similar to refresh
but are
more limited in scope, and may be preferable in some instances. For example,
if you want to flush just the log files, flush-logs
is a better choice
than refresh
.
The shutdown
command shuts down the server.
The processlist
command displays information about the threads
executing within the server. The kill
command kills server threads.
You can always display or kill your own threads, but you need the
process privilege to display or kill threads initiated by other
users. See section 4.5.4 KILL
Syntax.
It is a good idea in general to grant privileges only to those users who need them, but you should exercise particular caution in granting certain privileges:
SELECT
. This includes the contents of all databases
hosted by the server!
mysql
database can be used to change passwords
and other access privilege information. (Passwords are stored
encrypted, so a malicious user cannot simply read them to know the plain
text password). If they can access the mysql.user
password
column, they can use it to log into the MySQL server
for the given user. (With sufficient privileges, the same user can
replace a password with a different one.)
There are some things that you cannot do with the MySQL privilege system:
MySQL client programs generally require that you specify connection
parameters when you want to access a MySQL server: the host you want
to connect to, your user name, and your password. For example, the
mysql
client can be started like this (optional arguments are enclosed
between `[' and `]'):
shell> mysql [-h host_name] [-u user_name] [-pyour_pass]
Alternate forms of the -h
, -u
, and -p
options are
--host=host_name
, --user=user_name
, and
--password=your_pass
. Note that there is no space between
-p
or --password=
and the password following it.
NOTE: Specifying a password on the command line is not secure!
Any user on your system may then find out your password by typing a command
like: ps auxww
. See section 4.1.2 my.cnf Option Files.
mysql
uses default values for connection parameters that are missing
from the command line:
localhost
.
-p
is missing.
Thus, for a Unix user joe
, the following commands are equivalent:
shell> mysql -h localhost -u joe shell> mysql -h localhost shell> mysql -u joe shell> mysql
Other MySQL clients behave similarly.
On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways:
[client]
section of the
`.my.cnf' configuration file in your home directory. The relevant
section of the file might look like this:
[client] host=host_name user=user_name password=your_passSee section 4.1.2 my.cnf Option Files.
mysql
using MYSQL_HOST
. The
MySQL user name can be specified using USER
(this is for
Windows only). The password can be specified using MYSQL_PWD
(but this is insecure; see the next section). See section B Environment Variables.
When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether or not you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters Stage 2 and waits for requests.
Your identity is based on two pieces of information:
Identity checking is performed using the three user
table scope fields
(Host
, User
, and Password
). The server accepts the
connection only if a user
table entry matches your hostname and user
name, and you supply the correct password.
Values in the user
table scope fields may be specified as follows:
Host
value may be a hostname or an IP number, or 'localhost'
to indicate the local host.
Host
field.
Host
value of '%'
matches any hostname.
Host
value means that the privilege should be anded
with the entry in the host
table that matches the given host name.
You can find more information about this in the next chapter.
Host
values specified as
IP numbers, you can specify a netmask indicating how many address bits to
use for the network number. For example:
GRANT ALL PRIVILEGES on db.* to david@'192.58.197.0/255.255.255.0';This will allow everyone to connect from an IP where the following is true:
user_ip & netmask = host_ip.In the above example all IP:s in the interval 192.58.197.0 - 192.58.197.255 can connect to the MySQL server.
User
field, but you can
specify a blank value, which matches any name. If the user
table
entry that matches an incoming connection has a blank user name, the user is
considered to be the anonymous user (the user with no name), rather than the
name that the client actually specified. This means that a blank user name
is used for all further access checking for the duration of the connection
(that is, during Stage 2).
Password
field can be blank. This does not mean that any password
matches, it means the user must connect without specifying a password.
Non-blank Password
values represent encrypted passwords.
MySQL does not store passwords in plaintext form for anyone to
see. Rather, the password supplied by a user who is attempting to
connect is encrypted (using the PASSWORD()
function). The
encrypted password is then used when the client/server is checking if
the password is correct (This is done without the encrypted password
ever traveling over the connection.) Note that from MySQL's
point of view the encrypted password is the REAL password, so you should
not give anyone access to it! In particular, don't give normal users
read access to the tables in the mysql
database!
The examples below show how various combinations of Host
and
User
values in user
table entries apply to incoming
connections:
Host value | User value | Connections matched by entry |
'thomas.loc.gov' | 'fred' | fred , connecting from thomas.loc.gov
|
'thomas.loc.gov' | '' | Any user, connecting from thomas.loc.gov
|
'%' | 'fred' | fred , connecting from any host
|
'%' | '' | Any user, connecting from any host |
'%.loc.gov' | 'fred' | fred , connecting from any host in the loc.gov domain
|
'x.y.%' | 'fred' | fred , connecting from x.y.net , x.y.com ,x.y.edu , etc. (this is probably not useful)
|
'144.155.166.177' | 'fred' | fred , connecting from the host with IP address 144.155.166.177
|
'144.155.166.%' | 'fred' | fred , connecting from any host in the 144.155.166 class C subnet
|
'144.155.166.0/255.255.255.0' | 'fred' | Same as previous example |
Because you can use IP wild-card values in the Host
field (for example,
'144.155.166.%'
to match every host on a subnet), there is the
possibility that someone might try to exploit this capability by naming a
host 144.155.166.somewhere.com
. To foil such attempts, MySQL
disallows matching on hostnames that start with digits and a dot. Thus, if
you have a host named something like 1.2.foo.com
, its name will never
match the Host
column of the grant tables. Only an IP number can
match an IP wild-card value.
An incoming connection may be matched by more than one entry in the
user
table. For example, a connection from thomas.loc.gov
by
fred
would be matched by several of the entries just shown above. How
does the server choose which entry to use if more than one matches? The
server resolves this question by sorting the user
table after reading
it at startup time, then looking through the entries in sorted order when a
user attempts to connect. The first matching entry is the one that is used.
user
table sorting works as follows. Suppose the user
table
looks like this:
+-----------+----------+- | Host | User | ... +-----------+----------+- | % | root | ... | % | jeffrey | ... | localhost | root | ... | localhost | | ... +-----------+----------+-
When the server reads in the table, it orders the entries with the
most-specific Host
values first ('%'
in the Host
column
means ``any host'' and is least specific). Entries with the same Host
value are ordered with the most-specific User
values first (a blank
User
value means ``any user'' and is least specific). The resulting
sorted user
table looks like this:
+-----------+----------+- | Host | User | ... +-----------+----------+- | localhost | root | ... | localhost | | ... | % | jeffrey | ... | % | root | ... +-----------+----------+-
When a connection is attempted, the server looks through the sorted entries
and uses the first match found. For a connection from localhost
by
jeffrey
, the entries with 'localhost'
in the Host
column
match first. Of those, the entry with the blank user name matches both the
connecting hostname and user name. (The '%'/'jeffrey'
entry would
have matched, too, but it is not the first match in the table.)
Here is another example. Suppose the user
table looks like this:
+----------------+----------+- | Host | User | ... +----------------+----------+- | % | jeffrey | ... | thomas.loc.gov | | ... +----------------+----------+-
The sorted table looks like this:
+----------------+----------+- | Host | User | ... +----------------+----------+- | thomas.loc.gov | | ... | % | jeffrey | ... +----------------+----------+-
A connection from thomas.loc.gov
by jeffrey
is matched by the
first entry, whereas a connection from whitehouse.gov
by
jeffrey
is matched by the second.
A common misconception is to think that for a given user name, all entries
that explicitly name that user will be used first when the server attempts to
find a match for the connection. This is simply not true. The previous
example illustrates this, where a connection from thomas.loc.gov
by
jeffrey
is first matched not by the entry containing 'jeffrey'
as the User
field value, but by the entry with no user name!
If you have problems connecting to the server, print out the user
table and sort it by hand to see where the first match is being made.
Once you establish a connection, the server enters Stage 2. For each request
that comes in on the connection, the server checks whether you have
sufficient privileges to perform it, based on the type of operation you wish
to perform. This is where the privilege fields in the grant tables come into
play. These privileges can come from any of the user
, db
,
host
, tables_priv
, or columns_priv
tables. The grant
tables are manipulated with GRANT
and REVOKE
commands.
See section 4.3.1 GRANT
and REVOKE
Syntax. (You may find it helpful to refer to
section 4.2.5 How the Privilege System Works, which lists the fields present in each of the grant
tables.)
The user
table grants privileges that are assigned to you on a global
basis and that apply no matter what the current database is. For example, if
the user
table grants you the delete privilege, you can
delete rows from any database on the server host! In other words,
user
table privileges are superuser privileges. It is wise to grant
privileges in the user
table only to superusers such as server or
database administrators. For other users, you should leave the privileges
in the user
table set to 'N'
and grant privileges on a
database-specific basis only, using the db
and host
tables.
The db
and host
tables grant database-specific privileges.
Values in the scope fields may be specified as follows:
Host
and Db
fields of either table.
'%'
Host
value in the db
table means ``any host.'' A
blank Host
value in the db
table means ``consult the
host
table for further information.''
'%'
or blank Host
value in the host
table means ``any
host.''
'%'
or blank Db
value in either table means ``any database.''
User
value in either table matches the anonymous user.
The db
and host
tables are read in and sorted when the server
starts up (at the same time that it reads the user
table). The
db
table is sorted on the Host
, Db
, and User
scope
fields, and the host
table is sorted on the Host
and Db
scope fields. As with the user
table, sorting puts the most-specific
values first and least-specific values last, and when the server looks for
matching entries, it uses the first match that it finds.
The tables_priv
and columns_priv
tables grant table- and
column-specific privileges. Values in the scope fields may be specified as
follows:
Host
field of either table.
'%'
or blank Host
value in either table means ``any host.''
Db
, Table_name
and Column_name
fields cannot contain
wild cards or be blank in either table.
The tables_priv
and columns_priv
tables are sorted on
the Host
, Db
, and User
fields. This is similar to
db
table sorting, although the sorting is simpler because
only the Host
field may contain wild cards.
The request verification process is described below. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.)
For administrative requests (shutdown, reload, etc.), the
server checks only the user
table entry, because that is the only table
that specifies administrative privileges. Access is granted if the entry
allows the requested operation and denied otherwise. For example, if you
want to execute mysqladmin shutdown
but your user
table entry
doesn't grant the shutdown privilege to you, access is denied
without even checking the db
or host
tables. (They
contain no Shutdown_priv
column, so there is no need to do so.)
For database-related requests (insert, update, etc.), the
server first checks the user's global (superuser) privileges by looking in
the user
table entry. If the entry allows the requested operation,
access is granted. If the global privileges in the user
table are
insufficient, the server determines the user's database-specific privileges
by checking the db
and host
tables:
db
table for a match on the Host
,
Db
, and User
fields. The Host
and User
fields are
matched to the connecting user's hostname and MySQL user name. The
Db
field is matched to the database the user wants to access. If
there is no entry for the Host
and User
, access is denied.
db
table entry and its Host
field is
not blank, that entry defines the user's database-specific privileges.
db
table entry's Host
field is blank, it
signifies that the host
table enumerates which hosts should be allowed
access to the database. In this case, a further lookup is done in the
host
table to find a match on the Host
and Db
fields.
If no host
table entry matches, access is denied. If there is a
match, the user's database-specific privileges are computed as the
intersection (not the union!) of the privileges in the db
and
host
table entries, that is, the privileges that are 'Y'
in both
entries. (This way you can grant general privileges in the db
table
entry and then selectively restrict them on a host-by-host basis using the
host
table entries.)
After determining the database-specific privileges granted by the db
and host
table entries, the server adds them to the global privileges
granted by the user
table. If the result allows the requested
operation, access is granted. Otherwise, the server checks the user's
table and column privileges in the tables_priv
and columns_priv
tables and adds those to the user's privileges. Access is allowed or denied
based on the result.
Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this:
global privileges OR (database privileges AND host privileges) OR table privileges OR column privileges
It may not be apparent why, if the global user
entry privileges are
initially found to be insufficient for the requested operation, the server
adds those privileges to the database-, table-, and column-specific privileges
later. The reason is that a request might require more than one type of
privilege. For example, if you execute an INSERT ... SELECT
statement, you need both insert and select privileges.
Your privileges might be such that the user
table entry grants one
privilege and the db
table entry grants the other. In this case, you
have the necessary privileges to perform the request, but the server cannot
tell that from either table by itself; the privileges granted by the entries
in both tables must be combined.
The host
table can be used to maintain a list of secure servers.
At TcX, the host
table contains a list of all machines on the local
network. These are granted all privileges.
You can also use the host
table to indicate hosts that are not
secure. Suppose you have a machine public.your.domain
that is located
in a public area that you do not consider secure. You can allow access to
all hosts on your network except that machine by using host
table
entries
like this:
+--------------------+----+- | Host | Db | ... +--------------------+----+- | public.your.domain | % | ... (all privileges set to 'N') | %.your.domain | % | ... (all privileges set to 'Y') +--------------------+----+-
Naturally, you should always test your entries in the grant tables (for
example, using mysqlaccess
) to make sure your access privileges are
actually set up the way you think they are.
Access denied
Errors
If you encounter Access denied
errors when you try to connect to the
MySQL server, the list below indicates some courses of
action you can take to correct the problem:
mysql_install_db
script to set up the initial grant table contents? If not, do so.
See section 4.3.4 Setting Up the Initial MySQL Privileges. Test the initial privileges by executing
this command:
shell> mysql -u root testThe server should let you connect without error. You should also make sure you have a file `user.MYD' in the MySQL database directory. Ordinarily, this is `PATH/var/mysql/user.MYD', where
PATH
is the
pathname to the MySQL installation root.
shell> mysql -u root mysqlThe server should let you connect because the MySQL
root
user
has no password initially. That is also a security risk, so setting the
root
password is something you should do while you're setting up
your other MySQL users.
If you try to connect as root
and get this error:
Access denied for user: '@unknown' to database mysqlthis means that you don't have an entry in the
user
table with a
User
column value of 'root'
and that mysqld
cannot
resolve the hostname for your client. In this case, you must restart the
server with the --skip-grant-tables
option and edit your
`/etc/hosts' or `\windows\hosts' file to add an entry for your
host.
shell> mysqladmin -u root -pxxxx ver Access denied for user: 'root@localhost' (Using password: YES)It means that you are using a wrong password. See section 4.3.6 Setting Up Passwords. If you have forgot the root password, you can restart
mysqld
with
--skip-grant-tables
to change the password. You can find more
about this option later on in this manual section.
If you get the above error even if you haven't specified a password,
this means that you a wrong password in some my.ini
file. See section 4.1.2 my.cnf Option Files. You can avoid using option files with the --no-defaults
option, as follows:
shell> mysqladmin --no-defaults -u root ver
mysql_fix_privilege_tables
script? If not, do so. The structure of
the grant tables changed with MySQL Version 3.22.11 when the
GRANT
statement became functional.
PASSWORD()
function if you set the password with the
INSERT
, UPDATE
, or SET PASSWORD
statements. The
PASSWORD()
function is unnecessary if you specify the password using
the GRANT ... INDENTIFIED BY
statement or the mysqladmin
password
command.
See section 4.3.6 Setting Up Passwords.
localhost
is a synonym for your local hostname, and is also the
default host to which clients try to connect if you specify no host
explicitly. However, connections to localhost
do not work if you are
running on a system that uses MIT-pthreads (localhost
connections are
made using Unix sockets, which are not supported by MIT-pthreads). To avoid
this problem on such systems, you should use the --host
option to name
the server host explicitly. This will make a TCP/IP connection to the
mysqld
server. In this case, you must have your real hostname in
user
table entries on the server host. (This is true even if you are
running a client program on the same host as the server.)
Access denied
error when trying to connect to the
database with mysql -u user_name db_name
, you may have a problem
with the user
table. Check this by executing mysql -u root
mysql
and issuing this SQL statement:
mysql> SELECT * FROM user;The result should include an entry with the
Host
and User
columns matching your computer's hostname and your MySQL user name.
Access denied
error message will tell you who you are trying
to log in as, the host from which you are trying to connect, and whether
or not you were using a password. Normally, you should have one entry in
the user
table that exactly matches the hostname and user name
that were given in the error message. For example if you get an error
message that contains Using password: NO
, this means that you
tried to login without an password.
user
table that matches that host:
Host ... is not allowed to connect to this MySQL serverYou can fix this by using the command-line tool
mysql
(on the
server host!) to add a row to the user
, db
, or host
table for the user/hostname combination from which you are trying to
connect and then execute mysqladmin flush-privileges
. If you are
not running MySQL Version 3.22 and you don't know the IP number or
hostname of the machine from which you are connecting, you should put an
entry with '%'
as the Host
column value in the user
table and restart mysqld
with the --log
option on the
server machine. After trying to connect from the client machine, the
information in the MySQL log will indicate how you really did
connect. (Then replace the '%'
in the user
table entry
with the actual hostname that shows up in the log. Otherwise, you'll
have a system that is insecure.)
Another reason for this error on Linux is that you are using a binary
MySQL version that is compiled with a different glibc version
than the one you are using. In this case you should either upgrade your
OS/glibc or download the source MySQL version and compile this
yourself. A source RPM is normally trivial to compile and install, so
this isn't a big problem.
shell> mysqladmin -u root -pxxxx -h some-hostname ver Access denied for user: 'root' (Using password: YES)This means that MySQL got some error when trying to resolve the IP to a hostname. In this case you can execute
mysqladmin
flush-hosts
to reset the internal DNS cache. See section 5.5.5 How MySQL uses DNS.
Some permanent solutions are:
mysqld
with --skip-name-resolve
.
mysqld
with --skip-host-cache
.
localhost
if you are running the server and the client
on the same machine.
/etc/hosts
.
mysql -u root test
works but mysql -h your_hostname -u root
test
results in Access denied
, then you may not have the correct name
for your host in the user
table. A common problem here is that the
Host
value in the user table entry specifies an unqualified hostname,
but your system's name resolution routines return a fully qualified domain
name (or vice-versa). For example, if you have an entry with host
'tcx'
in the user
table, but your DNS tells MySQL that
your hostname is 'tcx.subnet.se'
, the entry will not work. Try adding
an entry to the user
table that contains the IP number of your host as
the Host
column value. (Alternatively, you could add an entry to the
user
table with a Host
value that contains a wild card--for
example, 'tcx.%'
. However, use of hostnames ending with `%' is
insecure and is not recommended!)
mysql -u user_name test
works but mysql -u user_name
other_db_name
doesn't work, you don't have an entry for other_db_name
listed in the db
table.
mysql -u user_name db_name
works when executed on the server
machine, but mysql -u host_name -u user_name db_name
doesn't work when
executed on another client machine, you don't have the client machine listed
in the user
table or the db
table.
Access denied
, remove from the
user
table all entries that have Host
values containing
wild cards (entries that contain `%' or `_'). A very common error
is to insert a new entry with Host
='%'
and
User
='some user'
, thinking that this will allow you to specify
localhost
to connect from the same machine. The reason that this
doesn't work is that the default privileges include an entry with
Host
='localhost'
and User
=''
. Because that entry
has a Host
value 'localhost'
that is more specific than
'%'
, it is used in preference to the new entry when connecting from
localhost
! The correct procedure is to insert a second entry with
Host
='localhost'
and User
='some_user'
, or to
remove the entry with Host
='localhost'
and
User
=''
.
db
or
host
table:
Access to database deniedIf the entry selected from the
db
table has an empty value in the
Host
column, make sure there are one or more corresponding entries in
the host
table specifying which hosts the db
table entry
applies to.
If you get the error when using the SQL commands SELECT ...
INTO OUTFILE
or LOAD DATA INFILE
, your entry in the user
table
probably doesn't have the file privilege enabled.
Access denied
when you run a client without any options, make
sure you haven't specified an old password in any of your option files!
See section 4.1.2 my.cnf Option Files.
INSERT
or
UPDATE
statement) and your changes seem to be ignored, remember
that you must issue a FLUSH PRIVILEGES
statement or execute a
mysqladmin flush-privileges
command to cause the server to re-read
the privilege tables. Otherwise your changes have no effect until the
next time the server is restarted. Remember that after you set the
root
password with an UPDATE
command, you won't need to
specify it until after you flush the privileges, because the server
won't know you've changed the password yet!
mysql -u user_name db_name
or mysql
-u user_name -pyour_pass db_name
. If you are able to connect using the
mysql
client, there is a problem with your program and not with the
access privileges. (Note that there is no space between -p
and the
password; you can also use the --password=your_pass
syntax to specify
the password. If you use the -p
option alone, MySQL will
prompt you for the password.)
mysqld
daemon with the
--skip-grant-tables
option. Then you can change the MySQL
grant tables and use the mysqlaccess
script to check whether or not
your modifications have the desired effect. When you are satisfied with your
changes, execute mysqladmin flush-privileges
to tell the mysqld
server to start using the new grant tables. Note: Reloading the
grant tables overrides the --skip-grant-tables
option. This allows
you to tell the server to begin using the grant tables again without bringing
it down and restarting it.
mysqld
daemon with a debugging
option (for example, --debug=d,general,query
). This will print host and
user information about attempted connections, as well as information about
each command issued. See section H.1.2 Creating trace files.
mysqldump mysql
command. As always, post your problem using
the mysqlbug
script. See section 1.2.22.3 How to Report Bugs or Problems. In some cases you may need
to restart mysqld
with --skip-grant-tables
to run
mysqldump
.
GRANT
and REVOKE
SyntaxGRANT priv_type [(column_list)] [, priv_type [(column_list)] ...] ON {tbl_name | * | *.* | db_name.*} TO user_name [IDENTIFIED BY 'password'] [, user_name [IDENTIFIED BY 'password'] ...] [WITH GRANT OPTION] REVOKE priv_type [(column_list)] [, priv_type [(column_list)] ...] ON {tbl_name | * | *.* | db_name.*} FROM user_name [, user_name ...]
GRANT
is implemented in MySQL Version 3.22.11 or later. For
earlier MySQL versions, the GRANT
statement does nothing.
The GRANT
and REVOKE
commands allow system administrators
to create users and grant and revoke rights to MySQL users at
four privilege levels:
mysql.user
table.
mysql.db
and mysql.host
tables.
mysql.tables_priv
table.
mysql.columns_priv
table.
If you give a grant for a users that doesn't exists, that user is created.
For examples of how GRANT
works, see section 4.3.5 Adding New Users to MySQL.
For the GRANT
and REVOKE
statements, priv_type
may be
specified as any of the following:
ALL PRIVILEGES FILE RELOAD ALTER INDEX SELECT CREATE INSERT SHUTDOWN DELETE PROCESS UPDATE DROP REFERENCES USAGE
ALL
is a synonym for ALL PRIVILEGES
. REFERENCES
is not
yet implemented. USAGE
is currently a synonym for ``no privileges.''
It can be used when you want to create a user that has no privileges.
To revoke the grant privilege from a user, use a priv_type
value of GRANT OPTION
:
REVOKE GRANT OPTION ON ... FROM ...;
The only priv_type
values you can specify for a table are SELECT
,
INSERT
, UPDATE
, DELETE
, CREATE
, DROP
,
GRANT
, INDEX
, and ALTER
.
The only priv_type
values you can specify for a column (that is, when
you use a column_list
clause) are SELECT
, INSERT
, and
UPDATE
.
You can set global privileges by using ON *.*
syntax. You can set
database privileges by using ON db_name.*
syntax. If you specify
ON *
and you have a current database, you will set the privileges for
that database. (WARNING: If you specify ON *
and you
don't have a current database, you will affect the global privileges!)
In order to accommodate granting rights to users from arbitrary hosts,
MySQL supports specifying the user_name
value in the form
user@host
. If you want to specify a user
string
containing special characters (such as `-'), or a host
string
containing special characters or wild-card characters (such as `%'), you
can quote the user or host name (for example, 'test-user'@'test-hostname'
).
You can specify wild cards in the hostname. For example,
user@"%.loc.gov"
applies to user
for any host in the
loc.gov
domain, and user@"144.155.166.%"
applies to user
for any host in the 144.155.166
class C subnet.
The simple form user
is a synonym for user@"%"
.
NOTE: If you allow anonymous users to connect to the MySQL
server (which is the default), you should also add all local users as
user@localhost
because otherwise the anonymous user entry for the
local host in the mysql.user
table will be used when the user tries to
log into the MySQL server from the local machine! Anonymous users
are defined by inserting entries with User=''
into the
mysql.user
table. You can verify if this applies to you by executing
this query:
mysql> SELECT Host,User FROM mysql.user WHERE User='';
For the moment, GRANT
only supports host, table, database, and
column names up to 60 characters long. A user name can be up to 16
characters.
The privileges for a table or column are formed from the
logical OR of the privileges at each of the four privilege
levels. For example, if the mysql.user
table specifies that a
user has a global select privilege, this can't be denied by an
entry at the database, table, or column level.
The privileges for a column can be calculated as follows:
global privileges OR (database privileges AND host privileges) OR table privileges OR column privileges
In most cases, you grant rights to a user at only one of the privilege levels, so life isn't normally as complicated as above. The details of the privilege-checking procedure are presented in section 4.2 General Security Issues and the MySQL Access Privilege System.
If you grant privileges for a user/hostname combination that does not exist
in the mysql.user
table, an entry is added and remains there until
deleted with a DELETE
command. In other words, GRANT
may
create user
table entries, but REVOKE
will not remove them;
you must do that explicitly using DELETE
.
In MySQL Version 3.22.12 or later,
if a new user is created or if you have global grant privileges, the user's
password will be set to the password specified by the IDENTIFIED BY
clause, if one is given. If the user already had a password, it is replaced
by the new one.
WARNING: If you create a new user but do not specify an
IDENTIFIED BY
clause, the user has no password. This is insecure.
Passwords can also be set with the SET PASSWORD
command.
See section 5.5.6 SET
Syntax.
If you grant privileges for a database, an entry in the mysql.db
table is created if needed. When all privileges for the database have been
removed with REVOKE
, this entry is deleted.
If a user doesn't have any privileges on a table, the table is not displayed
when the user requests a list of tables (for example, with a SHOW TABLES
statement).
The WITH GRANT OPTION
clause gives the user the ability to give
to other users any privileges the user has at the specified privilege level.
You should be careful to whom you give the grant privilege, as two
users with different privileges may be able to join privileges!
You cannot grant another user a privilege you don't have yourself; the grant privilege allows you to give away only those privileges you possess.
Be aware that when you grant a user the grant privilege at a
particular privilege level, any privileges the user already possesses (or
is given in the future!) at that level are also grantable by that user.
Suppose you grant a user the insert privilege on a database. If
you then grant the select privilege on the database and specify
WITH GRANT OPTION
, the user can give away not only the select
privilege, but also insert. If you then grant the update
privilege to the user on the database, the user can give away the
insert, select and update.
You should not grant alter privileges to a normal user. If you do that, the user can try to subvert the privilege system by renaming tables!
Note that if you are using table or column privileges for even one user, the server examines table and column privileges for all users and this will slow down MySQL a bit.
When mysqld
starts, all privileges are read into memory.
Database, table, and column privileges take effect at once, and
user-level privileges take effect the next time the user connects.
Modifications to the grant tables that you perform using GRANT
or
REVOKE
are noticed by the server immediately.
If you modify the grant tables manually (using INSERT
, UPDATE
,
etc.), you should execute a FLUSH PRIVILEGES
statement or run
mysqladmin flush-privileges
to tell the server to reload the grant
tables.
See section 4.3.3 When Privilege Changes Take Effect.
The biggest differences between the ANSI SQL and MySQL versions of
GRANT
are:
TRIGGER
, EXECUTE
or
UNDER
privileges.
INSERT
grant on only part of the
columns in a table, you can execute INSERT
statements on the
table; The columns for which you don't have the INSERT
privilege
will set to their default values. ANSI SQL requires you to have the
INSERT
privilege on all columns.
REVOKE
commands or by manipulating the
MySQL grant tables.
There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows:
-u
or
--user
options. This means that you can't make a database secure in
any way unless all MySQL user names have passwords. Anyone may
attempt to connect to the server using any name, and they will succeed if
they specify any name that doesn't have a password.
PASSWORD()
and ENCRYPT()
functions in section 6.3.5.2 Miscellaneous Functions. Note that even if the password is stored 'scrambled', and
knowing your 'scrambled' password is enough to be able to connect to
the MySQL server!
MySQL users and they privileges are normally created with the
GRANT
command. See section 4.3.1 GRANT
and REVOKE
Syntax.
When you login to a MySQL server with a command line client you
should specify the password with --password=your-password
.
See section 4.2.7 Connecting to the MySQL Server.
mysql --user=monty --password=guess database_name
If you want the client to prompt for a password, you should use
--password
without any argument
mysql --user=monty --password database_name
or the short form:
mysql -u monty -p database_name
Note that in the last example the password is NOT 'database_name'.
If you want to use the -p
option to supply a password you should do like this:
mysql -u monty -pguess database_name
On some system the library call that MySQL uses to prompt for a password will automatically cut the password to 8 characters. Internally MySQL doesn't have any limit for the length of the password.
When mysqld
starts, all grant table contents are read into memory and
become effective at that point.
Modifications to the grant tables that you perform using GRANT
,
REVOKE
, or SET PASSWORD
are noticed by the server immediately.
If you modify the grant tables manually (using INSERT
, UPDATE
,
etc.), you should execute a FLUSH PRIVILEGES
statement or run
mysqladmin flush-privileges
or mysqladmin reload
to tell the
server to reload the grant tables. Otherwise your changes will have no
effect until you restart the server. If you change the grant tables manually
but forget to reload the privileges, you will be wondering why your changes
don't seem to make any difference!
When the server notices that the grant tables have been changed, existing client connections are affected as follows:
USE db_name
command.
Global privilege changes and password changes take effect the next time the client connects.
After installing MySQL, you set up the initial access privileges by
running scripts/mysql_install_db
.
See section 2.3.1 Quick Installation Overview.
The mysql_install_db
script starts up the mysqld
server, then initializes the grant tables to contain the following set
of privileges:
root
user is created as a superuser who can do
anything. Connections must be made from the local host.
NOTE:
The initial root
password is empty, so anyone can connect as root
without a password and be granted all privileges.
'test'
or starting with 'test_'
. Connections must be
made from the local host. This means any local user can connect without a
password and be treated as the anonymous user.
mysqladmin shutdown
or mysqladmin processlist
.
NOTE: The default privileges are different for Windows. See section 2.6.2.3 Running MySQL on Windows.
Because your installation is initially wide open, one of the first things you
should do is specify a password for the MySQL
root
user. You can do this as follows (note that you specify the
password using the PASSWORD()
function):
shell> mysql -u root mysql mysql> UPDATE user SET Password=PASSWORD('new_password') WHERE user='root'; mysql> FLUSH PRIVILEGES;
You can, in MySQL Version 3.22 and above, use the SET PASSWORD
statement:
shell> mysql -u root mysql mysql> SET PASSWORD FOR root=PASSWORD('new_password');
Another way to set the password is by using the mysqladmin
command:
shell> mysqladmin -u root password new_password
Only users with write/update access to the mysql
database can change the
password for others users. All normal users (not anonymous ones) can only
change their own password with either of the above commands or with
SET PASSWORD=PASSWORD('new password')
.
Note that if you update the password in the user
table directly using
the first method, you must tell the server to re-read the grant tables (with
FLUSH PRIVILEGES
), because the change will go unnoticed otherwise.
Once the root
password has been set, thereafter you must supply that
password when you connect to the server as root
.
You may wish to leave the root
password blank so that you don't need
to specify it while you perform additional setup or testing. However, be sure
to set it before using your installation for any real production work.
See the scripts/mysql_install_db
script to see how it sets up
the default privileges. You can use this as a basis to see how to
add other users.
If you want the initial privileges to be different than those just described
above, you can modify mysql_install_db
before you run it.
To re-create the grant tables completely, remove all the `.frm',
`.MYI', and `.MYD' files in the directory containing the
mysql
database. (This is the directory named `mysql' under
the database directory, which is listed when you run mysqld
--help
.) Then run the mysql_install_db
script, possibly after
editing it first to have the privileges you want.
NOTE: For MySQL versions older than Version 3.22.10,
you should NOT delete the `.frm' files. If you accidentally do this,
you should copy them back from your MySQL distribution before
running mysql_install_db
.
You can add users two different ways: by using GRANT
statements
or by manipulating the MySQL grant tables directly. The
preferred method is to use GRANT
statements, because they are
more concise and less error-prone. See section 4.3.1 GRANT
and REVOKE
Syntax.
There is also a lot of contributed programs like phpmyadmin
that
can be used to create and administrate users. See section E Contributed Programs.
The examples below show how to use the mysql
client to set up new
users. These examples assume that privileges are set up according to the
defaults described in the previous section. This means that to make changes,
you must be on the same machine where mysqld
is running, you must
connect as the MySQL root
user, and the root
user must
have the insert privilege for the mysql
database and the
reload administrative privilege. Also, if you have changed the
root
user password, you must specify it for the mysql
commands
below.
You can add new users by issuing GRANT
statements:
shell> mysql --user=root mysql mysql> GRANT ALL PRIVILEGES ON *.* TO monty@localhost IDENTIFIED BY 'some_pass' WITH GRANT OPTION; mysql> GRANT ALL PRIVILEGES ON *.* TO monty@"%" IDENTIFIED BY 'some_pass' WITH GRANT OPTION; mysql> GRANT RELOAD,PROCESS ON *.* TO admin@localhost; mysql> GRANT USAGE ON *.* TO dummy@localhost;
These GRANT
statements set up three new users:
monty
'some_pass'
to do so. Note that we must issue
GRANT
statements for both monty@localhost
and
monty@"%"
. If we don't add the entry with localhost
, the
anonymous user entry for localhost
that is created by
mysql_install_db
will take precedence when we connect from the local
host, because it has a more specific Host
field value and thus comes
earlier in the user
table sort order.
admin
localhost
without a password and who is
granted the reload and process administrative privileges.
This allows the user to execute the mysqladmin reload
,
mysqladmin refresh
, and mysqladmin flush-*
commands, as well as
mysqladmin processlist
. No database-related privileges are granted.
(They can be granted later by issuing additional GRANT
statements.)
dummy
'N'
-- the USAGE
privilege
type allows you to create a user with no privileges. It is assumed that you
will grant database-specific privileges later.
You can also add the same user access information directly by issuing
INSERT
statements and then telling the server to reload the grant
tables:
shell> mysql --user=root mysql mysql> INSERT INTO user VALUES('localhost','monty',PASSWORD('some_pass'), 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO user VALUES('%','monty',PASSWORD('some_pass'), 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO user SET Host='localhost',User='admin', Reload_priv='Y', Process_priv='Y'; mysql> INSERT INTO user (Host,User,Password) VALUES('localhost','dummy',''); mysql> FLUSH PRIVILEGES;
Depending on your MySQL version, you may have to use a different
number of 'Y'
values above (versions prior to Version 3.22.11 had fewer
privilege columns). For the admin
user, the more readable extended
INSERT
syntax that is available starting with Version 3.22.11 is used.
Note that to set up a superuser, you need only create a user
table
entry with the privilege fields set to 'Y'
. No db
or
host
table entries are necessary.
The privilege columns in the user
table were not set explicitly in the
last INSERT
statement (for the dummy
user), so those columns
are assigned the default value of 'N'
. This is the same thing that
GRANT USAGE
does.
The following example adds a user custom
who can connect from hosts
localhost
, server.domain
, and whitehouse.gov
. He wants
to access the bankaccount
database only from localhost
,
the expenses
database only from whitehouse.gov
, and
the customer
database from all three hosts. He wants
to use the password stupid
from all three hosts.
To set up this user's privileges using GRANT
statements, run these
commands:
shell> mysql --user=root mysql mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON bankaccount.* TO custom@localhost IDENTIFIED BY 'stupid'; mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON expenses.* TO custom@whitehouse.gov IDENTIFIED BY 'stupid'; mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP ON customer.* TO custom@'%' IDENTIFIED BY 'stupid';
The reason that we do to grant statements for the user 'custom' is that we want the give the user access to MySQL both from the local machine with Unix sockets and from the remote machine 'whitehouse.gov' over TCP/IP.
To set up the user's privileges by modifying the grant tables directly,
run these commands (note the FLUSH PRIVILEGES
at the end):
shell> mysql --user=root mysql mysql> INSERT INTO user (Host,User,Password) VALUES('localhost','custom',PASSWORD('stupid')); mysql> INSERT INTO user (Host,User,Password) VALUES('server.domain','custom',PASSWORD('stupid')); mysql> INSERT INTO user (Host,User,Password) VALUES('whitehouse.gov','custom',PASSWORD('stupid')); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES ('localhost','bankaccount','custom','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES ('whitehouse.gov','expenses','custom','Y','Y','Y','Y','Y','Y'); mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv, Create_priv,Drop_priv) VALUES('%','customer','custom','Y','Y','Y','Y','Y','Y'); mysql> FLUSH PRIVILEGES;
The first three INSERT
statements add user
table entries that
allow user custom
to connect from the various hosts with the given
password, but grant no permissions to him (all privileges are set to the
default value of 'N'
). The next three INSERT
statements add
db
table entries that grant privileges to custom
for the
bankaccount
, expenses
, and customer
databases, but only
when accessed from the proper hosts. As usual, when the grant tables are
modified directly, the server must be told to reload them (with
FLUSH PRIVILEGES
) so that the privilege changes take effect.
If you want to give a specific user access from any machine in a given
domain, you can issue a GRANT
statement like the following:
mysql> GRANT ... ON *.* TO myusername@"%.mydomainname.com" IDENTIFIED BY 'mypassword';
To do the same thing by modifying the grant tables directly, do this:
mysql> INSERT INTO user VALUES ('%.mydomainname.com', 'myusername', PASSWORD('mypassword'),...); mysql> FLUSH PRIVILEGES;
You can also use xmysqladmin
, mysql_webadmin
, and even
xmysql
to insert, change, and update values in the grant tables.
You can find these utilities in the
Contrib directory of the MySQL
Website.
In most cases you should use GRANT
to set up your users/passwords,
so the following only applies for advanced users. See section 4.3.1 GRANT
and REVOKE
Syntax.
The examples in the preceding sections illustrate an important principle:
when you store a non-empty password using INSERT
or UPDATE
statements, you must use the PASSWORD()
function to encrypt it. This
is because the user
table stores passwords in encrypted form, not as
plaintext. If you forget that fact, you are likely to attempt to set
passwords like this:
shell> mysql -u root mysql mysql> INSERT INTO user (Host,User,Password) VALUES('%','jeffrey','biscuit'); mysql> FLUSH PRIVILEGES;
The result is that the plaintext value 'biscuit'
is stored as the
password in the user
table. When the user jeffrey
attempts to
connect to the server using this password, the mysql
client encrypts
it with PASSWORD()
, generates an authentification vector
based on encrypted password and a random number,
obtained from server, and sends the result to the server.
The server uses the password
value in the user
table
(that is not encrypted value 'biscuit'
)
to perform the same calculations, and compares results.
The comparison fails and the server rejects the
connection:
shell> mysql -u jeffrey -pbiscuit test Access denied
Passwords must be encrypted when they are inserted in the user
table, so the INSERT
statement should have been specified like this
instead:
mysql> INSERT INTO user (Host,User,Password) VALUES('%','jeffrey',PASSWORD('biscuit'));
You must also use the PASSWORD()
function when you use SET
PASSWORD
statements:
mysql> SET PASSWORD FOR jeffrey@"%" = PASSWORD('biscuit');
If you set passwords using the GRANT ... IDENTIFIED BY
statement
or the mysqladmin password
command, the PASSWORD()
function
is unnecessary. They both take care of encrypting the password for you,
so you would specify a password of 'biscuit'
like this:
mysql> GRANT USAGE ON *.* TO jeffrey@"%" IDENTIFIED BY 'biscuit';
or
shell> mysqladmin -u jeffrey password biscuit
NOTE: PASSWORD()
does not perform password encryption in the
same way that Unix passwords are encrypted. You should not assume that if
your Unix password and your MySQL password are the same, that
PASSWORD()
will result in the same encrypted value as is stored in the
Unix password file. See section 4.3.2 MySQL User Names and Passwords.
It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed below, along with an assessment of the risks of each method:
mysql.user
table. Knowing
the encrypted password for a user makes it possible to login as this
user. The passwords are only scrambled so that one shouldn't be able to
see the real password you used (if you happen to use a similar password
with your other applications).
-pyour_pass
or --password=your_pass
option on the command
line. This is convenient but insecure, because your password becomes visible
to system status programs (such as ps
) that may be invoked by other
users to display command lines. (MySQL clients typically overwrite
the command-line argument with zeroes during their initialization sequence,
but there is still a brief interval during which the value is visible.)
-p
or --password
option (with no your_pass
value
specified). In this case, the client program solicits the password from
the terminal:
shell> mysql -u user_name -p Enter password: ********The `*' characters represent your password. It is more secure to enter your password this way than to specify it on the command line because it is not visible to other users. However, this method of entering a password is suitable only for programs that you run interactively. If you want to invoke a client from a script that runs non-interactively, there is no opportunity to enter the password from the terminal. On some systems, you may even find that the first line of your script is read and interpreted (incorrectly) as your password!
[client]
section of the `.my.cnf' file in your
home directory:
[client] password=your_passIf you store your password in `.my.cnf', the file should not be group or world readable or writable. Make sure the file's access mode is
400
or 600
.
See section 4.1.2 my.cnf Option Files.
MYSQL_PWD
environment variable, but
this method must be considered extremely insecure and should not be used.
Some versions of ps
include an option to display the environment of
running processes; your password will be in plain sight for all to see if
you set MYSQL_PWD
. Even on systems without such a version of
ps
, it is unwise to assume there is no other method to observe process
environments. See section B Environment Variables.
All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected `.my.cnf' file.
Because MySQL tables are stored as files, it is easy to do a
backup. To get a consistent backup, do a LOCK TABLES
on the
relevant tables followed by FLUSH TABLES
for the tables.
See section 6.7.2 LOCK TABLES/UNLOCK TABLES
Syntax.
See section 4.5.3 FLUSH
Syntax.
You only need a read lock; this allows other threads to continue to
query the tables while you are making a copy of the files in the
database directory. The FLUSH TABLE
is needed to ensure that
the all active index pages is written to disk before you start the backup.
If you want to make a SQL level backup of a table, you can use
SELECT INTO OUTFILE
or BACKUP TABLE
. See section 6.4.1 SELECT
Syntax.
See section 4.4.2 BACKUP TABLE
Syntax.
Another way to back up a database is to use the mysqldump
program or
the mysqlhotcopy script
. See section 4.8.5 mysqldump, Dumping Table Structure and Data.
See section 4.8.6 mysqlhotcopy, Copying MySQL Databases and Tables.
shell> mysqldump --tab=/path/to/some/dir --opt --full or shell> mysqlhotcopy database /path/to/some/dirYou can also simply copy all table files (`*.frm', `*.MYD', and `*.MYI' files) as long as the server isn't updating anything. The script
mysqlhotcopy
does use this method.
mysqld
if it's running, then start it with the
--log-update[=file_name]
option. See section 4.9.3 The Update Log. The update
log file(s) provide you with the information you need to replicate
changes to the database that are made subsequent to the point at which
you executed mysqldump
.
If you have to restore something, try to recover your tables using
REPAIR TABLE
or myisamchk -r
first. That should work in
99.9% of all cases. If myisamchk
fails, try the following
procedure: (This will only work if you have started MySQL with
--log-update
. See section 4.9.3 The Update Log.):
mysqldump
backup.
shell> mysqlbinlog hostname-bin.[0-9]* | mysqlIf you are using the update log you can use:
shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql
ls
is used to get all the update log files in the right order.
You can also do selective backups with SELECT * INTO OUTFILE 'file_name'
FROM tbl_name
and restore with LOAD DATA INFILE 'file_name' REPLACE
...
To avoid duplicate records, you need a PRIMARY KEY
or a
UNIQUE
key in the table. The REPLACE
keyword causes old records
to be replaced with new ones when a new record duplicates an old record on
a unique key value.
If you get performance problems in making backups on your system, you can solve this by setting up replication and do the backups on the slave instead of on the master. See section 4.10.1 Introduction.
If you are using a Veritas file system, you can do:
FLUSH TABLES WITH READ LOCK
mount vxfs snapshot
.
UNLOCK TABLES
BACKUP TABLE
SyntaxBACKUP TABLE tbl_name[,tbl_name...] TO '/path/to/backup/directory'
Make a copy of all the table files to the backup directory that are the
minimum needed to restore it. Currenlty only works for MyISAM
tables. For MyISAM
table, copies .frm
(definition) and
.MYD
(data) files. The index file can be rebuilt from those two.
Before using this command, please see See section 4.4.1 Database Backups.
During the backup, read lock will be held for each table, one at time,
as they are being backed up. If you want to backup several tables as
a snapshot, you must first issue LOCK TABLES
obtaining a read
lock for each table in the group.
The command returns a table with the following columns:
Column | Value |
Table | Table name |
Op | Always ``backup'' |
Msg_type | One of status , error , info or warning .
|
Msg_text | The message. |
Note that BACKUP TABLE
is only available in MySQL
version 3.23.25 and later.
RESTORE TABLE
SyntaxRESTORE TABLE tbl_name[,tbl_name...] FROM '/path/to/backup/directory'
Restores the table(s) from the backup that was made with
BACKUP TABLE
. Existing tables will not be overwritten - if you
try to restore over an existing table, you will get an error. Restore
will take longer than BACKUP due to the need to rebuilt the index. The
more keys you have, the longer it is going to take. Just as
BACKUP TABLE
, currently only works of MyISAM
tables.
The command returns a table with the following columns:
Column | Value |
Table | Table name |
Op | Always ``restore'' |
Msg_type | One of status , error , info or warning .
|
Msg_text | The message. |
CHECK TABLE
SyntaxCHECK TABLE tbl_name[,tbl_name...] [option [option...]] option = QUICK | FAST | MEDIUM | EXTEND | CHANGED
CHECK TABLE
only works on MyISAM
tables. On
MyISAM
tables it's the same thing as running myisamchk -m
table_name
on the table.
If you don't specify any option MEDIUM
is used.
Checks the table(s) for errors. For MyISAM
tables the key statistics
is updated. The command returns a table with the following columns:
Column | Value |
Table | Table name. |
Op | Always ``check''. |
Msg_type | One of status , error , info , or warning .
|
Msg_text | The message. |
Note that you can get many rows of information for each checked
table. The last row will be of Msg_type status
and should
normally be OK
. If you don't get OK
, or Not
checked
you should normally run a repair of the table. See section 4.4.6 Using myisamchk
for Table Maintenance and Crash Recovery. Not checked
means that the table the given TYPE
told MySQL that there wasn't any need to check the table.
The different check types stand for the following:
Type | Meaning |
QUICK | Don't scan the rows to check for wrong links. |
FAST | Only check tables which haven't been closed properly. |
CHANGED | Only check tables which have been changed since last check or haven't been closed properly. |
MEDIUM | Scan rows to verify that deleted links are ok. This also calculates a key checksum for the rows and verifies this with a calcualted checksum for the keys. |
EXTENDED | Do a full key lookup for all keys for each row. This ensures that the table is 100 % consistent, but will take a long time! |
For dynamic sized MyISAM
tables a started check will always
do a MEDIUM
check. For static size rows we skip the row scan
for QUICK
and FAST
as the rows are very seldom corrupted.
You can combine check options as in:
CHECK TABLE test_table FAST QUICK;
Which only would do a quick check on the table if it wasn't closed properly.
NOTE: that in some case CHECK TABLE
will change the
table! This happens if the table is marked as 'corrupted' or 'not
closed properly' but CHECK TABLE
didn't find any problems in the
table. In this case CHECK TABLE
will mark the table as ok.
If a table is corrupted, then it's most likely that the problem is in the indexes and not in the data part. All of the above check types checks the indexes throughly and should thus find most errors.
If you just want to check a table that you assume is ok, you should use
no check options or the QUICK
option. The later should be used
when you are in a hurry and can take the very small risk that
QUICK
didn't find an error in the data file (In most cases
MySQL should find, under normal usage, any error in the data
file. If this happens then the table will be marked as 'corrupted',
in which case the table can't be used until it's repaired).
FAST
and CHANGED
are mostly intended to be used from a
script (for example to be executed from cron) if you want to check your
table from time to time. In most cases you FAST
is to be prefered
over CHANGED
. (The only case when it isn't is when you suspect a
bug you have found a bug in the MyISAM
code.).
EXTENDED
is only to be used after you have run a normal check but
still get strange errors from a table when MySQL tries to
update a row or find a row by key (this is VERY unlikely to happen if a
normal check has succeeded!).
Some things reported by check table, can't be corrected automatically:
Found row where the auto_increment column has the value 0
.
This means that you have in the table a row where the
auto_increment
index column contains the value 0.
(It's possible to create a row where the auto_increment column is 0 by
explicitely setting the column to 0 with an UPDATE
statement)
This isn't an error in itself, but could cause trouble if you decide to
dump the table and restore it or do an ALTER TABLE
on the
table. In this case the auto_increment column will change value,
according to the rules of auto_increment columns, which could cause
problems like a duplicate key error.
To get rid of the warning, just execute an UPDATE
statement
to set the column to some other value than 0.
REPAIR TABLE
SyntaxREPAIR TABLE tbl_name[,tbl_name...] [QUICK] [EXTENDED]
REPAIR TABLE
only works on MyISAM
tables and is the same
as running myisamchk -r table_name
on the table.
Normally you should never have to run this command, but if disaster strikes
you are very likely to get back all your data from a MyISAM table with
REPAIR TABLE
. If your tables get corrupted a lot you should
try to find the reason for this! See section A.2 What to Do if MySQL Keeps Crashing. See section 7.1.3 MyISAM table problems..
REPAIR TABLE
repairs a possible corrupted table. The command returns a
table with the following columns:
Column | Value |
Table | Table name |
Op | Always ``repair'' |
Msg_type | One of status , error , info or warning .
|
Msg_text | The message. |
Note that you can get many rows of information for each repaired
table. The last one row will be of Msg_type status
and should
normally be OK
. If you don't get OK
, you should try
repairing the table with myisamchk -o
, as REPAIR TABLE
does not yet implement all the options of myisamchk
. In the near
future, we will make it more flexible.
If QUICK
is given then MySQL will try to do a
REPAIR
of only the index tree.
If you use EXTENDED
then MySQL will create the index row
by row instead of creating one index at a time with sorting; This may be
better than sorting on fixed-length keys if you have long char()
keys that compress very good.
myisamchk
for Table Maintenance and Crash Recovery
Starting with MySQL Version 3.23.13, you can check MyISAM
tables with the CHECK TABLE
command. See section 4.4.4 CHECK TABLE
Syntax. You can
repair tables with the REPAIR TABLE
command. See section 4.4.5 REPAIR TABLE
Syntax.
To check/repair MyISAM tables (.MYI
and .MYD
) you should
use the myisamchk
utility. To check/repair ISAM tables
(.ISM
and .ISD
) you should use the isamchk
utility. See section 7 MySQL Table Types.
In the following text we will talk about myisamchk
, but everything
also applies to the old isamchk
.
You can use the myisamchk
utility to get information about your
database tables, check and repair them, or optimize them. The following
sections describe how to invoke myisamchk
(including a
description of its options), how to set up a table maintenance schedule,
and how to use myisamchk
to perform its various functions.
You can, in most cases, also use the command OPTIMIZE TABLES
to
optimize and repair tables, but this is not as fast or reliable (in case
of real fatal errors) as myisamchk
. On the other hand,
OPTIMIZE TABLE
is easier to use and you don't have to worry about
flushing tables.
See section 4.5.1 OPTIMIZE TABLE
Syntax.
Even that the repair in myisamchk
is quite secure, it's always a
good idea to make a backup BEFORE doing a repair (or anything that could
make a lot of changes to a table)
myisamchk
Invocation Syntax
myisamchk
is invoked like this:
shell> myisamchk [options] tbl_name
The options
specify what you want myisamchk
to do. They are
described below. (You can also get a list of options by invoking
myisamchk --help
.) With no options, myisamchk
simply checks your
table. To get more information or to tell myisamchk
to take corrective
action, specify options as described below and in the following sections.
tbl_name
is the database table you want to check/repair. If you run
myisamchk
somewhere other than in the database directory, you must
specify the path to the file, because myisamchk
has no idea where your
database is located. Actually, myisamchk
doesn't care whether or not
the files you are working on are located in a database directory; you can
copy the files that correspond to a database table into another location and
perform recovery operations on them there.
You can name several tables on the myisamchk
command line if you
wish. You can also specify a name as an index file
name (with the `.MYI' suffix), which allows you to specify all
tables in a directory by using the pattern `*.MYI'.
For example, if you are in a database directory, you can check all the
tables in the directory like this:
shell> myisamchk *.MYI
If you are not in the database directory, you can check all the tables there by specifying the path to the directory:
shell> myisamchk /path/to/database_dir/*.MYI
You can even check all tables in all databases by specifying a wild card with the path to the MySQL data directory:
shell> myisamchk /path/to/datadir/*/*.MYI
The recommended way to quickly check all tables is:
myisamchk --silent --fast /path/to/datadir/*/*.MYI isamchk --silent /path/to/datadir/*/*.ISM
If you want to check all tables and repair all tables that are corrupted, you can use the following line:
myisamchk --silent --force --fast --update-state -O key_buffer=64M -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.MYI isamchk --silent --force -O key_buffer=64M -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.ISM
The above assumes that you have more than 64 M free.
Note that if you get an error like:
myisamchk: warning: 1 clients is using or hasn't closed the table properly
This means that you are trying to check a table that has been updated by
the another program (like the mysqld
server) that hasn't yet closed
the file or that has died without closing the file properly.
If you mysqld
is running, you must force a sync/close of all
tables with FLUSH TABLES
and ensure that no one is using the
tables while you are running myisamchk
. In MySQL Version 3.23
the easiest way to avoid this problem is to use CHECK TABLE
instead of myisamchk
to check tables.
myisamchk
myisamchk
supports the following options.
-# or --debug=debug_options
debug_options
string often is
'd:t:o,filename'
.
-? or --help
-O var=option, --set-variable var=option
myisamchk --help
:
key_buffer_size | 523264 |
read_buffer_size | 262136 |
write_buffer_size | 262136 |
sort_buffer_size | 2097144 |
sort_key_blocks | 16 |
decode_bits | 9 |
sort_buffer_size
is used when the keys are repaired by sorting
keys, which is the normal case when you use --recover
.
key_buffer_size
is used when you are checking the table with
--extended-check
or when the keys are repaired by inserting key
row by row in to the table (like when doing normal inserts). Repairing
through the key buffer is used in the following cases:
--safe-recover
.
FULLTEXT
index.
CHAR
, VARCHAR
or TEXT
keys as the
sort needs to store the whole keys during sorting. If you have lots
of temporary space and you can force myisamchk
to repair by sorting
you can use the --sort-recover
option.
-s or --silent
-s
twice (-ss
) to make myisamchk
very silent.
-v or --verbose
-d
and
-e
. Use -v
multiple times (-vv
, -vvv
) for more
verbosity!
-V or --version
myisamchk
version and exit.
-w or, --wait
mysqld
on the table with --skip-locking
, the table can only be locked
by another myisamchk
command.
myisamchk
-c or --check
myisamchk
any options that override this.
-e or --extend-check
myisamchk
or myisamchk --medium-check
should, in most
cases, be able to find out if there are any errors in the table.
If you are using --extended-check
and have much memory, you should
increase the value of key_buffer_size
a lot!
-F or --fast
-C or --check-only-changed
-f or --force
myisamchk
with -r
(repair) on the table, if
myisamchk
finds any errors in the table.
-i or --information
-m or --medium-check
-U or --update-state
--check-only-changed
option, but you shouldn't use this
option if the mysqld
server is using the table and you are
running mysqld
with --skip-locking
.
-T or --read-only
myisamchk
to check a table that is in use by some other application that doesn't
use locking (like mysqld --skip-locking
).
The following options are used if you start myisamchk
with
-r
or -o
:
-D # or --data-file-length=#
-e or --extend-check
-f or --force
table_name.TMD
) instead of aborting.
-k # or keys-used=#
#
indexes. If you are using MyISAM
, tells which keys
to use, where each binary bit stands for one key (first key is bit 0).
This can be used to get faster inserts! Deactivated indexes can be
reactivated by using myisamchk -r
. keys.
-l or --no-symlinks
myisamchk
repairs the
table a symlink points at. This option doesn't exist in MySQL 4.0,
as MySQL 4.0 will not remove symlinks during repair.
-r or --recover
-r
, you
should then try -o
. (Note that in the unlikely case that -r
fails, the data file is still intact.)
If you have lots of memory, you should increase the size of
sort_buffer_size
!
-o or --safe-recover
-r
, but can handle a couple of very unlikely cases that
-r
cannot handle. This recovery method also uses much less disk
space than -r
. Normally one should always first repair with
-r
, and only if this fails use -o
.
If you have lots of memory, you should increase the size of
key_buffer_size
!
-n or --sort-recover
myisamchk
to use sorting to resolve the keys even if the
temporary files should be very big. This will not have any effect if you have
fulltext keys in the table.
--character-sets-dir=...
--set-character-set=name
.t or --tmpdir=path
myisamchk
will
use the environment variable TMPDIR
for this.
-q or --quick
-q
to force myisamchk
to modify the original datafile in case
of duplicate keys
-u or --unpack
myisamchk
Other actions that myisamchk
can do, besides repair and check tables:
-a or --analyze
myisamchk --describe --verbose table_name'
or using SHOW KEYS
in
MySQL.
-d or --description
-A or --set-auto-increment[=value]
-S or --sort-index
-R or --sort-records=#
SELECT
and ORDER BY
operations on
this index. (It may be VERY slow to do a sort the first time!)
To find out a table's index numbers, use SHOW INDEX
, which shows a
table's indexes in the same order that myisamchk
sees them. Indexes are
numbered beginning with 1.
myisamchk
Memory Usage
Memory allocation is important when you run myisamchk
.
myisamchk
uses no more memory than you specify with the -O
options. If you are going to use myisamchk
on very large files,
you should first decide how much memory you want it to use. The default
is to use only about 3M to fix things. By using larger values, you can
get myisamchk
to operate faster. For example, if you have more
than 32M RAM, you could use options such as these (in addition to any
other options you might specify):
shell> myisamchk -O sort=16M -O key=16M -O read=1M -O write=1M ...
Using -O sort=16M
should probably be enough for most cases.
Be aware that myisamchk
uses temporary files in TMPDIR
. If
TMPDIR
points to a memory file system, you may easily get out of
memory errors. If this happens, set TMPDIR
to point at some directory
with more space and restart myisamchk
.
When repairing, myisamchk
will also need a lot of disk space:
--quick
, as in this
case only the index file will be re-created. This space is needed on the
same disk as the original record file!
--recover
or --sort-recover
(but not when using --safe-recover
, you will need space for a
sort buffer for:
(largest_key + row_pointer_length)*number_of_rows * 2
.
You can check the length of the keys and the row_pointer_length with
myisamchk -dv table
.
This space is allocated on the temporary disk (specified by TMPDIR
or
--tmpdir=#
).
If you have a problem with disk space during repair, you can try to use
--safe-recover
instead of --recover
.
myisamchk
for Crash Recovery
If you run mysqld
with --skip-locking
(which is the default on
some systems, like Linux), you can't reliably use myisamchk
to
check a table when mysqld
is using the same table. If you
can be sure that no one is accessing the tables through mysqld
while you run myisamchk
, you only have to do mysqladmin
flush-tables
before you start checking the tables. If you can't
guarantee the above, then you must take down mysqld
while you
check the tables. If you run myisamchk
while mysqld
is updating
the tables, you may get a warning that a table is corrupt even if it
isn't.
If you are not using --skip-locking
, you can use myisamchk
to check tables at any time. While you do this, all clients that try
to update the table will wait until myisamchk
is ready before
continuing.
If you use myisamchk
to repair or optimize tables, you
MUST always ensure that the mysqld
server is not using
the table (this also applies if you are using --skip-locking
).
If you don't take down mysqld
you should at least do a
mysqladmin flush-tables
before you run myisamchk
.
This chapter describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted a lot you should try to find the reason for this! See section A.2 What to Do if MySQL Keeps Crashing.
The MyISAM
table section contains reason for why a table could be
corrupted. See section 7.1.3 MyISAM table problems..
When performing crash recovery, it is important to understand that each table
tbl_name
in a database corresponds to three files in the database
directory:
File | Purpose |
`tbl_name.frm' | Table definition (form) file |
`tbl_name.MYD' | Data file |
`tbl_name.MYI' | Index file |
Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.
myisamchk
works by creating a copy of the `.MYD' (data) file
row by row. It ends the repair stage by removing the old `.MYD'
file and renaming the new file to the original file name. If you use
--quick
, myisamchk
does not create a temporary `.MYD'
file, but instead assumes that the `.MYD' file is correct and only
generates a new index file without touching the `.MYD' file. This
is safe, because myisamchk
automatically detects if the
`.MYD' file is corrupt and aborts the repair in this case. You can
also give two --quick
options to myisamchk
. In this case,
myisamchk
does not abort on some errors (like duplicate key) but
instead tries to resolve them by modifying the `.MYD'
file. Normally the use of two --quick
options is useful only if
you have too little free disk space to perform a normal repair. In this
case you should at least make a backup before running myisamchk
.
To check a MyISAM table, use the following commands:
myisamchk tbl_name
myisamchk
without options or
with either the -s
or --silent
option.
myisamchk -m tbl_name
myisamchk -e tbl_name
-e
means
``extended check''). It does a check-read of every key for each row to verify
that they indeed point to the correct row. This may take a LONG time on a
big table with many keys. myisamchk
will normally stop after the first
error it finds. If you want to obtain more information, you can add the
--verbose
(-v
) option. This causes myisamchk
to keep
going, up through a maximum of 20 errors. In normal usage, a simple
myisamchk
(with no arguments other than the table name) is sufficient.
myisamchk -e -i tbl_name
-i
option tells myisamchk
to
print some informational statistics, too.
In the following section we only talk about using myisamchk
on
MyISAM
tables (extensions .MYI
and .MYD
). If you
are using ISAM
tables (extensions .ISM
and .ISD
),
you should use isamchk
instead.
Starting with MySQL Version 3.23.14, you can repair MyISAM
tables with the REPAIR TABLE
command. See section 4.4.5 REPAIR TABLE
Syntax.
The symptoms of a corrupted table include queries that abort unexpectedly and observable errors such as these:
perror ###
. Here
is the most common errors that indicates a problem with the table:
shell> perror 126 127 132 134 135 136 141 144 145 126 = Index file is crashed / Wrong file format 127 = Record-file is crashed 132 = Old database file 134 = Record was already deleted (or record file crashed) 135 = No more room in record file 136 = No more room in index file 141 = Duplicate unique key or constraint on write or update 144 = Table is crashed and last repair failed 145 = Table was marked as crashed and should be repairedNote that error 135, no more room in record file, is not an error that can be fixed by a simple repair. In this case you have to do:
ALTER TABLE table MAX_ROWS=xxx AVG_ROW_LENGTH=yyy;
In the other cases, you must repair your tables. myisamchk
can usually detect and fix most things that go wrong.
The repair process involves up to four stages, described below. Before you
begin, you should cd
to the database directory and check the
permissions of the table files. Make sure they are readable by the Unix user
that mysqld
runs as (and to you, because you need to access the files
you are checking). If it turns out you need to modify files, they must also
be writable by you.
If you are using MySQL Version 3.23.16 and above, you can (and
should) use the CHECK
and REPAIR
commands to check and repair
MyISAM
tables. See section 4.4.4 CHECK TABLE
Syntax. See section 4.4.5 REPAIR TABLE
Syntax.
The manual section about table maintenance includes the options to
isamchk
/myisamchk
. See section 4.4.6 Using myisamchk
for Table Maintenance and Crash Recovery.
The following section is for the cases where the above command fails or
if you want to use the extended features that isamchk
/myisamchk
provides.
If you are going to repair a table from the command line, you must first
take down the mysqld
server. Note that when you do
mysqladmin shutdown
on a remote server, the mysqld
server
will still be alive for a while after mysqladmin
returns, until
all queries are stopped and all keys have been flushed to disk.
Stage 1: Checking your tables
Run myisamchk *.MYI
or myisamchk -e *.MYI
if you have
more time. Use the -s
(silent) option to suppress unnecessary
information.
If the mysqld
server is done you should use the --update option to tell
myisamchk
to mark the table as 'checked'.
You have to repair only those tables for which myisamchk
announces an
error. For such tables, proceed to Stage 2.
If you get weird errors when checking (such as out of
memory
errors), or if myisamchk
crashes, go to Stage 3.
Stage 2: Easy safe repair
NOTE: If you want repairing to go much faster, you should add: -O
sort_buffer=# -O key_buffer=#
(where # is about 1/4 of the available
memory) to all isamchk/myisamchk
commands.
First, try myisamchk -r -q tbl_name
(-r -q
means ``quick
recovery mode''). This will attempt to repair the index file without
touching the data file. If the data file contains everything that it
should and the delete links point at the correct locations within the
data file, this should work, and the table is fixed. Start repairing the
next table. Otherwise, use the following procedure:
myisamchk -r tbl_name
(-r
means ``recovery mode''). This will
remove incorrect records and deleted records from the data file and
reconstruct the index file.
myisamchk --safe-recover tbl_name
.
Safe recovery mode uses an old recovery method that handles a few cases that
regular recovery mode doesn't (but is slower).
If you get weird errors when repairing (such as out of
memory
errors), or if myisamchk
crashes, go to Stage 3.
Stage 3: Difficult repair
You should only reach this stage if the first 16K block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:
shell> mysql db_name mysql> SET AUTOCOMMIT=1; mysql> TRUNCATE TABLE table_name; mysql> quitIf your SQL version doesn't have
TRUNCATE TABLE
, use DELETE FROM
table_name
instead.
Go back to Stage 2. myisamchk -r -q
should work now. (This shouldn't
be an endless loop.)
Stage 4: Very difficult repair
You should reach this stage only if the description file has also crashed. That should never happen, because the description file isn't changed after the table is created:
myisamchk -r
.
To coalesce fragmented records and eliminate wasted space resulting from
deleting or updating records, run myisamchk
in recovery mode:
shell> myisamchk -r tbl_name
You can optimize a table in the same way using the SQL OPTIMIZE TABLE
statement. OPTIMIZE TABLE
does a repair of the table, a key
analyzes and also sorts the index tree to give faster key lookups.
There is also no possibility of unwanted interaction between a utility
and the server, because the server does all the work when you use
OPTIMIZE TABLE
. See section 4.5.1 OPTIMIZE TABLE
Syntax.
myisamchk
also has a number of other options you can use to improve
the performance of a table:
-S, --sort-index
-R index_num, --sort-records=index_num
-a, --analyze
For a full description of the option. See section 4.4.6.1 myisamchk
Invocation Syntax.
Starting with MySQL Version 3.23.13, you can check MyISAM
tables with the CHECK TABLE
command. See section 4.4.4 CHECK TABLE
Syntax. You can
repair tables with the REPAIR TABLE
command. See section 4.4.5 REPAIR TABLE
Syntax.
It is a good idea to perform table checks on a regular basis rather than
waiting for problems to occur. For maintenance purposes, you can use
myisamchk -s
to check tables. The -s
option (short for
--silent
) causes myisamchk
to run in silent mode, printing
messages only when errors occur.
It's also a good idea to check tables when the server starts up.
For example, whenever the machine has done a reboot in the middle of an
update, you usually need to check all the tables that could have been
affected. (This is an ``expected crashed table''.) You could add a test to
safe_mysqld
that runs myisamchk
to check all tables that have
been modified during the last 24 hours if there is an old `.pid'
(process ID) file left after a reboot. (The `.pid' file is created by
mysqld
when it starts up and removed when it terminates normally. The
presence of a `.pid' file at system startup time indicates that
mysqld
terminated abnormally.)
An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file.
You should also check your tables regularly during normal system
operation. At MySQL AB, we run a cron
job to check all
our important tables once a week, using a line like this in a `crontab'
file:
35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI
This prints out information about crashed tables so we can examine and repair them when needed.
As we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.
We recommend that to start with, you execute myisamchk -s
each
night on all tables that have been updated during the last 24 hours,
until you come to trust MySQL as much as we do.
Normally you don't need to maintain MySQL tables that much. If
you are changing tables with dynamic size rows (tables with VARCHAR
,
BLOB
or TEXT
columns) or have tables with many deleted rows
you may want to from time to time (once a month?) defragment/reclaim space
from the tables.
You can do this by using OPTIMIZE TABLE
on the tables in question or
if you can take the mysqld
server down for a while do:
isamchk -r --silent --sort-index -O sort_buffer_size=16M */*.ISM myisamchk -r --silent --sort-index -O sort_buffer_size=16M */*.MYI
To get a description of a table or statistics about it, use the commands shown below. We explain some of the information in more detail later:
myisamchk -d tbl_name
myisamchk
in ``describe mode'' to produce a description of
your table. If you start the MySQL server using the
--skip-locking
option, myisamchk
may report an error for a
table that is updated while it runs. However, because myisamchk
doesn't change the table in describe mode, there isn't any risk of
destroying data.
myisamchk -d -v tbl_name
myisamchk
is doing, add -v
to tell it to run in verbose mode.
myisamchk -eis tbl_name
myisamchk -eiv tbl_name
-eis
, but tells you what is being done.
Example of myisamchk -d
output:
MyISAM file: company.MYI Record format: Fixed length Data records: 1403698 Deleted blocks: 0 Recordlength: 226 table description: Key Start Len Index Type 1 2 8 unique double 2 15 10 multip. text packed stripped 3 219 8 multip. double 4 63 10 multip. text packed stripped 5 167 2 multip. unsigned short 6 177 4 multip. unsigned long 7 155 4 multip. text 8 138 4 multip. unsigned long 9 177 4 multip. unsigned long 193 1 text
Example of myisamchk -d -v
output:
MyISAM file: company Record format: Fixed length File-version: 1 Creation time: 1999-10-30 12:12:51 Recover time: 1999-10-31 19:13:01 Status: checked Data records: 1403698 Deleted blocks: 0 Datafile parts: 1403698 Deleted data: 0 Datafilepointer (bytes): 3 Keyfile pointer (bytes): 3 Max datafile length: 3791650815 Max keyfile length: 4294967294 Recordlength: 226 table description: Key Start Len Index Type Rec/key Root Blocksize 1 2 8 unique double 1 15845376 1024 2 15 10 multip. text packed stripped 2 25062400 1024 3 219 8 multip. double 73 40907776 1024 4 63 10 multip. text packed stripped 5 48097280 1024 5 167 2 multip. unsigned short 4840 55200768 1024 6 177 4 multip. unsigned long 1346 65145856 1024 7 155 4 multip. text 4995 75090944 1024 8 138 4 multip. unsigned long 87 85036032 1024 9 177 4 multip. unsigned long 178 96481280 1024 193 1 text
Example of myisamchk -eis
output:
Checking MyISAM file: company Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4 Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4 Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4 Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3 Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4 Total: Keyblocks used: 98% Packed: 17% Records: 1403698 M.recordlength: 226 Packed: 0% Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00 Record blocks: 1403698 Delete blocks: 0 Recorddata: 317235748 Deleted data: 0 Lost space: 0 Linkdata: 0 User time 1626.51, System time 232.36 Maximum resident set size 0, Integral resident set size 0 Non physical pagefaults 0, Physical pagefaults 627, Swaps 0 Blocks in 0 out 0, Messages in 0 out 0, Signals 0 Voluntary context switches 639, Involuntary context switches 28966
Example of myisamchk -eiv
output:
Checking MyISAM file: company Data records: 1403698 Deleted blocks: 0 - check file-size - check delete-chain block_size 1024: index 1: index 2: index 3: index 4: index 5: index 6: index 7: index 8: index 9: No recordlinks - check index reference - check data record references index: 1 Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4 - check data record references index: 2 Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4 - check data record references index: 3 Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4 - check data record references index: 4 Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3 - check data record references index: 5 Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 6 Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 7 Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 8 Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 9 Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4 Total: Keyblocks used: 9% Packed: 17% - check records and index references [LOTS OF ROW NUMBERS DELETED] Records: 1403698 M.recordlength: 226 Packed: 0% Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00 Record blocks: 1403698 Delete blocks: 0 Recorddata: 317235748 Deleted data: 0 Lost space: 0 Linkdata: 0 User time 1639.63, System time 251.61 Maximum resident set size 0, Integral resident set size 0 Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0 Blocks in 4 out 0, Messages in 0 out 0, Signals 0 Voluntary context switches 10604, Involuntary context switches 122798
Here are the sizes of the data and index files for the table used in the preceding examples:
-rw-rw-r-- 1 monty tcx 317235748 Jan 12 17:30 company.MYD -rw-rw-r-- 1 davida tcx 96482304 Jan 12 18:35 company.MYM
Explanations for the types of information myisamchk
produces are
given below. The ``keyfile'' is the index file. ``Record'' and ``row''
are synonymous:
ISAM file
Isam-version
Creation time
Recover time
Data records
Deleted blocks
Datafile: Parts
Data
records
.
Deleted data
Datafile pointer
Keyfile pointer
Max datafile length
.MYD
file) can become, in bytes.
Max keyfile length
.MYI
file) can become, in bytes.
Recordlength
Record format
Fixed length
.
Other possible values are Compressed
and Packed
.
table description
Key
Start
Len
Index
unique
or multip.
(multiple). Indicates whether or not one value
can exist multiple times in this index.
Type
packed
, stripped
or empty
.
Root
Blocksize
Rec/key
myisamchk -a
. If this is not updated at all, a default
value of 30 is given.
Keyblocks used
myisamchk
, the values are very
high (very near the theoretical maximum).
Packed
CHAR
/VARCHAR
/DECIMAL
keys. For long strings like
names, this can significantly reduce the space used. In the third example
above, the 4th key is 10 characters long and a 60% reduction in space is
achieved.
Max levels
Records
M.recordlength
Packed
Packed
value indicates the percentage of savings achieved by doing this.
Recordspace used
Empty space
Blocks/Record
myisamchk
.
See section 4.4.6.10 Table Optimization.
Recordblocks
Deleteblocks
Recorddata
Deleted data
Lost space
Linkdata
Linkdata
is the sum of the amount of
storage used by all such pointers.
If a table has been compressed with myisampack
, myisamchk
-d
prints additional information about each table column. See
section 4.7.4 myisampack, The MySQL Compressed Read-only Table Generator, for an example of this
information and a description of what it means.
OPTIMIZE TABLE
SyntaxOPTIMIZE TABLE tbl_name[,tbl_name]...
OPTIMIZE TABLE
should be used if you have deleted a large part of a
table or if you have made many changes to a table with variable-length rows
(tables that have VARCHAR
, BLOB
, or TEXT
columns).
Deleted records are maintained in a linked list and subsequent INSERT
operations reuse old record positions. You can use OPTIMIZE TABLE
to
reclaim the unused space and to defragment the data file.
For the moment OPTIMIZE TABLE
only works on MyISAM and
BDB
tables. For BDB
tables, OPTIMIZE TABLE
is
currently mapped to ANALYZE TABLE
. See section 4.5.2 ANALYZE TABLE
Syntax.
You can get optimize table to work on other table types by starting
mysqld
with --skip-new
or --safe-mode
, but in this
case OPTIMIZE TABLE
is just mapped to ALTER TABLE
.
OPTIMIZE TABLE
works the following way:
OPTIMIZE TABLE
for MyISAM
tables is equvialent of running
myisamchk --quick --check-changed-tables --sort-index --analyze
on the table.
Note that the table is locked during the time OPTIMIZE TABLE
is
running!
ANALYZE TABLE
SyntaxANALYZE TABLE tbl_name[,tbl_name...]
Analyze and store the key distribution for the table. During the
analyze the table is locked with a read lock. This works on
MyISAM
and BDB
tables.
This is equivalent to running myisamchk -a
on the table.
MySQL uses the stored key distribution to decide in which order tables should be joined when one does a join on something else than a constant.
The command returns a table with the following columns:
Column | Value |
Table | Table name |
Op | Always ``analyze'' |
Msg_type | One of status , error , info or warning .
|
Msg_text | The message. |
You can check the stored key distribution with the SHOW INDEX
command.
See section 4.5.5.1 Retrieving information about Database, Tables, Columns, and Indexes.
If the table hasn't changed since the last ANALYZE TABLE
command,
the table will not be analyzed again.
FLUSH
SyntaxFLUSH flush_option [,flush_option]
You should use the FLUSH
command if you want to clear some of the
internal caches MySQL uses. To execute FLUSH
, you must have
the RELOAD privilege.
flush_option
can be any of the following:
HOSTS | Empties the host cache tables. You should flush the
host tables if some of your hosts change IP number or if you get the
error message Host ... is blocked . When more than
max_connect_errors errors occur in a row for a given host while
connection to the MySQL server, MySQL assumes
something is wrong and blocks the host from further connection requests.
Flushing the host tables allows the host to attempt to connect
again. See section A.4.4 Host '...' is blocked Error.) You can start mysqld with
-O max_connection_errors=999999999 to avoid this error message.
|
LOGS | Closes and reopens all log files.
If you have specified the update log file or a binary log file without
an extension, the extension number of the log file will be incremented
by one relative to the previous file. If you have used an extension in
the file name, MySQL will close and reopen the update log file.
See section 4.9.3 The Update Log. This is the same thing as sending the SIGHUP
signal to the mysqld server.
|
PRIVILEGES | Reloads the privileges from the grant tables in
the mysql database.
|
TABLES | Closes all open tables and force all tables in use to be closed. |
[TABLE | TABLES] table_name [,table_name...] | Flushes only the given tables. |
TABLES WITH READ LOCK | Closes all open tables and locks all tables for all databases with a read until one executes UNLOCK TABLES . This is very convenient way to get backups if you have a file system, like Veritas,that can take snapshots in time.
|
STATUS | Resets most status variables to zero. This is something one should only use when debugging a query. |
You can also access each of the commands shown above with the mysqladmin
utility, using the flush-hosts
, flush-logs
, reload
,
or flush-tables
commands.
Take also a look at the RESET
command used with
replication. See section 4.10.6 SQL Commands Related to Replication.
KILL
SyntaxKILL thread_id
Each connection to mysqld
runs in a separate thread. You can see
which threads are running with the SHOW PROCESSLIST
command and kill
a thread with the KILL thread_id
command.
If you have the process privilege, you can see and kill all threads. Otherwise, you can see and kill only your own threads.
You can also use the mysqladmin processlist
and mysqladmin kill
commands to examine and kill threads.
When you do a KILL
, a thread specific kill flag
is set for
the thread.
In most cases it may take some time for the thread to die as the kill flag is only checked at specific intervals.
SELECT
, ORDER BY
and GROUP BY
loops, the flag is
checked after reading a block of rows. If the kill flag is set the
statement is aborted
ALTER TABLE
the kill flag is checked before each block of
rows are read from the original table. If the kill flag was set the command
is aborted and the temporary table is deleted.
UPDATE TABLE
and DELETE TABLE
, the kill flag
is checked after each block read and after each updated or delete
row. If the kill flag is set the statement is aborted. Note that if you
are not using transactions, the changes will not be rolled back!
GET_LOCK()
will abort with NULL
.
INSERT DELAYED
thread will quickly flush all rows it has in
memory and die.
Locked
),
the table lock will be quickly aborted.
write
call, the
write is aborted with an disk full error message.
SHOW
SyntaxSHOW DATABASES [LIKE wild] or SHOW [OPEN] TABLES [FROM db_name] [LIKE wild] or SHOW [FULL] COLUMNS FROM tbl_name [FROM db_name] [LIKE wild] or SHOW INDEX FROM tbl_name [FROM db_name] or SHOW TABLE STATUS [FROM db_name] [LIKE wild] or SHOW STATUS [LIKE wild] or SHOW VARIABLES [LIKE wild] or SHOW LOGS or SHOW [FULL] PROCESSLIST or SHOW GRANTS FOR user or SHOW CREATE TABLE table_name or SHOW MASTER STATUS or SHOW MASTER LOGS or SHOW SLAVE STATUS
SHOW
provides information about databases, tables, columns, or
status information about the server. If the LIKE wild
part is
used, the wild
string can be a string that uses the SQL `%'
and `_' wild-card characters.
You can use db_name.tbl_name
as an alternative to the tbl_name
FROM db_name
syntax. These two statements are equivalent:
mysql> SHOW INDEX FROM mytable FROM mydb; mysql> SHOW INDEX FROM mydb.mytable;
SHOW DATABASES
lists the databases on the MySQL server
host. You can also get this list using the mysqlshow
command.
SHOW TABLES
lists the tables in a given database. You can also
get this list using the mysqlshow db_name
command.
NOTE: If a user doesn't have any privileges for a table, the table
will not show up in the output from SHOW TABLES
or mysqlshow
db_name
.
SHOW OPEN TABLES
lists the tables that are currently open in
the table cache. See section 5.4.6 How MySQL Opens and Closes Tables. The Comment
field tells
how many times the table is cached
and in_use
.
SHOW COLUMNS
lists the columns in a given table. If you specify
the FULL
option, you will also get the privileges you have for
each column. If the column types are different than you expect them to
be based on a CREATE TABLE
statement, note that MySQL
sometimes changes column types. See section 6.5.3.1 Silent Column Specification Changes.
The DESCRIBE
statement provides information similar to
SHOW COLUMNS
.
See section 6.6.2 DESCRIBE
Syntax (Get Information About Columns).
SHOW FIELDS
is a synonym for SHOW COLUMNS
, and
SHOW KEYS
is a synonym for SHOW INDEX
. You can also
list a table's columns or indexes with mysqlshow db_name tbl_name
or mysqlshow -k db_name tbl_name
.
SHOW INDEX
returns the index information in a format that closely
resembles the SQLStatistics
call in ODBC. The following columns
are returned:
Column | Meaning |
Table | Name of the table. |
Non_unique | 0 if the index can't contain duplicates. |
Key_name | Name of the index. |
Seq_in_index | Column sequence number in index, starting with 1. |
Column_name | Column name. |
Collation | How the column is sorted in the index.
In MySQL, this can have values
`A' (Ascending) or NULL (Not
sorted).
|
Cardinality | Number of unique values in the index.
This is updated by running
isamchk -a .
|
Sub_part | Number of indexed characters if the
column is only partly indexed.
NULL if the entire key is indexed.
|
Comment | Various remarks. For now, it tells whether index is FULLTEXT or not. |
Note that as the Cardinality
is counted based on statistics
stored as integers, it's not necessarily accurate for small tables.
SHOW TABLE STATUS
SHOW TABLE STATUS [FROM db_name] [LIKE wild]
SHOW TABLE STATUS
(new in Version 3.23) works likes SHOW
STATUS
, but provides a lot of information about each table. You can
also get this list using the mysqlshow --status db_name
command.
The following columns are returned:
Column | Meaning |
Name | Name of the table. |
Type | Type of table. See section 7 MySQL Table Types. |
Row_format | The row storage format (Fixed, Dynamic, or Compressed). |
Rows | Number of rows. |
Avg_row_length | Average row length. |
Data_length | Length of the data file. |
Max_data_length | Max length of the data file. |
Index_length | Length of the index file. |
Data_free | Number of allocated but not used bytes. |
Auto_increment | Next autoincrement value. |
Create_time | When the table was created. |
Update_time | When the data file was last updated. |
Check_time | When the table was last checked. |
Create_options | Extra options used with CREATE TABLE .
|
Comment | The comment used when creating the table (or some information why MySQL couldn't access the table information). |
InnoDB
tables will report the free space in the tablespace
in the table comment.
SHOW STATUS
SHOW STATUS
provides server status information
(like mysqladmin extended-status
). The output resembles that shown
below, though the format and numbers probably differ:
+--------------------------+------------+ | Variable_name | Value | +--------------------------+------------+ | Aborted_clients | 0 | | Aborted_connects | 0 | | Bytes_received | 155372598 | | Bytes_sent | 1176560426 | | Connections | 30023 | | Created_tmp_disk_tables | 0 | | Created_tmp_tables | 8340 | | Created_tmp_files | 60 | | Delayed_insert_threads | 0 | | Delayed_writes | 0 | | Delayed_errors | 0 | | Flush_commands | 1 | | Handler_delete | 462604 | | Handler_read_first | 105881 | | Handler_read_key | 27820558 | | Handler_read_next | 390681754 | | Handler_read_prev | 6022500 | | Handler_read_rnd | 30546748 | | Handler_read_rnd_next | 246216530 | | Handler_update | 16945404 | | Handler_write | 60356676 | | Key_blocks_used | 14955 | | Key_read_requests | 96854827 | | Key_reads | 162040 | | Key_write_requests | 7589728 | | Key_writes | 3813196 | | Max_used_connections | 0 | | Not_flushed_key_blocks | 0 | | Not_flushed_delayed_rows | 0 | | Open_tables | 1 | | Open_files | 2 | | Open_streams | 0 | | Opened_tables | 44600 | | Questions | 2026873 | | Select_full_join | 0 | | Select_full_range_join | 0 | | Select_range | 99646 | | Select_range_check | 0 | | Select_scan | 30802 | | Slave_running | OFF | | Slave_open_temp_tables | 0 | | Slow_launch_threads | 0 | | Slow_queries | 0 | | Sort_merge_passes | 30 | | Sort_range | 500 | | Sort_rows | 30296250 | | Sort_scan | 4650 | | Table_locks_immediate | 1920382 | | Table_locks_waited | 0 | | Threads_cached | 0 | | Threads_created | 30022 | | Threads_connected | 1 | | Threads_running | 1 | | Uptime | 80380 | +--------------------------+------------+
The status variables listed above have the following meaning:
Variable | Meaning |
Aborted_clients | Number of connections aborted because the client died without closing the connection properly. See section A.4.9 Communication Errors / Aborted Connection. |
Aborted_connects | Number of tries to connect to the MySQL server that failed. See section A.4.9 Communication Errors / Aborted Connection. |
Bytes_received | Number of bytes received from all clients. |
Bytes_sent | Number of bytes sent to all clients. |
Connections | Number of connection attempts to the MySQL server. |
Created_tmp_disk_tables | Number of implicit temporary tables on disk created while executing statements. |
Created_tmp_tables | Number of implicit temporary tables in memory created while executing statements. |
Created_tmp_files | How many temporary files mysqld have created.
|
Delayed_insert_threads | Number of delayed insert handler threads in use. |
Delayed_writes | Number of rows written with INSERT DELAYED .
|
Delayed_errors | Number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key ).
|
Flush_commands | Number of executed FLUSH commands.
|
Handler_delete | Number of times a row was deleted from a table. |
Handler_read_first | Number of times the first entry was read from an index.
If this is high, it suggests that the server is doing a lot of full index scans, for example,
SELECT col1 FROM foo , assuming that col1 is indexed.
|
Handler_read_key | Number of requests to read a row based on a key. If this is high, it is a good indication that your queries and tables are properly indexed. |
Handler_read_next | Number of requests to read next row in key order. This will be incremented if you are querying an index column with a range constraint. This also will be incremented if you are doing an index scan. |
Handler_read_rnd | Number of requests to read a row based on a fixed position. This will be high if you are doing a lot of queries that require sorting of the result. |
Handler_read_rnd_next | Number of requests to read the next row in the datafile. This will be high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have. |
Handler_update | Number of requests to update a row in a table. |
Handler_write | Number of requests to insert a row in a table. |
Key_blocks_used | The number of used blocks in the key cache. |
Key_read_requests | The number of requests to read a key block from the cache. |
Key_reads | The number of physical reads of a key block from disk. |
Key_write_requests | The number of requests to write a key block to the cache. |
Key_writes | The number of physical writes of a key block to disk. |
Max_used_connections | The maximum number of connections in use simultaneously. |
Not_flushed_key_blocks | Keys blocks in the key cache that has changed but hasn't yet been flushed to disk. |
Not_flushed_delayed_rows | Number of rows waiting to be written in INSERT DELAY queues.
|
Open_tables | Number of tables that are open. |
Open_files | Number of files that are open. |
Open_streams | Number of streams that are open (used mainly for logging). |
Opened_tables | Number of tables that have been opened. |
Select_full_join | Number of joins without keys (Should be 0). |
Select_full_range_join | Number of joins where we used a range search on reference table. |
Select_range | Number of joins where we used ranges on the first table. (It's normally not critical even if this is big.) |
Select_scan | Number of joins where we scanned the first table. |
Select_range_check | Number of joins without keys where we check for key usage after each row (Should be 0). |
Questions | Number of queries sent to the server. |
Slave_open_temp_tables | Number of temporary tables currently open by the slave thread |
Slow_launch_threads | Number of threads that have taken more than slow_launch_time to connect.
|
Slow_queries | Number of queries that have taken more than long_query_time . See section 4.9.5 The Slow Query Log.
|
Sort_merge_passes | Number of merges the sort has to do. If this value is large you should consider increasing sort_buffer .
|
Sort_range | Number of sorts that where done with ranges. |
Sort_rows | Number of sorted rows. |
Sort_scan | Number of sorts that where done by scanning the table. |
Table_locks_immediate | Number of times a table lock was acquired immediately. Available after 3.23.33. |
Table_locks_waited | Number of times a table lock could not be acquired immediately and a wait was needed. If this is high, and you have performance problems, you should first optimize your queries, and then either split your table(s) or use replication. Available after 3.23.33. |
Threads_cached | Number of threads in the thread cache. |
Threads_connected | Number of currently open connections. |
Threads_created | Number of threads created to handle connections. |
Threads_running | Number of threads that are not sleeping. |
Uptime | How many seconds the server has been up. |
Some comments about the above:
Opened_tables
is big, then your table_cache
variable is probably too small.
key_reads
is big, then your key_cache
is probably too
small. The cache hit rate can be calculated with
key_reads
/key_read_requests
.
Handler_read_rnd
is big, then you probably have a lot of
queries that require MySQL to scan whole tables or you have
joins that don't use keys properly.
Threads_created
is big, you may want to increase the
thread_cache_size
variable.
SHOW VARIABLES
SHOW VARIABLES [LIKE wild]
SHOW VARIABLES
shows the values of some MySQL system
variables. You can also get this information using the mysqladmin
variables
command. If the default values are unsuitable, you can set most
of these variables using command-line options when mysqld
starts up.
See section 4.1.1 mysqld Command-line Options.
The output resembles that shown below, though the format and numbers may differ somewhat:
+-------------------------+---------------------------+ | Variable_name | Value | +-------------------------+---------------------------+ | ansi_mode | OFF | | back_log | 50 | | basedir | /my/monty/ | | bdb_cache_size | 16777216 | | bdb_log_buffer_size | 32768 | | bdb_home | /my/monty/data/ | | bdb_max_lock | 10000 | | bdb_logdir | | | bdb_shared_data | OFF | | bdb_tmpdir | /tmp/ | | binlog_cache_size | 32768 | | concurrent_insert | ON | | connect_timeout | 5 | | datadir | /my/monty/data/ | | delay_key_write | ON | | delayed_insert_limit | 100 | | delayed_insert_timeout | 300 | | delayed_queue_size | 1000 | | flush | OFF | | flush_time | 0 | | have_bdb | YES | | have_innodb | YES | | have_raid | YES | | have_ssl | NO | | init_file | | | interactive_timeout | 28800 | | join_buffer_size | 131072 | | key_buffer_size | 16776192 | | language | /my/monty/share/english/ | | large_files_support | ON | | log | OFF | | log_update | OFF | | log_bin | OFF | | log_slave_updates | OFF | | long_query_time | 10 | | low_priority_updates | OFF | | lower_case_table_names | 0 | | max_allowed_packet | 1048576 | | max_binlog_cache_size | 4294967295 | | max_connections | 100 | | max_connect_errors | 10 | | max_delayed_threads | 20 | | max_heap_table_size | 16777216 | | max_join_size | 4294967295 | | max_sort_length | 1024 | | max_tmp_tables | 32 | | max_write_lock_count | 4294967295 | | myisam_recover_options | DEFAULT | | myisam_sort_buffer_size | 8388608 | | net_buffer_length | 16384 | | net_read_timeout | 30 | | net_retry_count | 10 | | net_write_timeout | 60 | | open_files_limit | 0 | | pid_file | /my/monty/data/donna.pid | | port | 3306 | | protocol_version | 10 | | record_buffer | 131072 | | query_buffer_size | 0 | | safe_show_database | OFF | | server_id | 0 | | skip_locking | ON | | skip_networking | OFF | | skip_show_database | OFF | | slow_launch_time | 2 | | socket | /tmp/mysql.sock | | sort_buffer | 2097116 | | table_cache | 64 | | table_type | MYISAM | | thread_cache_size | 4 | | thread_stack | 65536 | | tmp_table_size | 1048576 | | tmpdir | /tmp/ | | version | 3.23.29a-gamma-debug | | wait_timeout | 28800 | +-------------------------+---------------------------+
Each option is described below. Values for buffer sizes, lengths, and stack
sizes are given in bytes. You can specify values with a suffix of `K'
or `M' to indicate kilobytes or megabytes. For example, 16M
indicates 16 megabytes. The case of suffix letters does not matter;
16M
and 16m
are equivalent:
ansi_mode
.
ON
if mysqld
was started with --ansi
.
See section 1.4.3 Running MySQL in ANSI Mode.
back_log
back_log
value indicates how many requests can be
stacked during this short time before MySQL momentarily stops
answering new requests. You need to increase this only if you expect a large
number of connections in a short period of time.
In other words, this value is the size of the listen queue for incoming
TCP/IP connections. Your operating system has its own limit on the size
of this queue. The manual page for the Unix listen(2)
system
call should have more details. Check your OS documentation for the
maximum value for this variable. Attempting to set back_log
higher than your operating system limit will be ineffective.
basedir
--basedir
option.
bdb_cache_size
BDB
tables. If you don't use BDB
tables, you should start
mysqld
with --skip-bdb
to not waste memory for this
cache.
bdb_log_buffer_size
BDB
tables. If you don't use BDB
tables, you should set this to 0 or
start mysqld
with --skip-bdb
to not waste memory for this
cache.
bdb_home
--bdb-home
option.
bdb_max_lock
bdb:
Lock table is out of available locks
or Got error 12 from ...
when you have do long transactions or when mysqld
has to examine
a lot of rows to calculate the query.
bdb_logdir
--bdb-logdir
option.
bdb_shared_data
ON
if you are using --bdb-shared-data
.
bdb_tmpdir
--bdb-tmpdir
option.
binlog_cache_size
. The size of the cache to hold the SQL
BEGIN/COMMIT/ROLLBACK
Syntax.
character_set
character_sets
concurrent_inserts
ON
(the default), MySQL will allow you to use INSERT
on MyISAM
tables at the same time as you run SELECT
queries
on them. You can turn this option off by starting mysqld
with --safe
or --skip-new
.
connect_timeout
mysqld
server is waiting for a connect
packet before responding with Bad handshake
.
datadir
--datadir
option.
delay_key_write
delay_key_write
option CREATE TABLE
. This means that the
key buffer for tables with this option will not get flushed on every
index update, but only when a table is closed. This will speed up
writes on keys a lot, but you should add automatic checking of all tables
with myisamchk --fast --force
if you use this. Note that if you
start mysqld
with the --delay-key-write-for-all-tables
option this means that all tables will be treated as if they were
created with the delay_key_write
option. You can clear this flag
by starting mysqld
with --skip-new
or --safe-mode
.
delayed_insert_limit
delayed_insert_limit
rows, the INSERT
DELAYED
handler will check if there are any SELECT
statements
pending. If so, it allows these to execute before continuing.
delayed_insert_timeout
INSERT DELAYED
thread should wait for INSERT
statements before terminating.
delayed_queue_size
INSERT
DELAYED
. If the queue becomes full, any client that does INSERT
DELAYED
will wait until there is room in the queue again.
flush
ON
if you have started MySQL with the --flush
option.
flush_time
flush_time
seconds all
tables will be closed (to free up resources and sync things to disk). We
only recommend this option on Win95, Win98, or on systems where you have
very little resources.
have_bdb
YES
if mysqld
supports Berkeley DB tables. DISABLED
if --skip-bdb
is used.
have_innodb
YES
if mysqld
supports InnoDB tables. DISABLED
if --skip-innodb
is used.
have_raid
YES
if mysqld
supports the RAID
option.
have_ssl
YES
if mysqld
supports SSL (encryption) on the client/server
protocol.
init_file
--init-file
option when
you start the server. This is a file of SQL statements you want the
server to execute when it starts.
interactive_timeout
CLIENT_INTERACTIVE
option to
mysql_real_connect()
. See also wait_timeout
.
join_buffer_size
key_buffer_size
key_buffer_size
is the size of the buffer used for index blocks.
Increase this to get better index handling (for all reads and multiple
writes) to as much as you can afford; 64M on a 256M machine that mainly
runs MySQL is quite common. If you, however, make this too big
(more than 50% of your total memory?) your system may start to page and
become REALLY slow. Remember that because MySQL does not cache
data read, that you will have to leave some room for the OS filesystem
cache.
You can check the performance of the key buffer by doing show
status
and examine the variables Key_read_requests
,
Key_reads
, Key_write_requests
, and Key_writes
. The
Key_reads/Key_read_request
ratio should normally be < 0.01.
The Key_write/Key_write_requests
is usually near 1 if you are
using mostly updates/deletes but may be much smaller if you tend to
do updates that affect many at the same time or if you are
using delay_key_write
. See section 4.5.5 SHOW
Syntax.
To get even more speed when writing many rows at the same time, use
LOCK TABLES
. See section 6.7.2 LOCK TABLES/UNLOCK TABLES
Syntax.
language
large_file_support
mysqld
was compiled with options for big file support.
locked_in_memory
mysqld
was locked in memory with --memlock
log
log_update
log_bin
log_slave_updates
long_query_time
Slow_queries
counter
will be incremented. If you are using --log-slow-queries
, the query
will be logged to the slow query logfile. See section 4.9.5 The Slow Query Log.
lower_case_table_names
max_allowed_packet
net_buffer_length
bytes, but can grow up to max_allowed_packet
bytes when needed. This value by default is small, to catch big (possibly
wrong) packets. You must increase this value if you are using big
BLOB
columns. It should be as big as the biggest BLOB
you want
to use. The current protocol limits max_allowed_packet
to 16M.
max_binlog_cache_size
max_binlog_size
max_connections
mysqld
requires. See below for
comments on file descriptor limits. See section A.4.5 Too many connections
Error.
max_connect_errors
FLUSH HOSTS
.
max_delayed_threads
INSERT DELAYED
statements. If you try to insert data into a new table after all INSERT
DELAYED
threads are in use, the row will be inserted as if the
DELAYED
attribute wasn't specified.
max_heap_table_size
max_join_size
max_join_size
records return an error. Set this value if your users tend to perform joins
that lack a WHERE
clause, that take a long time, and that return
millions of rows.
max_sort_length
BLOB
or TEXT
values (only the first max_sort_length
bytes of each value
are used; the rest are ignored).
max_user_connections
max_tmp_tables
max_write_lock_count
myisam_recover_options
--myisam-recover
option.
myisam_sort_buffer_size
REPAIR
or when creating indexes with CREATE INDEX
or
ALTER TABLE
.
myisam_max_extra_sort_file_size
.
myisam_max_sort_file_size
REPAIR
, ALTER TABLE
or LOAD DATA INFILE
. If the file size would be bigger than this,
the index will be created through the key cache (which is slower).
NOTE that this parameter is given in megabytes!
net_buffer_length
max_allowed_packet
bytes.)
net_read_timeout
write_timeout
. See also slave_read_timeout
.
net_retry_count
FreeBSD
as
internal interrupts are sent to all threads.
net_write_timeout
open_files_limit
mysqld
will use this value to reserve file
descriptors to use with setrlimit()
. If this value is 0 then
mysqld
will reserve max_connections*5
or
max_connections + table_cache*2
(whichever is larger) number of
files. You should try increasing this if mysqld
gives you the
error 'Too many open files'.
pid_file
--pid-file
option.
port
--port
option.
protocol_version
record_buffer
record_rnd_buffer
record_buffer
.
query_buffer_size
safe_show_databases
skip_show_databases
.
server_id
--server-id
option.
skip_locking
mysqld
uses external locking.
skip_networking
skip_show_databases
SHOW DATABASES
if they don't have
the PROCESS_PRIV
privilege. This can improve security if you're
concerned about people being able to see what databases other users
have. See also safe_show_databases
.
slave_read_timeout
slow_launch_time
Slow_launch_threads
counter will be incremented.
socket
sort_buffer
ORDER BY
or GROUP BY
operations.
See section A.6 Where MySQL Stores Temporary Files.
table_cache
mysqld
requires.
MySQL needs two file descriptors for each unique open table.
See below for comments on file descriptor limits. You can check if you
need to increase the table cache by checking the Opened_tables
variable. See section 4.5.5 SHOW
Syntax. If this variable is big and you don't do
FLUSH TABLES
a lot (which just forces all tables to be closed and
reopenend), then you should increase the value of this variable.
Make sure that your operating system can handle the number of open file
descriptors implied by the table_cache
setting. If table_cache
is set too high, MySQL may run out of file descriptors and refuse
connections, fail to perform queries, and be very unreliable.
For information about how the table cache works, see section 5.4.6 How MySQL Opens and Closes Tables.
table_type
thread_cache_size
thread_cache_size
threads from before. All new
threads are first taken from the cache, and only when the cache is empty
is a new thread created. This variable can be increased to improve
performance if you have a lot of new connections. (Normally this doesn't
give a notable performance improvement if you have a good
thread implementation.) By examing the difference between
the Connections
and Threads_created
you can see how efficient
the current thread cache is for you.
thread_concurrency
mysqld
will call thr_setconcurrency()
with
this value. thr_setconcurrency()
permits the application to give
the threads system a hint for the desired number of threads that should
be run at the same time.
thread_stack
crash-me
test are dependent on this value. The default is
large enough for normal operation. See section 5.1.4 The MySQL Benchmark Suite.
timezone
tmp_table_size
MyISAM
table.
Increase the value of tmp_table_size
if you do many advanced
GROUP BY
queries and you have lots of memory.
tmpdir
version
wait_timeout
interactive_timeout
.
The manual section that describes tuning MySQL contains some information of how to tune the above variables. See section 5.5.2 Tuning Server Parameters.
SHOW LOGS
SHOW LOGS
shows you status information about existing log
files. It currently only displays information about Berkeley DB log
files.
File
shows the full path to the log file
Type
shows the type of the log file (BDB
for Berkeley
DB log files)
Status
shows the status of the log file (FREE
if the
file can be removed, or IN USE
if the file is needed by the transaction
subsystem)
SHOW PROCESSLIST
SHOW PROCESSLIST
shows you which threads are running. You can
also get this information using the mysqladmin processlist
command. If you have the process privilege, you can see all
threads. Otherwise, you can see only your own threads. See section 4.5.4 KILL
Syntax. If you don't use the FULL
option, then only
the first 100 characters of each query will be shown.
This command is very useful if you get the 'too many connections' error
message and want to find out what's going on. MySQL reserves
one extra connection for a client with the Process_priv
privilege
to ensure that you should always be able to login and check the system
(assuming you are not giving this privilege to all your users).
SHOW GRANTS
SHOW GRANTS FOR user
lists the grant commands that must be issued to
duplicate the grants for a user.
mysql> SHOW GRANTS FOR root@localhost; +---------------------------------------------------------------------+ | Grants for root@localhost | +---------------------------------------------------------------------+ | GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION | +---------------------------------------------------------------------+
SHOW CREATE TABLE
Shows a CREATE TABLE
statement that will create the given table:
mysql> show create table t\G *************************** 1. row *************************** Table: t Create Table: CREATE TABLE t ( id int(11) default NULL auto_increment, s char(60) default NULL, PRIMARY KEY (id) ) TYPE=MyISAM
SHOW CREATE TABLE
will quote table and column names according to
SQL_QUOTE_SHOW_CREATE
option.
section 5.5.6 SET
Syntax.
By default, MySQL uses the ISO-8859-1 (Latin1) character set with sorting according to Swedish/Finnish. This is the character set suitable in the USA and western Europe.
All standard MySQL binaries are compiled with
--with-extra-charsets=complex
. This will add code to all
standard programs to be able to handle latin1
and all multi-byte
character sets within the binary. Other character sets will be
loaded from a character-set definition file when needed.
The character set determines what characters are allowed in names and how
things are sorted by the ORDER BY
and GROUP BY
clauses of
the SELECT
statement.
You can change the character set with the --default-character-set
option when you start the server. The character sets available depend
on the --with-charset=charset
and --with-extra-charset=
list-of-charset | complex | all
options to configure
, and the
character set configuration files listed in
`SHAREDIR/charsets/Index'. See section 2.3.3 Typical configure
Options.
If you change the character set when running MySQL (which may also change the sort order), you must run myisamchk -r -q on all tables. Otherwise your indexes may not be ordered correctly.
When a client connects to a MySQL server, the server sends the default character set in use to the client. The client will switch to use this character set for this connection.
One should use mysql_real_escape_string()
when escaping strings
for a SQL query. mysql_real_escape_string()
is identical to the
old mysql_escape_string()
function, except that it takes the MYSQL
connection handle as the first parameter.
If the client is compiled with different paths than where the server is installed and the user who configured MySQL didn't included all character sets in the MySQL binary, one must specify for the client where it can find the additional character sets it will need if the server runs with a different character set than the client.
One can specify this by putting in a MySQL option file:
[client] character-sets-dir=/usr/local/mysql/share/mysql/charsets
where the path points to where the dynamic MySQL character sets are stored.
One can force the client to use specific character set by specifying:
[client] default-character-set=character-set-name
but normally this is never needed.
mysqld
can issue error messages in the following languages:
Czech, Danish, Dutch, English (the default), Estonian, French, German, Greek,
Hungarian, Italian, Japanese, Korean, Norwegian, Norwegian-ny, Polish,
Portuguese, Romanian, Russian, Slovak, Spanish, and Swedish.
To start mysqld
with a particular language, use either the
--language=lang
or -L lang
options. For example:
shell> mysqld --language=swedish
or:
shell> mysqld --language=/usr/local/share/swedish
Note that all language names are specified in lowercase.
The language files are located (by default) in `mysql_base_dir/share/LANGUAGE/'.
To update the error message file, you should edit the `errmsg.txt' file and execute the following command to generate the `errmsg.sys' file:
shell> comp_err errmsg.txt errmsg.sys
If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file.
To add another character set to MySQL, use the following procedure.
Decide if the set is simple or complex. If the character set does not need to use special string collating routines for sorting and does not need multi-byte character support, it is simple. If it needs either of those features, it is complex.
For example, latin1
and danish
are simple charactersets while
big5
or czech
are complex character sets.
In the following section, we have assumed that you name your character
set MYSET
.
For a simple character set do the following:
ctype
array takes up the first 257 words. The
to_lower
, to_upper
and sort_order
arrays take up
256 words each after that.
CHARSETS_AVAILABLE
and
COMPILED_CHARSETS
lists in configure.in
.
For a complex character set do the following:
ctype_MYSET
,
to_lower_MYSET
, and so on. This corresponds to the arrays
in the simple character set. See section 4.6.4 The character definition arrays. For a complex
character set
/* * This comment is parsed by configure to create ctype.c, * so don't change it unless you know what you are doing. * * .configure. number_MYSET=MYNUMBER * .configure. strxfrm_multiply_MYSET=N * .configure. mbmaxlen_MYSET=N */The
configure
program uses this comment to include
the character set into the MySQL library automatically.
The strxfrm_multiply and mbmaxlen lines will be explained in
the following sections. Only include them if you the string
collating functions or the multi-byte character set functions,
respectively.
my_strncoll_MYSET()
my_strcoll_MYSET()
my_strxfrm_MYSET()
my_like_range_MYSET()
CHARSETS_AVAILABLE
and
COMPILED_CHARSETS
lists in configure.in
.
The file `sql/share/charsets/README' includes some more instructions.
If you want to have the character set included in the MySQL distribution, mail a patch to internals@lists.mysql.com.
to_lower[]
and to_upper[]
are simple arrays that hold the
lowercase and uppercase characters corresponding to each member of the
character set. For example:
to_lower['A'] should contain 'a' to_upper['a'] should contain 'A'
sort_order[]
is a map indicating how characters should be ordered for
comparison and sorting purposes. For many character sets, this is the same as
to_upper[]
(which means sorting will be case insensitive).
MySQL will sort characters based on the value of
sort_order[character]
. For more complicated sorting rules, see
the discussion of string collating below. See section 4.6.5 String Collating Support.
ctype[]
is an array of bit values, with one element for one character.
(Note that to_lower[]
, to_upper[]
, and sort_order[]
are indexed by character value, but ctype[]
is indexed by character
value + 1. This is an old legacy to be able to handle EOF.)
You can find the following bitmask definitions in `m_ctype.h':
#define _U 01 /* Uppercase */ #define _L 02 /* Lowercase */ #define _N 04 /* Numeral (digit) */ #define _S 010 /* Spacing character */ #define _P 020 /* Punctuation */ #define _C 040 /* Control character */ #define _B 0100 /* Blank */ #define _X 0200 /* heXadecimal digit */
The ctype[]
entry for each character should be the union of the
applicable bitmask values that describe the character. For example,
'A'
is an uppercase character (_U
) as well as a
hexadecimal digit (_X
), so ctype['A'+1]
should contain the
value:
_U + _X = 01 + 0200 = 0201
If the sorting rules for your language are too complex to be handled
with the simple sort_order[]
table, you need to use the string
collating functions.
Right now the best documentation on this is the character sets that are already implemented. Look at the big5, czech, gbk, sjis, and tis160 character sets for examples.
You must specify the strxfrm_multiply_MYSET=N
value in the
special comment at the top of the file. N
should be set to
the maximum ratio the strings may grow during my_strxfrm_MYSET
(it
must be a positive integer).
If your want to add support for a new character set that includes multi-byte characters, you need to use the multi-byte character functions.
Right now the best documentation on this is the character sets that are
already implemented. Look at the euc_kr, gb2312, gbk, sjis and ujis
character sets for examples. These are implemented in the
ctype-'charset'.c
files in the `strings' directory.
You must specify the mbmaxlen_MYSET=N
value in the special
comment at the top of the source file. N
should be set to the
size in bytes of the largest character in the set.
All MySQL clients that communicate with the server using the
mysqlclient
library use the following environment variables:
Name | Description |
MYSQL_UNIX_PORT | The default socket; used for connections to localhost
|
MYSQL_TCP_PORT | The default TCP/IP port |
MYSQL_PWD | The default password |
MYSQL_DEBUG | Debug-trace options when debugging |
TMPDIR | The directory where temporary tables/files are created |
Use of MYSQL_PWD
is insecure.
See section 4.2.7 Connecting to the MySQL Server.
The `mysql' client uses the file named in the MYSQL_HISTFILE
environment variable to save the command-line history. The default value for
the history file is `$HOME/.mysql_history', where $HOME
is the
value of the HOME
environment variable. See section B Environment Variables.
All MySQL programs take many different options. However, every
MySQL program provides a --help
option that you can use
to get a full description of the program's different options. For example, try
mysql --help
.
You can override default options for all standard client programs with an option file. section 4.1.2 my.cnf Option Files.
The list below briefly describes the MySQL programs:
myisamchk
myisamchk
has many functions, it is described in its own
chapter. See section 4 MySQL Database Administration.
make_binary_distribution
support.mysql.com
for the
convenience of other MySQL users.
msql2mysql
mSQL
programs to MySQL. It doesn't
handle all cases, but it gives a good start when converting.
mysqlaccess
mysqladmin
mysqladmin
can also be used to retrieve version,
process, and status information from the server.
See section 4.8.3 mysqladmin, Administrating a MySQL Server.
mysqlbug
mysqld
mysqldump
mysqlimport
LOAD DATA
INFILE
. See section 4.8.7 mysqlimport, Importing Data from Text Files.
mysqlshow
mysql_install_db
replace
msql2mysql
, but that has more
general applicability as well. replace
changes strings in place in
files or on the standard input. Uses a finite state machine to match longer
strings first. Can be used to swap strings. For example, this command
swaps a
and b
in the given files:
shell> replace a b b a -- file1 file2 ...
safe_mysqld
is the recommended way to start a mysqld
daemon on Unix. safe_mysqld
adds some safety features such as
restarting the server when an error occurs and logging run-time
information to a log file.
If you don't use --mysqld=#
or --mysqld-version=#
safe_mysqld
will use an executable named mysqld-max
if it
exists. If not, safe_mysqld
will start mysqld
.
This makes it very easy to test to use mysqld-max
instead of
mysqld
; Just copy mysqld-max
to where you have
mysqld
and it will be used.
Normally one should never edit the safe_mysqld
script, but
instead put the options to safe_mysqld
in the
[safe_mysqld]
section in the my.cnf
file. safe_mysqld
will read all options from the [mysqld]
,
[server]
and [safe_mysqld]
sections from the option files.
See section 4.1.2 my.cnf Option Files.
Note that all options on the command line to safe_mysqld
are passed
to mysqld
. If you wants to use any options in safe_mysqld
that
mysqld
doesn't support, you must specify these in the option file.
Most of the options to safe_mysqld
are the same as the options to
mysqld
. See section 4.1.1 mysqld Command-line Options.
safe_mysqld
supports the following options:
--basedir=path
--core-file-size=#
mysqld
should be able to create. Passed to ulimit -c
.
--datadir=path
--defaults-extra-file=path
--defaults-file=path
--err-log=path
--ledir=path
mysqld
--log=path
--mysqld=mysqld-version
mysqld
version in the ledir
directory you want to start.
--mysqld-version=version
--mysqld=
but here you only give the suffix for mysqld
.
For example if you use --mysqld-version=max
, safe_mysqld
will
start the ledir/mysqld-max
version. If the argument to
--mysqld-version
is empty, ledir/mysqld
will be used.
--no-defaults
--open-files-limit=#
mysqld
should be able to open. Passed to ulimit -n
. Note that you need to start safe_mysqld
as root for this to work properly!
--pid-file=path
--port=#
--socket=path
--timezone=#
TZ
) variable to the value of this parameter.
--user=#
The safe_mysqld
script is written so that it normally is able to start
a server that was installed from either a source or a binary version of
MySQL, even if these install the server in slightly different
locations. safe_mysqld
expects one of these conditions to be true:
safe_mysqld
is invoked. safe_mysqld
looks under its working
directory for `bin' and `data' directories (for binary
distributions) or for `libexec' and `var' directories (for source
distributions). This condition should be met if you execute
safe_mysqld
from your MySQL installation directory (for
example, `/usr/local/mysql' for a binary distribution).
safe_mysqld
attempts to locate them by absolute pathnames. Typical
locations are `/usr/local/libexec' and `/usr/local/var'.
The actual locations are determined when the distribution was built from which
safe_mysqld
comes. They should be correct if
MySQL was installed in a standard location.
Because safe_mysqld
will try to find the server and databases relative
to its own working directory, you can install a binary distribution of
MySQL anywhere, as long as you start safe_mysqld
from the
MySQL installation directory:
shell> cd mysql_installation_directory shell> bin/safe_mysqld &
If safe_mysqld
fails, even when invoked from the MySQL
installation directory, you can modify it to use the path to mysqld
and the pathname options that are correct for your system. Note that if you
upgrade MySQL in the future, your modified version of
safe_mysqld
will be overwritten, so you should make a copy of your
edited version that you can reinstall.
mysqld_multi
is meant for managing several mysqld
processes running in different UNIX sockets and TCP/IP ports.
The program will search for group(s) named [mysqld#] from my.cnf (or the
given --config-file=...), where # can be any positive number starting
from 1. These groups should be the same as the usual [mysqld]
group (e.g. options to mysqld, see MySQL manual for detailed
information about this group), but with those port, socket etc. options
that are wanted for each separate mysqld
processes. The number in
the group name has another function; it can be used for starting,
stopping, or reporting some specific mysqld
servers with this
program. See the usage and options below for more information.
Usage: mysqld_multi [OPTIONS] {start|stop|report} [GNR,GNR,GNR...] or mysqld_multi [OPTIONS] {start|stop|report} [GNR-GNR,GNR,GNR-GNR,...]
The GNR above means the group number. You can start, stop or report any GNR, or several of them at the same time. (See --example) The GNRs list can be comma separated, or a dash combined, of which the latter means that all the GNRs between GNR1-GNR2 will be affected. Without GNR argument all the found groups will be either started, stopped, or reported. Note that you must not have any white spaces in the GNR list. Anything after a white space is ignored.
mysqld_multi
supports the following options:
--config-file=...
[mysqld_multi]
), but only groups
[mysqld#]. Without this option everything will be searched from the
ordinary my.cnf file.
--example
--help
--log=...
--mysqladmin=...
mysqladmin
binary to be used for a server shutdown.
--mysqld=...
mysqld
binary to be used. Note that you can give
safe_mysqld
to this option also. The options are passed to
mysqld
. Just make sure you have mysqld
in your environment
variable PATH
or fix safe_mysqld
.
--no-log
--password=...
mysqladmin
.
--tcp-ip
--user=...
mysqladmin
.
--version
Some notes about mysqld_multi
:
mysqld
services (e.g using the mysqladmin
) have the same
password and username for all the data directories accessed (to the
'mysql' database) And make sure that the user has the 'Shutdown_priv'
privilege! If you have many data- directories and many different 'mysql'
databases with different passwords for the MySQL 'root' user,
you may want to create a common 'multi_admin' user for each using the
same password (see below). Example how to do it:
shell> mysql -u root -S /tmp/mysql.sock -proot_password -e "GRANT SHUTDOWN ON *.* TO multi_admin@localhost IDENTIFIED BY 'multipass'" See section 4.2.5 How the Privilege System Works.You will have to do the above for each
mysqld
running in each
data directory, that you have (just change the socket, -S=...)
pid-file
is very important, if you are using safe_mysqld
to start mysqld
(e.g. --mysqld=safe_mysqld) Every mysqld
should have its own pid-file
. The advantage using
safe_mysqld
instead of mysqld
directly here is, that
safe_mysqld
'guards' every mysqld
process and will restart
it, if a mysqld
process fails due to signal kill -9, or
similar. (Like segmentation fault, which MySQL should never do,
of course ;) Please note that safe_mysqld
script may require that
you start it from a certain place. This means that you may have to CD to
a certain directory, before you start the mysqld_multi
. If
you have problems starting, please see the safe_mysqld
script. Check especially the lines:
-------------------------------------------------------------------------- MY_PWD=`pwd` Check if we are starting this relative (for the binary release) if test -d /data/mysql -a -f ./share/mysql/english/errmsg.sys -a -x ./bin/mysqld -------------------------------------------------------------------------- See section 4.7.2 safe_mysqld, the wrapper around mysqld.The above test should be successful, or you may encounter problems.
mysqlds
in the same data
directory. Use separate data directories, unless you KNOW what
you are doing!
mysqld
.
mysqld
group were intentionally left out from
the example. You may have 'gaps' in the config file. This gives you
more flexibility. The order in which the mysqlds
are started or
stopped depends on the order in which they appear in the config file.
mysqld
, but in order to
do this you need to be root when you start the mysqld_multi
script. Having the option in the config file doesn't matter; you will
just get a warning, if you are not the superuser and the mysqlds
are started under YOUR UNIX account. IMPORTANT: Make
sure that the pid-file
and the data directory are
read+write(+execute for the latter one) accessible for THAT
UNIX user, who the specific mysqld
process is started
as. DON'T use the UNIX root account for this, unless you
KNOW what you are doing!
mysqlds
and why WOULD
YOU WANT to have separate mysqld
processes. Starting multiple
mysqlds
in one data directory WILL NOT give you extra
performance in a threaded system!
See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.
This is an example of the config file on behalf of mysqld_multi
.
# This file should probably be in your home dir (~/.my.cnf) or /etc/my.cnf # Version 2.1 by Jani Tolonen [mysqld_multi] mysqld = /usr/local/bin/safe_mysqld mysqladmin = /usr/local/bin/mysqladmin user = multi_admin password = multipass [mysqld2] socket = /tmp/mysql.sock2 port = 3307 pid-file = /usr/local/mysql/var2/hostname.pid2 datadir = /usr/local/mysql/var2 language = /usr/local/share/mysql/english user = john [mysqld3] socket = /tmp/mysql.sock3 port = 3308 pid-file = /usr/local/mysql/var3/hostname.pid3 datadir = /usr/local/mysql/var3 language = /usr/local/share/mysql/swedish user = monty [mysqld4] socket = /tmp/mysql.sock4 port = 3309 pid-file = /usr/local/mysql/var4/hostname.pid4 datadir = /usr/local/mysql/var4 language = /usr/local/share/mysql/estonia user = tonu [mysqld6] socket = /tmp/mysql.sock6 port = 3311 pid-file = /usr/local/mysql/var6/hostname.pid6 datadir = /usr/local/mysql/var6 language = /usr/local/share/mysql/japanese user = jani
See section 4.1.2 my.cnf Option Files.
myisampack
is used to compress MyISAM tables, and pack_isam
is used to compress ISAM tables. Because ISAM tables are deprecated, we
will only discuss myisampack
here, but everything said about
myisampack
should also be true for pack_isam
.
myisampack
works by compressing each column in the table separately.
The information needed to decompress columns is read into memory when the
table is opened. This results in much better performance when accessing
individual records, because you only have to uncompress exactly one record, not
a much larger disk block as when using Stacker on MS-DOS.
Usually, myisampack
packs the data file 40%-70%.
MySQL uses memory mapping (mmap()
) on compressed tables and
falls back to normal read/write file usage if mmap()
doesn't work.
There are currently two limitations with myisampack
:
myisampack
can also pack BLOB
or TEXT
columns. The
older pack_isam
could not do this.
Fixing these limitations is on our TODO list but with low priority.
myisampack
is invoked like this:
shell> myisampack [options] filename ...
Each filename should be the name of an index (`.MYI') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.MYI' extension.
myisampack
supports the following options:
-b, --backup
tbl_name.OLD
.
-#, --debug=debug_options
debug_options
string often is
'd:t:o,filename'
.
-f, --force
myisampack
creates a temporary file named `tbl_name.TMD'
while it compresses the table. If you kill myisampack
, the `.TMD'
file may not be deleted. Normally, myisampack
exits with an error if
it finds that `tbl_name.TMD' exists. With --force
,
myisampack
packs the table anyway.
-?, --help
-j big_tbl_name, --join=big_tbl_name
big_tbl_name
. All tables that are to be combined
MUST be identical (same column names and types, same indexes, etc.).
-p #, --packlength=#
myisampack
stores all rows with length pointers of 1, 2, or 3
bytes. In most normal cases, myisampack
can determine the right length
value before it begins packing the file, but it may notice during the packing
process that it could have used a shorter length. In this case,
myisampack
will print a note that the next time you pack the same file,
you could use a shorter record length.)
-s, --silent
-t, --test
-T dir_name, --tmp_dir=dir_name
-v, --verbose
-V, --version
-w, --wait
mysqld
server was
invoked with the --skip-locking
option, it is not a good idea to
invoke myisampack
if the table might be updated during the
packing process.
The sequence of commands shown below illustrates a typical table compression session:
shell> ls -l station.* -rw-rw-r-- 1 monty my 994128 Apr 17 19:00 station.MYD -rw-rw-r-- 1 monty my 53248 Apr 17 19:00 station.MYI -rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm shell> myisamchk -dvv station MyISAM file: station Isam-version: 2 Creation time: 1996-03-13 10:08:58 Recover time: 1997-02-02 3:06:43 Data records: 1192 Deleted blocks: 0 Datafile: Parts: 1192 Deleted data: 0 Datafile pointer (bytes): 2 Keyfile pointer (bytes): 2 Max datafile length: 54657023 Max keyfile length: 33554431 Recordlength: 834 Record format: Fixed length table description: Key Start Len Index Type Root Blocksize Rec/key 1 2 4 unique unsigned long 1024 1024 1 2 32 30 multip. text 10240 1024 1 Field Start Length Type 1 1 1 2 2 4 3 6 4 4 10 1 5 11 20 6 31 1 7 32 30 8 62 35 9 97 35 10 132 35 11 167 4 12 171 16 13 187 35 14 222 4 15 226 16 16 242 20 17 262 20 18 282 20 19 302 30 20 332 4 21 336 4 22 340 1 23 341 8 24 349 8 25 357 8 26 365 2 27 367 2 28 369 4 29 373 4 30 377 1 31 378 2 32 380 8 33 388 4 34 392 4 35 396 4 36 400 4 37 404 1 38 405 4 39 409 4 40 413 4 41 417 4 42 421 4 43 425 4 44 429 20 45 449 30 46 479 1 47 480 1 48 481 79 49 560 79 50 639 79 51 718 79 52 797 8 53 805 1 54 806 1 55 807 20 56 827 4 57 831 4 shell> myisampack station.MYI Compressing station.MYI: (1192 records) - Calculating statistics normal: 20 empty-space: 16 empty-zero: 12 empty-fill: 11 pre-space: 0 end-space: 12 table-lookups: 5 zero: 7 Original trees: 57 After join: 17 - Compressing file 87.14% shell> ls -l station.* -rw-rw-r-- 1 monty my 127874 Apr 17 19:00 station.MYD -rw-rw-r-- 1 monty my 55296 Apr 17 19:04 station.MYI -rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm shell> myisamchk -dvv station MyISAM file: station Isam-version: 2 Creation time: 1996-03-13 10:08:58 Recover time: 1997-04-17 19:04:26 Data records: 1192 Deleted blocks: 0 Datafile: Parts: 1192 Deleted data: 0 Datafilepointer (bytes): 3 Keyfile pointer (bytes): 1 Max datafile length: 16777215 Max keyfile length: 131071 Recordlength: 834 Record format: Compressed table description: Key Start Len Index Type Root Blocksize Rec/key 1 2 4 unique unsigned long 10240 1024 1 2 32 30 multip. text 54272 1024 1 Field Start Length Type Huff tree Bits 1 1 1 constant 1 0 2 2 4 zerofill(1) 2 9 3 6 4 no zeros, zerofill(1) 2 9 4 10 1 3 9 5 11 20 table-lookup 4 0 6 31 1 3 9 7 32 30 no endspace, not_always 5 9 8 62 35 no endspace, not_always, no empty 6 9 9 97 35 no empty 7 9 10 132 35 no endspace, not_always, no empty 6 9 11 167 4 zerofill(1) 2 9 12 171 16 no endspace, not_always, no empty 5 9 13 187 35 no endspace, not_always, no empty 6 9 14 222 4 zerofill(1) 2 9 15 226 16 no endspace, not_always, no empty 5 9 16 242 20 no endspace, not_always 8 9 17 262 20 no endspace, no empty 8 9 18 282 20 no endspace, no empty 5 9 19 302 30 no endspace, no empty 6 9 20 332 4 always zero 2 9 21 336 4 always zero 2 9 22 340 1 3 9 23 341 8 table-lookup 9 0 24 349 8 table-lookup 10 0 25 357 8 always zero 2 9 26 365 2 2 9 27 367 2 no zeros, zerofill(1) 2 9 28 369 4 no zeros, zerofill(1) 2 9 29 373 4 table-lookup 11 0 30 377 1 3 9 31 378 2 no zeros, zerofill(1) 2 9 32 380 8 no zeros 2 9 33 388 4 always zero 2 9 34 392 4 table-lookup 12 0 35 396 4 no zeros, zerofill(1) 13 9 36 400 4 no zeros, zerofill(1) 2 9 37 404 1 2 9 38 405 4 no zeros 2 9 39 409 4 always zero 2 9 40 413 4 no zeros 2 9 41 417 4 always zero 2 9 42 421 4 no zeros 2 9 43 425 4 always zero 2 9 44 429 20 no empty 3 9 45 449 30 no empty 3 9 46 479 1 14 4 47 480 1 14 4 48 481 79 no endspace, no empty 15 9 49 560 79 no empty 2 9 50 639 79 no empty 2 9 51 718 79 no endspace 16 9 52 797 8 no empty 2 9 53 805 1 17 1 54 806 1 3 9 55 807 20 no empty 3 9 56 827 4 no zeros, zerofill(2) 2 9 57 831 4 no zeros, zerofill(1) 2 9
The information printed by myisampack
is described below:
normal
empty-space
empty-zero
empty-fill
INTEGER
column may be changed to MEDIUMINT
).
pre-space
end-space
table-lookup
ENUM
before Huffman compression.
zero
Original trees
After join
After a table has been compressed, myisamchk -dvv
prints additional
information about each field:
Type
constant
no endspace
no endspace, not_always
no endspace, no empty
table-lookup
ENUM
.
zerofill(n)
n
bytes in the value are always 0 and are not
stored.
no zeros
always zero
Huff tree
Bits
After you have run pack_isam
/myisampack
you must run
isamchk
/myisamchk
to re-create the index. At this time you
can also sort the index blocks and create statistics needed for
the MySQL optimizer to work more efficiently:
myisamchk -rq --analyze --sort-index table_name.MYI isamchk -rq --analyze --sort-index table_name.ISM
After you have installed the packed table into the MySQL database
directory you should do mysqladmin flush-tables
to force mysqld
to start using the new table.
If you want to unpack a packed table, you can do this with the
--unpack
option to isamchk
or myisamchk
.
mysqld-max
is the MySQL server (mysqld
) configured with
the following configure options:
Option | Comment |
--with-server-suffix=-max | Add a suffix to the mysqld version string.
|
--with-bdb | Support for Berkeley DB (BDB) tables |
--with-innodb | Support for InnoDB tables. |
CFLAGS=-DUSE_SYMDIR | Symbolic links support for Windows. |
You can find the MySQL-max binaries at http://www.mysql.com/downloads/mysql-max-3.23.html.
The Windows MySQL 3.23 binary distribution includes both the
standard mysqld.exe binary and the mysqld-max.exe
binary.
http://www.mysql.com/downloads/mysql-3.23.html.
See section 2.1.2 Installing MySQL on Windows.
Note that as Berkeley DB and InnoDB are not available for all platforms,
some of the Max
binaries may not have support for both of these.
You can check which table types are supported by doing the following
query:
mysql> show variables like "have_%"; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | have_bdb | YES | | have_innodb | NO | | have_isam | YES | | have_raid | YES | | have_ssl | NO | +---------------+-------+
The meaning of the values are:
Value | Meaning. |
YES | The option is activated and usable. |
NO | MySQL is not compiled with support for this option. |
DISABLED | The xxxx option is disabled because one started mysqld with --skip-xxxx or because one didn't start mysqld with all needed options to enable the option. In this case the hostname.err file should contain a reason for why the option is disabled.
|
NOTE: To be able to create InnoDB tables you MUST edit
your startup options to include at least the innodb_data_file_path
option. See section 7.6.2 InnoDB startup options.
To get better performance for BDB tables, you should add some configuration options for these too. See section 7.5.3 BDB startup options.
safe_mysqld
will automatically try to start any mysqld
binary
with the -max
prefix. This makes it very easy to test out a
another mysqld
binary in an existing installation. Just
run configure
with the options you want and then install the
new mysqld
binary as mysqld-max
in the same directory
where your old mysqld
binary is. See section 4.7.2 safe_mysqld, the wrapper around mysqld.
The mysqld-max
RPM uses the above mentioned safe_mysqld
feature. It just installs the mysqld-max
executable and
safe_mysqld
will automatically use this executable when
safe_mysqld
is restarted.
The following table shows which table types our standard MySQL-Max binaries includes:
System | BDB | InnoDB |
AIX 4.3 | N | Y |
HP-UX 11.0 | N | Y |
Linux-Alpha | N | Y |
Linux-Intel | Y | Y |
Linux-Ia64 | N | Y |
Solaris-intel | N | Y |
Solaris-sparc | Y | Y |
SCO OSR5 | Y | Y |
UnixWare | Y | Y |
Windows/NT | Y | Y |
All MySQL clients that communicate with the server using the
mysqlclient
library use the following environment variables:
Name | Description |
MYSQL_UNIX_PORT | The default socket; used for connections to localhost
|
MYSQL_TCP_PORT | The default TCP/IP port |
MYSQL_PWD | The default password |
MYSQL_DEBUG | Debug-trace options when debugging |
TMPDIR | The directory where temporary tables/files are created |
Use of MYSQL_PWD
is insecure.
See section 4.2.7 Connecting to the MySQL Server.
The `mysql' client uses the file named in the MYSQL_HISTFILE
environment variable to save the command-line history. The default value for
the history file is `$HOME/.mysql_history', where $HOME
is the
value of the HOME
environment variable. See section B Environment Variables.
All MySQL programs take many different options. However, every
MySQL program provides a --help
option that you can use
to get a full description of the program's different options. For example, try
mysql --help
.
You can override default options for all standard client programs with an option file. section 4.1.2 my.cnf Option Files.
The list below briefly describes the MySQL programs:
myisamchk
myisamchk
has many functions, it is described in its own
chapter. See section 4 MySQL Database Administration.
make_binary_distribution
support.mysql.com
for the
convenience of other MySQL users.
msql2mysql
mSQL
programs to MySQL. It doesn't
handle all cases, but it gives a good start when converting.
mysqlaccess
mysqladmin
mysqladmin
can also be used to retrieve version,
process, and status information from the server.
See section 4.8.3 mysqladmin, Administrating a MySQL Server.
mysqlbug
mysqld
mysqldump
mysqlimport
LOAD DATA
INFILE
. See section 4.8.7 mysqlimport, Importing Data from Text Files.
mysqlshow
mysql_install_db
replace
msql2mysql
, but that has more
general applicability as well. replace
changes strings in place in
files or on the standard input. Uses a finite state machine to match longer
strings first. Can be used to swap strings. For example, this command
swaps a
and b
in the given files:
shell> replace a b b a -- file1 file2 ...
mysql
is a simple SQL shell (with GNU readline
capabilities).
It supports interactive and non-interactive use. When used interactively,
query results are presented in an ASCII-table format. When used
non-interactively (for example, as a filter), the result is presented in
tab-separated format. (The output format can be changed using command-line
options.) You can run scripts simply like this:
shell> mysql database < script.sql > output.tab
If you have problems due to insufficient memory in the client, use the
--quick
option! This forces mysql
to use
mysql_use_result()
rather than mysql_store_result()
to
retrieve the result set.
Using mysql
is very easy. Just start it as follows:
mysql database
or mysql --user=user_name --password=your_password database
. Type a SQL statement, end it with `;', `\g', or `\G'
and press RETURN/ENTER.
mysql
supports the following options:
-?, --help
-A, --no-auto-rehash
-B, --batch
--character-sets-dir=...
-C, --compress
-#, --debug[=...]
-D, --database=...
my.cnf
file.
--default-character-set=...
-e, --execute=...
-E, --vertical
\G
.
-f, --force
-g, --no-named-commands
-G, --enable-named-commands
-i, --ignore-space
-h, --host=...
-H, --html
-L, --skip-line-numbers
--no-pager
--no-tee
-n, --unbuffered
-N, --skip-column-names
-O, --set-variable var=option
--help
lists variables.
-o, --one-database
--pager[=...]
ENV
variable PAGER
. Valid
pagers are less, more, cat [> filename], etc. See interactive help (\h)
also. This option does not work in batch mode. Pager works only in UNIX.
-p[password], --password[=...]
-p
you can't have a space between the option and the
password.
-P --port=...
-q, --quick
-r, --raw
--batch
-s, --silent
-S --socket=...
-t --table
-T, --debug-info
--tee=...
-u, --user=#
-U, --safe-updates[=#], --i-am-a-dummy[=#]
UPDATE
and DELETE
that uses keys. See below for
more information about this option. You can reset this option if you have
it in your my.cnf
file by using --safe-updates=0
.
-v, --verbose
-V, --version
-w, --wait
You can also set the following variables with -O
or
--set-variable
:
Variablename | Default | Description |
connect_timeout | 0 | Number of seconds before timeout connection. |
max_allowed_packet | 16777216 | Max packetlength to send/receive from to server |
net_buffer_length | 16384 | Buffer for TCP/IP and socket communication |
select_limit | 1000 | Automatic limit for SELECT when using --i-am-a-dummy |
max_join_size | 1000000 | Automatic limit for rows in a join when using --i-am-a-dummy. |
If you type 'help' on the command line, mysql
will print out the
commands that it supports:
mysql> help MySQL commands: help (\h) Display this text. ? (\h) Synonym for `help'. clear (\c) Clear command. connect (\r) Reconnect to the server. Optional arguments are db and host. edit (\e) Edit command with $EDITOR. ego (\G) Send command to mysql server, display result vertically. exit (\q) Exit mysql. Same as quit. go (\g) Send command to mysql server. nopager (\n) Disable pager, print to stdout. notee (\t) Don't write into outfile. pager (\P) Set PAGER [to_pager]. Print the query results via PAGER. print (\p) Print current command. quit (\q) Quit mysql. rehash (\#) Rebuild completion hash. source (\.) Execute a SQL script file. Takes a file name as an argument. status (\s) Get status information from the server. tee (\T) Set outfile [to_outfile]. Append everything into given outfile. use (\u) Use another database. Takes database name as argument.
From the above, pager only works in UNIX.
The status
command gives you some information about the
connection and the server you are using. If you are running in the
--safe-updates
mode, status
will also print the values for
the mysql
variables that affect your queries.
A useful startup option for beginners (introduced in MySQL
Version 3.23.11) is --safe-updates
(or --i-am-a-dummy
for
users that has at some time done a DELETE FROM table_name
but
forgot the WHERE
clause). When using this option, mysql
sends the following command to the MySQL server when opening
the connection:
SET SQL_SAFE_UPDATES=1,SQL_SELECT_LIMIT=#select_limit#, SQL_MAX_JOIN_SIZE=#max_join_size#"
where #select_limit#
and #max_join_size#
are variables that
can be set from the mysql
command line. See section 5.5.6 SET
Syntax.
The effect of the above is:
UPDATE
or DELETE
statement
if you don't have a key constraint in the WHERE
part. One can,
however, force an UPDATE/DELETE
by using LIMIT
:
UPDATE table_name SET not_key_column=# WHERE not_key_column=# LIMIT 1;
#select_limit#
rows.
SELECT
's that will probably need to examine more than
#max_join_size
row combinations will be aborted.
Some useful hints about the mysql
client:
Some data is much more readable when displayed vertically, instead of the usual horizontal box type output. For example longer text, which includes new lines, is often much easier to be read with vertical output.
mysql> select * from mails where length(txt) < 300 limit 300,1\G *************************** 1. row *************************** msg_nro: 3068 date: 2000-03-01 23:29:50 time_zone: +0200 mail_from: Monty reply: monty@no.spam.com mail_to: "Thimble Smith" <tim@no.spam.com> sbj: UTF-8 txt: >>>>> "Thimble" == Thimble Smith writes: Thimble> Hi. I think this is a good idea. Is anyone familiar with UTF-8 Thimble> or Unicode? Otherwise I'll put this on my TODO list and see what Thimble> happens. Yes, please do that. Regards, Monty file: inbox-jani-1 hash: 190402944 1 row in set (0.09 sec)
tee
option. The tee
can be
started with option --tee=...
, or from the command line
interactively with command tee
. All the data displayed on the
screen will also be appended into a given file. This can be very useful
for debugging purposes also. The tee
can be disabled from the
command line with command notee
. Executing tee
again
starts logging again. Without a parameter the previous file will be
used. Note that tee
will flush the results into the file after
each command, just before the command line appears again waiting for the
next command.
--pager[=...]
. Without argument, mysql
client will look
for environment variable PAGER and set pager
to that.
pager
can be started from the interactive command line with
command pager
and disabled with command nopager
. The
command takes an argument optionally and the pager
will be set to
that. Command pager
can be called without an argument, but this
requires that the option --pager
was used, or the pager
will default to stdout. pager
works only in UNIX, since it uses
the popen() function, which doesn't exist in Windows. In Windows, the
tee
option can be used instead, although it may not be as handy
as pager
can be in some situations.
pager
: You can use it to write to a file:
mysql> pager cat > /tmp/log.txtand the results will only go to a file. You can also pass any options for the programs that you want to use with the
pager
:
mysql> pager less -n -i -SFrom the above do note the option '-S'. You may find it very useful when browsing the results; try the option with horizontal output (end commands with '\g', or ';') and with vertical output (end commands with '\G'). Sometimes a very wide result set is hard to be read from the screen, with option -S to less you can browse the results within the interactive less from left to right, preventing lines longer than your screen from being continued to the next line. This can make the result set much more readable. You can swith the mode between on and off within the interactive less with '-S'. See the 'h' for more help about less.
mysql> pager cat | tee /dr1/tmp/res.txt | tee /dr2/tmp/res2.txt | less -n -i -S
tee
enabled, pager
set to 'less' and you will be able to browse the
results in unix 'less' and still have everything appended into a file
the same time. The difference between UNIX tee
used with the
pager
and the mysql
client in-built tee
, is that
the in-built tee
works even if you don't have the UNIX tee
available. The in-built tee
also logs everything that is printed
on the screen, where the UNIX tee
used with pager
doesn't
log quite that much. Last, but not least, the interactive tee
is
more handy to switch on and off, when you want to log something into a
file, but want to be able to turn the feature off sometimes.
A utility for performing administrative operations. The syntax is:
shell> mysqladmin [OPTIONS] command [command-option] command ...
You can get a list of the options your version of mysqladmin
supports
by executing mysqladmin --help
.
The current mysqladmin
supports the following commands:
create databasename
drop databasename
extended-status
flush-hosts
flush-logs
flush-tables
flush-privileges
kill id,id,...
password
ping
processlist
reload
refresh
shutdown
slave-start
slave-stop
status
variables
version
All commands can be shortened to their unique prefix. For example:
shell> mysqladmin proc stat +----+-------+-----------+----+-------------+------+-------+------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------+-----------+----+-------------+------+-------+------+ | 6 | monty | localhost | | Processlist | 0 | | | +----+-------+-----------+----+-------------+------+-------+------+ Uptime: 10077 Threads: 1 Questions: 9 Slow queries: 0 Opens: 6 Flush tables: 1 Open tables: 2 Memory in use: 1092K Max memory used: 1116K
The mysqladmin status
command result has the following columns:
Uptime | Number of seconds the MySQL server has been up. |
Threads | Number of active threads (clients). |
Questions | Number of questions from clients since mysqld was started.
|
Slow queries | Queries that have taken more than long_query_time seconds. See section 4.9.5 The Slow Query Log.
|
Opens | How many tables mysqld has opened.
|
Flush tables | Number of flush ... , refresh , and reload commands.
|
Open tables | Number of tables that are open now. |
Memory in use | Memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug=full).
|
Max memory used | Maximum memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug=full).
|
If you do myslqadmin shutdown
on a socket (in other words, on a
the computer where mysqld
is running), mysqladmin
will
wait until the MySQL pid-file
is removed to ensure that
the mysqld
server has stopped properly.
mysqlcheck
for Table Maintenance and Crash Recovery
Since MySQL version 3.23.38 you will be able to use a new
checking and repairing tool for MyISAM
tables. The difference to
myisamchk
is that mysqlcheck
should be used when the
mysqld
server is running, where as myisamchk
should be used
when it is not. The benefit is that you no longer have to take the
server down for checking or repairing your tables.
mysqlcheck
uses MySQL server commands CHECK
,
REPAIR
, ANALYZE
and OPTIMIZE
in a convenient way
for the user.
There are three alternative ways to invoke mysqlcheck
:
shell> mysqlcheck [OPTIONS] database [tables] shell> mysqlcheck [OPTIONS] --databases DB1 [DB2 DB3...] shell> mysqlcheck [OPTIONS] --all-databases
So it can be used in a similar way as mysqldump
when it
comes to what databases and tables you want to choose.
mysqlcheck
does have a special feature compared to the other
clients; the default behavior, checking tables (-c), can be changed by
renaming the binary. So if you want to have a tool that repairs tables
by default, you should just copy mysqlcheck
to your harddrive
with a new name, mysqlrepair
, or alternatively make a symbolic
link to mysqlrepair
and name the symbolic link as
mysqlrepair
. If you invoke mysqlrepair
now, it will repair
tables by default.
The names that you can use to change mysqlcheck
default behavior
are here:
mysqlrepair: The default option will be -r mysqlanalyze: The default option will be -a mysqloptimize: The default option will be -o
The options available for mysqlcheck
are listed here, please
check what your version supports with mysqlcheck --help
.
-A, --all-databases
-1, --all-in-1
-a, --analyze
--auto-repair
-#, --debug=...
--character-sets-dir=...
-c, --check
-C, --check-only-changed
--compress
-?, --help
-B, --databases
--default-character-set=...
-F, --fast
-f, --force
-e, --extended
-h, --host=...
-m, --medium-check
-o, --optimize
-p, --password[=...]
-P, --port=...
-q, --quick
-r, --repair
-s, --silent
-S, --socket=...
--tables
-u, --user=#
-v, --verbose
-V, --version
Utility to dump a database or a collection of database for backup or for transferring the data to another SQL server (not necessarily a MySQL server). The dump will contain SQL statements to create the table and/or populate the table.
If you are doing a backup on the server, you should consider using
the mysqlhotcopy
instead. See section 4.8.6 mysqlhotcopy, Copying MySQL Databases and Tables.
shell> mysqldump [OPTIONS] database [tables] OR mysqldump [OPTIONS] --databases [OPTIONS] DB1 [DB2 DB3...] OR mysqldump [OPTIONS] --all-databases [OPTIONS]
If you don't give any tables or use the --databases
or
--all-databases
, the whole database(s) will be dumped.
You can get a list of the options your version of mysqldump
supports
by executing mysqldump --help
.
Note that if you run mysqldump
without --quick
or
--opt
, mysqldump
will load the whole result set into
memory before dumping the result. This will probably be a problem if
you are dumping a big database.
Note that if you are using a new copy of the mysqldump
program
and you are going to do a dump that will be read into a very old MySQL
server, you should not use the --opt
or -e
options.
mysqldump
supports the following options:
--add-locks
LOCK TABLES
before and UNLOCK TABLE
after each table dump.
(To get faster inserts into MySQL.)
--add-drop-table
drop table
before each create statement.
-A, --all-databases
--databases
with all
databases selected.
-a, --all
--allow-keywords
-c, --complete-insert
-C, --compress
-B, --databases
USE db_name;
will be included in the output before each new database.
--delayed
INSERT DELAYED
command.
-e, --extended-insert
INSERT
syntax. (Gives more compact and
faster inserts statements.)
-#, --debug[=option_string]
--help
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--lines-terminated-by=...
-T
option and have the same
meaning as the corresponding clauses for LOAD DATA INFILE
.
See section 6.4.8 LOAD DATA INFILE
Syntax.
-F, --flush-logs
-f, --force,
-h, --host=..
localhost
.
-l, --lock-tables.
READ LOCAL
to allow concurrent inserts in the case of MyISAM
tables.
-n, --no-create-db
-t, --no-create-info
CREATE TABLE
statement.)
-d, --no-data
--opt
--quick --add-drop-table --add-locks --extended-insert
--lock-tables
. Should give you the fastest possible dump for reading
into a MySQL server.
-pyour_pass, --password[=your_pass]
mysqldump
you will be prompted for a password.
-P port_num, --port=port_num
localhost
, for which Unix sockets are
used.)
-q, --quick
mysql_use_result()
to do this.
-r, --result-file=...
-S /path/to/socket, --socket=/path/to/socket
localhost
(which is the
default host).
--tables
-T, --tab=path-to-some-directory
table_name.sql
file, that contains the SQL CREATE commands,
and a table_name.txt
file, that contains the data, for each give table.
NOTE: This only works if mysqldump
is run on the same
machine as the mysqld
daemon. The format of the .txt
file
is made according to the --fields-xxx
and --lines--xxx
options.
-u user_name, --user=user_name
-O var=option, --set-variable var=option
-v, --verbose
-V, --version
-w, --where='where-condition'
"--where=user='jimf'" "-wuserid>1" "-wuserid<1"
-O net_buffer_length=#, where # < 16M
--extended-insert
or --opt
), mysqldump
will create
rows up to net_buffer_length
length. If you increase this
variable, you should also ensure that the max_allowed_packet
variable in the MySQL server is bigger than the
net_buffer_length
.
The most normal use of mysqldump
is probably for making a backup of
whole databases. See section 4.4.1 Database Backups.
mysqldump --opt database > backup-file.sql
You can read this back into MySQL with:
mysql database < backup-file.sql
or
mysql -e "source /patch-to-backup/backup-file.sql" database
However, it's also very useful to populate another MySQL server with information from a database:
mysqldump --opt database | mysql --host=remote-host -C database
It is possible to dump several databases with one command:
mysqldump --databases database1 [database2 database3...] > my_databases.sql
If all the databases are wanted, one can use:
mysqldump --all-databases > all_databases.sql
mysqlhotcopy
is a perl script that uses LOCK TABLES
,
FLUSH TABLES
and cp
or scp
to quickly make a backup
of a database. It's the fastest way to make a backup of the database,
of single tables but it can only be run on the same machine where the
database directories are.
mysqlhotcopy db_name [/path/to/new_directory] mysqlhotcopy db_name_1 ... db_name_n /path/to/new_directory mysqlhotcopy db_name./regex/
mysqlhotcopy
supports the following options:
-?, --help
-u, --user=#
-p, --password=#
-P, --port=#
-S, --socket=#
--allowold
--keepold
--noindices
myisamchk -rq.
.
--method=#
cp
or scp
).
-q, --quiet
--debug
-n, --dryrun
--regexp=#
--suffix=#
--checkpoint=#
--flushlog
--tmpdir=#
You can use perldoc mysqlhotcopy
to get a more complete
documentation for mysqlhotcopy
.
mysqlhotcopy
reads the groups [client]
and [mysqlhotcopy]
from the option files.
To be able to execute mysqlhotcopy
you need write access to the
backup directory, SELECT
privilege to the tables you are about to
copy and the MySQL Reload
privilege (to be able to
execute FLUSH TABLES
).
mysqlimport
provides a command-line interface to the LOAD DATA
INFILE
SQL statement. Most options to mysqlimport
correspond
directly to the same options to LOAD DATA INFILE
.
See section 6.4.8 LOAD DATA INFILE
Syntax.
mysqlimport
is invoked like this:
shell> mysqlimport [options] database textfile1 [textfile2....]
For each text file named on the command line,
mysqlimport
strips any extension from the filename and uses the result
to determine which table to import the file's contents into. For example,
files named `patient.txt', `patient.text', and `patient' would
all be imported into a table named patient
.
mysqlimport
supports the following options:
-c, --columns=...
LOAD DATA INFILE
command,
which is then passed to MySQL. See section 6.4.8 LOAD DATA INFILE
Syntax.
-C, --compress
-#, --debug[=option_string]
-d, --delete
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--lines-terminated-by=...
LOAD DATA INFILE
. See section 6.4.8 LOAD DATA INFILE
Syntax.
-f, --force
--force
,
mysqlimport
exits if a table doesn't exist.
--help
-h host_name, --host=host_name
localhost
.
-i, --ignore
--replace
option.
-l, --lock-tables
-L, --local
localhost
(which is the default host).
-pyour_pass, --password[=your_pass]
mysqlimport
you will be prompted for a password.
-P port_num, --port=port_num
localhost
, for which Unix sockets are
used.)
-r, --replace
--replace
and --ignore
options control handling of input
records that duplicate existing records on unique key values. If you specify
--replace
, new rows replace existing rows that have the same unique key
value. If you specify --ignore
, input rows that duplicate an existing
row on a unique key value are skipped. If you don't specify either option, an
error occurs when a duplicate key value is found, and the rest of the text
file is ignored.
-s, --silent
-S /path/to/socket, --socket=/path/to/socket
localhost
(which is the
default host).
-u user_name, --user=user_name
-v, --verbose
-V, --version
Here is a sample run using mysqlimport
:
$ mysql --version mysql Ver 9.33 Distrib 3.22.25, for pc-linux-gnu (i686) $ uname -a Linux xxx.com 2.2.5-15 #1 Mon Apr 19 22:21:09 EDT 1999 i586 unknown $ mysql -e 'CREATE TABLE imptest(id INT, n VARCHAR(30))' test $ ed a 100 Max Sydow 101 Count Dracula . w imptest.txt 32 q $ od -c imptest.txt 0000000 1 0 0 \t M a x S y d o w \n 1 0 0000020 1 \t C o u n t D r a c u l a \n 0000040 $ mysqlimport --local test imptest.txt test.imptest: Records: 2 Deleted: 0 Skipped: 0 Warnings: 0 $ mysql -e 'SELECT * FROM imptest' test +------+---------------+ | id | n | +------+---------------+ | 100 | Max Sydow | | 101 | Count Dracula | +------+---------------+
mysqlshow
can be used to quickly look at which databases exist,
their tables, and the table's columns.
With the mysql
program you can get the same information with the
SHOW
commands. See section 4.5.5 SHOW
Syntax.
mysqlshow
is invoked like this:
shell> mysqlshow [OPTIONS] [database [table [column]]]
Note that in newer MySQL versions, you only see those database/tables/columns for which you have some privileges.
If the last argument contains a shell or SQL wild-card (*
, ?
,
%
or _
) then only what's matched by the wild card is shown.
This may cause some confusion when you try to display the columns for a
table with a _
as in this case mysqlshow
only shows you
the table names that match the pattern. This is easily fixed by
adding an extra %
last on the command line (as a separate
argument).
perror
can be used to print error message(s). perror
can
be invoked like this:
shell> perror [OPTIONS] [ERRORCODE [ERRORCODE...]] For example: shell> perror 64 79 Error code 64: Machine is not on the network Error code 79: Can not access a needed shared library
perror
can be used to display a description for a system error
code, or an MyISAM/ISAM table handler error code. The error messages
are mostly system dependent.
The mysql
client typically is used interactively, like this:
shell> mysql database
However, it's also possible to put your SQL commands in a file and tell
mysql
to read its input from that file. To do so, create a text
file `text_file' that contains the commands you wish to execute.
Then invoke mysql
as shown below:
shell> mysql database < text_file
You can also start your text file with a USE db_name
statement. In
this case, it is unnecessary to specify the database name on the command
line:
shell> mysql < text_file
See section 4.8 MySQL Client-Side Scripts and Utilities.
MySQL has several different log files that can help you find
out what's going on inside mysqld
:
The error log | Problems encountering starting, running or stopping mysqld .
|
The isam log | Logs all changes to the ISAM tables. Used only for debugging the isam code. |
The query log | Established connections and executed queries. |
The update log | Deprecated: Stores all statements that changes data |
The binary log | Stores all statements that changes something. Used also for replication |
The slow log | Stores all queries that took more than long_query_time to execute or didn't use indexes.
|
All logs can be found in the mysqld
data directory. You can
force mysqld
to reopen the log files (or in some cases
switch to a new log) by executing FLUSH LOGS
. See section 4.5.3 FLUSH
Syntax.
mysqld
writes all errors to the stderr, which the
safe_mysqld
script redirects to a file called
'hostname'.err
. (On Windows, mysqld
writes this directly
to `\mysql\data\mysql.err').
This contains information indicating when mysqld
was started and
stopped and also any critical errors found when running. If mysqld
dies unexpectedly and safe_mysqld
needs to restart mysqld
,
safe_mysqld
will write a restarted mysqld
row in this
file. This log also holds a warning if mysqld
notices a table
that needs to be automatically checked or repaired.
On some operating systems, the error log will contain a stack trace
for where mysqld
died. This can be used to find out where
mysqld
died. See section H.1.4 Using a stack trace.
If you want to know what happens within mysqld
, you should start
it with --log[=file]
. This will log all connections and queries
to the log file (by default named `'hostname'.log'). This log can
be very useful when you suspect an error in a client and want to know
exactly what mysqld
thought the client sent to it.
By default, the mysql.server
script starts the MySQL
server with the -l
option. If you need better performance when
you start using MySQL in a production environment, you can
remove the -l
option from mysql.server
or change it to
--log-binary
.
The entries in this log are written as mysqld
receives the questions.
This may be different than the order in which the statements are executed.
This is in contrast to the update log and the binary log which are written
after the query is executed, but before any locks are released.
NOTE: The update log is replaced by the binary log. See section 4.9.4 The Binary Update Log. With this you can do anything that you can do with the update log.
When started with the --log-update[=file_name]
option,
mysqld
writes a log file containing all SQL commands that update
data. If no filename is given, it defaults to the name of the host
machine. If a filename is given, but it doesn't contain a path, the file
is written in the data directory. If `file_name' doesn't have an
extension, mysqld
will create log file names like so:
`file_name.###', where ###
is a number that is incremented each
time you execute mysqladmin refresh
, execute mysqladmin
flush-logs
, execute the FLUSH LOGS
statement, or restart the server.
NOTE: For the above scheme to work, you should NOT create your own files with the same filename as the update log + some extensions that may be regarded as a number, in the directory used by the update log!
If you use the --log
or -l
options, mysqld
writes a
general log with a filename of `hostname.log', and restarts and
refreshes do not cause a new log file to be generated (although it is closed
and reopened). In this case you can copy it (on Unix) by doing:
mv hostname.log hostname-old.log mysqladmin flush-logs cp hostname-old.log to-backup-directory rm hostname-old.log
Update logging is smart because it logs only statements that really update
data. So an UPDATE
or a DELETE
with a WHERE
that finds no
rows is not written to the log. It even skips UPDATE
statements that
set a column to the value it already has.
The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.
If you want to update a database from update log files, you could do the following (assuming your update logs have names of the form `file_name.###'):
shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql
ls
is used to get all the log files in the right order.
This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.
In the future the binary log will replace the update log, so we recommend you to switch to this log format as soon as possible!
The binary log contains all information that is available in the update log in a more efficient format. It also contains information about how long every query that updated the database took.
The binary log is also used when you are replicating a slave from a master. See section 4.10 Replication in MySQL.
When started with the --log-bin[=file_name]
option, mysqld
writes a log file containing all SQL commands that update data. If no
file name is given, it defaults to the name of the host machine followed
by -bin
. If file name is given, but it doesn't contain a path, the
file is written in the data directory.
You can use the following options to mysqld
to affect what is logged
to the binary log:
binlog-do-db=database_name |
Tells the master it should log updates for the specified database, and
exclude all others not explicitly mentioned.
(Example: binlog-do-db=some_database )
|
binlog-ignore-db=database_name |
Tells the master that updates to the given database should not be logged
to the binary log (Example: binlog-ignore-db=some_database )
|
To the binary log filename mysqld
will append an extension that is a
number that is incremented each time you execute mysqladmin
refresh
, execute mysqladmin flush-logs
, execute the FLUSH LOGS
statement or restart the server.
To be able to know which different binary log files have been used,
mysqld
will also create a binary log index file that
contains the name of all used binary log files. By default this has the
same name as the binary log file, with the extension '.index'
.
You can change the name of the binary log index file with the
--log-bin-index=[filename]
option.
If you are using replication, you should not delete old binary log
files until you are sure that no slave will ever need to use them.
One way to do this is to do mysqladmin flush-logs
once a day and then
remove any logs that are more than 3 days old.
You can examine the binary log file with the mysqlbinlog
command.
For example, you can update a MySQL server from the binary log
as follows:
mysqlbinlog log-file | mysql -h server_name
You can also use the mysqlbinlog
program to read the binary log
directly from a remote MySQL server!
mysqlbinlog --help
will give you more information of how to use
this program!
If you are using BEGIN [WORK]
or SET AUTOCOMMIT=0
, you must
use the MySQL binary log for backups instead of the old update log.
The binary logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.
All updates (UPDATE
, DELETE
or INSERT
) that change
a transactional table (like BDB tables) are cached until a COMMIT
.
Any updates to a non-transactional table are stored in the binary log at
once. Every thread will, on start, allocate a buffer of
binlog_cache_size
to buffer queries. If a query is bigger than
this, the thread will open a temporary file to handle the bigger cache.
The temporary file will be deleted when the thread ends.
The max_binlog_cache_size
can be used to restrict the total size used
to cache a multi-transaction query.
If you are using the update or binary log, concurrent inserts will
not work together with CREATE ... INSERT
and INSERT ... SELECT
.
This is to ensure that you can recreate an exact copy of your tables by
applying the log on a backup.
When started with the --log-slow-queries[=file_name]
option,
mysqld
writes a log file containing all SQL commands that took
more than long_query_time
to execute. The time to get the initial
table locks are not counted as execution time.
The slow query log is logged after the query is executed and after all locks has been released. This may be different than the order in which the statements are executed.
If no file name is given, it defaults to the name of the host machine
suffixed with -slow.log
. If a filename is given, but doesn't
contain a path, the file is written in the data directory.
The slow query log can be used to find queries that take a long time to
execute and are thus candidates for optimization. With a large log, that
can become a difficult task. You can pipe the slow query log through the
mysqldumpslow
command to get a summary of the queries which
appear in the log.
You are using --log-long-format
then also queries that are not
using indexes are printed. See section 4.1.1 mysqld Command-line Options.
MySQL has a lot of log files which make it easy to see what is
going. See section 4.9 The MySQL Log Files. One must however from time to time clean up
after MysQL
to ensure that the logs don't take up too much disk
space.
When using MySQL with log files, you will, from time to time, want to remove/backup old log files and tell MySQL to start logging on new files. See section 4.4.1 Database Backups.
On a Linux (Redhat
) installation, you can use the
mysql-log-rotate
script for this. If you installed MySQL
from an RPM distribution, the script should have been installed
automatically. Note that you should be careful with this if you are using
the log for replication!
On other systems you must install a short script yourself that you
start from cron
to handle log files.
You can force MySQL to start using new log files by using
mysqladmin flush-logs
or by using the SQL command FLUSH LOGS
.
If you are using MySQL Version 3.21 you must use mysqladmin refresh
.
The above command does the following:
--log
) or slow query logging
(--log-slow-queries
) is used, closes and reopens the log file.
(`mysql.log' and ``hostname`-slow.log' as default).
--log-update
) is used, closes the update log and
opens a new log file with a higher sequence number.
If you are using only an update log, you only have to flush the logs and then move away the old update log files to a backup. If you are using the normal logging, you can do something like:
shell> cd mysql-data-directory shell> mv mysql.log mysql.old shell> mysqladmin flush-logs
and then take a backup and remove `mysql.old'.
This chapter describes the various replication features in MySQL. It serves as a reference to the options available with replication. You will be introduced to replication and learn how to implement it. Towards the end, there are some frequently asked questions and descriptions of problems and how to solve them.
One way replication can be used is to increase both robustness and speed. For robustness you can have two systems and can switch to the backup if you have problems with the master. The extra speed is achieved by sending a part of the non-updating queries to the replica server. Of course this only works if non-updating queries dominate, but that is the normal case.
Starting in Version 3.23.15, MySQL supports one-way replication internally. One server acts as the master, while the other acts as the slave. Note that one server could play the roles of master in one pair and slave in the other. The master server keeps a binary log of updates (See section 4.9.4 The Binary Update Log.) and an index file to binary logs to keep track of log rotation. The slave, upon connecting, informs the master where it left off since the last successfully propagated update, catches up on the updates, and then blocks and waits for the master to notify it of the new updates.
Note that if you are replicating a database, all updates to this database should be done through the master!
Another benefit of using replication is that one can get live backups of the system by doing a backup on a slave instead of doing it on the master. See section 4.4.1 Database Backups.
MySQL replication is based on the server keeping track of all changes to your database (updates, deletes, etc) in the binary log. (See section 4.9.4 The Binary Update Log.) and the slave server(s) reading the saved queries from the master server's binary log so that the slave can execute the same queries on its copy of the data.
It is very important to realize that the binary log is simply a record starting from a fixed point in time (the moment you enable binary logging). Any slaves which you set up will need copies of all the data from your master as it existed the moment that you enabled binary logging on the master. If you start your slaves with data that doesn't agree with what was on the master when the binary log was started, your slaves may fail.
A future version (4.0) of MySQL will remove the need to keep a (possibly large) snapshot of data for new slaves that you might wish to set up through the live backup functionality with no locking required. However, at this time, it is necessary to block all writes either with a global read lock or by shutting down the master while taking a snapshot.
Once a slave is properly configured and running, it will simply connect
to the master and wait for updates to process. If the master goes away
or the slave loses connectivity with your master, it will keep trying to
connect every master-connect-retry
seconds until it is able to
reconnect and resume listening for updates.
Each slave keeps track of where it left off. The master server has no knowledge of how many slaves there are or which ones are up-to-date at any given time.
The next section explains the master/slave setup process in more detail.
Below is a quick description of how to set up complete replication on your current MySQL server. It assumes you want to replicate all your databases and have not configured replication before. You will need to shutdown your master server briefly to complete the steps outlined below.
FILE
privilege and permission to connect from all the slaves. If the user is
only doing replication (which is recommended), you don't need to grant any
additional privileges.
For example, to create a user named repl
which can access your
master from any host, you might use this command:
GRANT FILE ON *.* TO repl@"%" IDENTIFIED BY '<password>';
mysqladmin -u root -p<password> shutdown
tar -cvf /tmp/mysql-snapshot.tar /path/to/data-dirWindows users can use WinZip or similar software to create an archive of the data directory.
my.cnf
on the master add log-bin
and
server-id=unique number
to the [mysqld]
section and
restart it. It is very important that the id of the slave is different from
the id of the master. Think of server-id
as something similar
to the IP address - it uniquely identifies the server instance in the
community of replication partners.
[mysqld] log-bin server-id=1
my.cnf
on the slave(s):
master-host=<hostname of the master> master-user=<replication user name> master-password=<replication user password> master-port=<TCP/IP port for master> server-id=<some unique number between 2 and 2^32-1>replacing the values in <> with what is relevant to your system.
server-id
must be different for each server participating in
replication. If you don't specify a server-id, it will be set to 1 if
you have not defined master-host
, else it will be set to 2. Note
that in the case of server-id
omission the master will refuse
connections from all slaves, and the slave will refuse to connect to a
master. Thus, omitting server-id
is only good for backup with a
binary log.
After you have done the above, the slave(s) should connect to the master and catch up on any updates which happened since the snapshot was taken.
If you have forgotten to set server-id
for the slave you will get
the following error in the error log file:
Warning: one should set server_id to a non-0 value if master_host is set. The server will not act as a slave.
If you have forgot to do this for the master, the slaves will not be able to connect to the master.
If a slave is not able to replicate for any reason, you will find error messages in the error log on the slave.
Once a slave is replicating, you will find a file called
master.info
in the same directory as your error log. The
master.info
file is used by the slave to keep track of how much
of the master's binary log is has processed. Do not remove or
edit the file, unless you really know what you are doing. Even in that case,
it is preferred that you use CHANGE MASTER TO
command.
Below is an explanation of what is supported and what is not:
AUTO_INCREMENT
,
LAST_INSERT_ID
, and TIMESTAMP
values.
RAND()
in updates does not replicate properly. Use
RAND(some_non_rand_expr)
if you are replicating updates with
RAND()
. You can, for example, use UNIX_TIMESTAMP()
for the
argument to RAND()
.
--default-character-set
)
on the master and the slave. If not, you may get duplicate key errors on
the slave, because a key that is regarded as unique on the master may
not be that in the other character set.
LOAD DATA INFILE
will be handled properly as long as the file
still resides on the master server at the time of update
propagation. LOAD LOCAL DATA INFILE
will be skipped.
FLUSH
commands are not stored in the binary log and are because
of this not replicated to the slaves. This is not normally a problem as
FLUSH
doesn't change anything. This does however mean that if you
update the MySQL
privilege tables directly without using
GRANT
statement and you replicate the MySQL
privilege
database, you must do a FLUSH PRIVILEGES
on your slaves to put
the new privileges into effect.
SLAVE STOP
, then
check Slave_open_temp_tables
variable to see if it is 0, then issue
mysqladmin shutdown
. If the number is not 0, restart the slave thread
with SLAVE START
and see
if you have better luck next time. There will be a cleaner solution, but it
has to wait until version 4.0.
In earlier versions temporary tables are not being replicated properly - we
recommend that you either upgrade, or execute SET SQL_LOG_BIN=0
on
your clients before all queries with temp tables.
log-slave-updates
enabled.
Note, however, that many queries will not work right in this kind of
setup unless your client code is written to take care of the potential
problems that can happen from updates that occur in different sequence
on different servers.
This means that you can do a setup like the following:
A -> B -> C -> AThis setup will only works if you only do non conflicting updates between the tables. In other words, if you insert data in A and C, you should never insert a row in A that may have a conflicting key with a row insert in C. You should also not update the sam rows on two servers if the order in which the updates are applied matters. Note that the log format has changed in Version 3.23.26 so that pre-3.23.26 slaves will not be able to read it.
.err
file. You should
then connect to the slave manually, fix the cause of the error (for
example, non-existent table), and then run SLAVE START
sql
command (available starting in Version 3.23.16). In Version 3.23.15, you
will have to restart the server.
master-connect-retry
(default
60) seconds. Because of this, it is safe to shut down the master, and
then restart it after a while. The slave will also be able to deal with
network connectivity outages.
master-port
parameter in my.cnf
.
replicate-do-db
directives in
my.cnf
or just exclude a set of databases with
replicate-ignore-db
. Note that up until Version 3.23.23, there was a bug
that did not properly deal with LOAD DATA INFILE
if you did it in
a database that was excluded from replication.
SET SQL_LOG_BIN = 0
will turn off
replication (binary) logging on the master, and SET SQL_LOG_BIN =
1
will turn in back on - you must have the process privilege to do
this.
FLUSH MASTER
and FLUSH SLAVE
commands. In Version 3.23.26 we have renamed them to
RESET MASTER
and RESET SLAVE
respectively to clarify
what they do. The old FLUSH
variants still work, though, for
compatibility.
LOAD TABLE FROM MASTER
for
network backup and to set up replication initially. We have recently
received a number of bug reports concerning it that we are investigating, so
we recommend that you use it only in testing until we make it more stable.
CHANGE MASTER TO
.
binlog-ignore-db
.
replicate-rewrite-db
to tell
the slave to apply updates from one database on the master to the one
with a different name on the slave.
PURGE MASTER LOGS TO 'log-name'
to get rid of old logs while the slave is running.
If you are using replication, we recommend you to use MySQL Version 3.23.30 or later. Older versions work, but they do have some bugs and are missing some features.
On both master and slave you need to use the server-id
option.
This sets an unique replication id. You should pick a unique value in the
range between 1 to 2^32-1 for each master and slave.
Example: server-id=3
The following table has the options you can use for the MASTER:
Option | Description |
log-bin=filename |
Write to a binary update log to the specified location. Note that if you
give it a parameter with an extension (for example,
log-bin=/mysql/logs/replication.log ) versions up to 3.23.24 will
not work right during replication if you do FLUSH LOGS . The
problem is fixed in Version 3.23.25. If you are using this kind of log
name, FLUSH LOGS will be ignored on binlog. To clear the log, run
FLUSH MASTER , and do not forget to run FLUSH SLAVE on all
slaves. In Version 3.23.26 and in later versions you should use
RESET MASTER and RESET SLAVE
|
log-bin-index=filename |
Because the user could issue the FLUSH LOGS command, we need to
know which log is currently active and which ones have been rotated out
and in what sequence. This information is stored in the binary log index file.
The default is `hostname`.index. You can use this option if you want to
be a rebel. (Example: log-bin-index=db.index )
|
sql-bin-update-same |
If set, setting SQL_LOG_BIN to a value will automatically set
SQL_LOG_UPDATE to the same value and vice versa.
|
binlog-do-db=database_name |
Tells the master it should log updates for the specified database, and
exclude all others not explicitly mentioned.
(Example: binlog-do-db=some_database )
|
binlog-ignore-db=database_name |
Tells the master that updates to the given database should not be logged
to the binary log (Example: binlog-ignore-db=some_database )
|
The following table has the options you can use for the SLAVE:
Option | Description |
master-host=host |
Master hostname or IP address for replication. If not set, the slave
thread will not be started.
(Example: master-host=db-master.mycompany.com )
|
master-user=username |
The user the slave thread will us for authentication when connecting to
the master. The user must have FILE privilege. If the master user
is not set, user test is assumed. (Example:
master-user=scott )
|
master-password=password |
The password the slave thread will authenticate with when connecting to
the master. If not set, an empty password is assumed. (Example:
master-password=tiger )
|
master-port=portnumber |
The port the master is listening on. If not set, the compiled setting of
MYSQL_PORT is assumed. If you have not tinkered with
configure options, this should be 3306. (Example:
master-port=3306 )
|
master-connect-retry=seconds |
The number of seconds the slave thread will sleep before retrying to
connect to the master in case the master goes down or the connection is
lost. Default is 60. (Example: master-connect-retry=60 )
|
master-info-file=filename |
The location of the file that remembers where we left off on the master
during the replication process. The default is master.info in the data
directory. Sasha: The only reason I see for ever changing the default
is the desire to be rebelious. (Example:
master-info-file=master.info )
|
replicate-do-table=db_name.table_name |
Tells the slave thread to restrict replication to the specified database.
To specify more than one table, use the directive multiple times,
once for each table. .
(Example: replicate-do-table=some_db.some_table )
|
replicate-ignore-table=db_name.table_name |
Tells the slave thread to not replicate to the specified table. To
specify more than one table to ignore, use the directive multiple
times, once for each table.(Example:
replicate-ignore-table=db_name.some_table )
|
replicate-wild-do-table=db_name.table_name |
Tells the slave thread to restrict replication to the tables that match the
specified wildcard pattern. .
To specify more than one table, use the directive multiple times,
once for each table. .
(Example: replicate-do-table=foo%.bar% will replicate only updates
to tables in all databases that start with foo and whose table names
start with bar)
|
replicate-wild-ignore-table=db_name.table_name |
Tells the slave thread to not replicate to the tables that match the given
wild card pattern. To
specify more than one table to ignore, use the directive multiple
times, once for each table.(Example:
replicate-ignore-table=foo%.bar% - will not upates to tables in all databases that start with foo and whose table names
start with bar)
|
replicate-ignore-db=database_name |
Tells the slave thread to not replicate to the specified database. To
specify more than one database to ignore, use the directive multiple
times, once for each database. This option will not work if you use cross
database updates. If you need cross database updates to work, make sure
you have 3.23.28 or later, and use
replicate-wild-ignore-table=db_name.% (Example:
replicate-ignore-db=some_db )
|
replicate-do-db=database_name |
Tells the slave thread to restrict replication to the specified database.
To specify more than one database, use the directive multiple times,
once for each database. Note that this will only work if you do not use
cross-database queries such as UPDATE some_db.some_table SET
foo='bar' while having selected a different or no database. If you need
cross database updates to work, make sure
you have 3.23.28 or later, and use
replicate-wild-do-table=db_name.%
(Example: replicate-do-db=some_db )
|
log-slave-updates | Tells the slave to log the updates from the slave thread to the binary log. Off by default. You will need to turn it on if you plan to daisy-chain the slaves. |
replicate-rewrite-db=from_name->to_name |
Updates to a database with a different name than the original (Example:
replicate-rewrite-db=master_db_name->slave_db_name
|
skip-slave-start |
Tells the slave server not to start the slave on the startup. The user
can start it later with SLAVE START .
|
slave_read_timeout=# | Number of seconds to wait for more data from the master before aborting the read. |
Replication can be controlled through the SQL interface. Below is the summary of commands:
Command | Description |
SLAVE START
| Starts the slave thread. (Slave) |
SLAVE STOP
| Stops the slave thread. (Slave) |
SET SQL_LOG_BIN=0
| Disables update logging if the user has process privilege. Ignored otherwise. (Master) |
SET SQL_LOG_BIN=1
| Re-enables update logging if the user has process privilege. Ignored otherwise. (Master) |
SET SQL_SLAVE_SKIP_COUNTER=n
| Skip the next n events from the master. Only valid when
the slave thread is not running, otherwise, gives an error. Useful for
recovering from replication glitches.
|
RESET MASTER
| Deletes all binary logs listed in the index file, resetting the binlog
index file to be empty. In pre-3.23.26 versions, FLUSH MASTER (Master)
|
RESET SLAVE
| Makes the slave forget its replication position in the master
logs. In pre 3.23.26 versions the command was called
FLUSH SLAVE (Slave)
|
LOAD TABLE tblname FROM MASTER
| Downloads a copy of the table from master to the slave. (Slave) |
CHANGE MASTER TO master_def_list
| Changes the master parameters to the values specified in
master_def_list and restarts the slave thread. master_def_list
is a comma-separated list of master_def where master_def is
one of the following: MASTER_HOST , MASTER_USER ,
MASTER_PASSWORD , MASTER_PORT , MASTER_CONNECT_RETRY ,
MASTER_LOG_FILE , MASTER_LOG_POS . Example:
CHANGE MASTER TO MASTER_HOST='master2.mycompany.com', MASTER_USER='replication', MASTER_PASSWORD='bigs3cret', MASTER_PORT=3306, MASTER_LOG_FILE='master2-bin.001', MASTER_LOG_POS=4;You only need to specify the values that need to be changed. The values that you omit will stay the same with the exception of when you change the host or the port. In that case, the slave will assume that since you are connecting to a different host or a different port, the master is different. Therefore, the old values of log and position are not applicable anymore, and will automatically be reset to an empty string and 0, respectively (the start values). Note that if you restart the slave, it will remember its last master. If this is not desirable, you should delete the `master.info' file before restarting, and the slave will read its master from my.cnf or the
command line. (Slave)
|
SHOW MASTER STATUS | Provides status information on the binlog of the master. (Master) |
SHOW SLAVE STATUS | Provides status information on essential parameters of the slave thread. (Slave) |
SHOW MASTER LOGS | Only available starting in Version 3.23.28. Lists the binary logs on the master. You should use this command prior to PURGE MASTER LOGS TO to find out how far you should go.
|
PURGE MASTER LOGS TO 'logname'
| Available starting in Version 3.23.28. Deletes all the
replication logs that are listed in the log
index as being prior to the specified log, and removed them from the
log index, so that the given log now becomes first. Example:
PURGE MASTER LOGS TO 'mysql-bin.010'This command will do nothing and fail with an error if you have an active slave that is currently reading one of the logs you are trying to delete. However, if you have a dormant slave, and happen to purge one of the logs it wants to read, the slave will be unable to replicate once it comes up. The command is safe to run while slaves are replicating - you do not need to stop them. You must first check all the slaves with SHOW SLAVE STATUS to
see which log they are on, then do a listing of the logs on the
master with SHOW MASTER LOGS , find the earliest log among all
the slaves (if all the slaves are up to date, this will be the
last log on the list), backup all the logs you are about to delete
(optional) and purge up to the target log.
|
Q: Why do I sometimes see more than one Binlog_Dump
thread on
the master after I have restarted the slave?
A: Binlog_Dump
is a continuous process that is handled by the
server in the following way:
pthread_cond_wait()
,
from which we can be awakened either by an update or a kill.
Binlog_dump
loop.
So if the slave thread stops on the slave, the corresponding
Binlog_Dump
thread on the master will not notice it until after
at least one update to the master (or a kill), which is needed to wake
it up from pthread_cond_wait()
. In the meantime, the slave
could have opened another connection, which resulted in another
Binlog_Dump
thread.
The above problem should not be present in Version 3.23.26 and later
versions. In Version 3.23.26 we added server-id
to each
replication server, and now all the old zombie threads are killed on the
master when a new replication thread connects from the same slave
Q: How do I rotate replication logs?
A: In Version 3.23.28 you should use PURGE MASTER LOGS
TO
command after determining which logs can be deleted, and optionally
backing them up first. In earlier versions the process is much more
painful, and cannot be safely done without stopping all the slaves in
the case that you plan to re-use log names. You will need to stop the
slave threads, edit the binary log index file, delete all the old logs,
restart the master, start slave threads, and then remove the old log files.
Q: How do I upgrade on a hot replication setup?
A: If you are upgrading pre-3.23.26 versions, you should just
lock the master tables, let the slave catch up, then run FLUSH
MASTER
on the master, and FLUSH SLAVE
on the slave to reset the
logs, then restart new versions of the master and the slave. Note that
the slave can stay down for some time - since the master is logging
all the updates, the slave will be able to catch up once it is up and
can connect.
After 3.23.26, we have locked the replication protocol for modifications, so you can upgrade masters and slave on the fly to a newer 3.23 version and you can have different versions of MySQL running on the slave and the master, as long as they are both newer than 3.23.26.
Q: What issues should I be aware of when setting up two-way replication?
A: MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In in other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that will make the update of client A work differently than it did on co-master 1. Thus when the update of client A will make it to co-master 2, it will produce tables that will be different than what you have on co-master 1, even after all the updates from co-master 2 have also propagated. So you should not co-chain two servers in a two-way replication relationship, unless you are sure that you updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code.
You must also realize that two-way replication actually does not improve performance very much, if at all, as far as updates are concerned. Both servers need to do the same amount of updates each, as you would have one server do. The only difference is that there will be a little less lock contention, because the updates originating on another server will be serialized in one slave thread. This benefit, though, might be offset by network delays.
Q: How can I use replication to improve performance of my system?
A: You should set up one server as the master, and direct all
writes to it, and configure as many slaves as you have the money and
rackspace for, distributing the reads among the master and the slaves.
You can also start the slaves with --skip-bdb
,
--low-priority-updates
and --delay-key-write-for-all-tables
to get speed improvements for the slave. In this case the slave will
use non-transactional MyISAM
tables instead of BDB
tables
to get more speed.
Q: What should I do to prepare my client code to use performance-enhancing replication?
A: If the part of your code that is responsible for database access has been properly abstracted/modularized, converting it to run with the replicated setup should be very smooth and easy - just change the implementation of your database access to read from some slave or the master, and to always write to the master. If your code does not have this level of abstraction, setting up a replicated system will give you an opportunity/motivation to it clean up. You should start by creating a wrapper library /module with the following functions:
safe_writer_connect()
safe_reader_connect()
safe_reader_query()
safe_writer_query()
safe_
means that the function will take care of handling all
the error conditions.
You should then convert your client code to use the wrapper library.
It may be a painful and scary process at first, but it will pay off in
the long run. All applications that follow the above pattern will be
able to take advantage of one-master/many slaves solution. The
code will be a lot easier to maintain, and adding troubleshooting
options will be trivial. You will just need to modify one or two
functions, for example, to log how long each query took, or which
query, among your many thousands, gave you an error. If you have written a lot of code already,
you may want to automate the conversion task by using Monty's
replace
utility, which comes with the standard distribution of
MySQL, or just write your own Perl script. Hopefully, your
code follows some recognizable pattern. If not, then you are probably
better off re-writing it anyway, or at least going through and manually
beating it into a pattern.
Note that, of course, you can use different names for the functions. What is important is having unified interface for connecting for reads, connecting for writes, doing a read, and doing a write.
Q: When and how much can MySQL replication improve the performance of my system?
A: MySQL replication is most beneficial for a system with frequent reads and not so frequent writes. In theory, by using a one master/many slaves setup you can scale by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it.
In order to determine how many slaves you can get before the added
benefits begin to level out, and how much you can improve performance
of your site, you need to know your query patterns, and empirically
(by benchmarking) determine the relationship between the throughput
on reads (reads per second, or max_reads
) and on writes
max_writes
) on a typical master and a typical slave. The
example below will show you a rather simplified calculation of what you
can get with replication for our imagined system.
Let's say our system load consists of 10% writes and 90% reads, and we
have determined that max_reads
= 1200 - 2 * max_writes
,
or in other words, our system can do 1200 reads per second with no
writes, our average write is twice as slow as average read,
and the relationship is
linear. Let us suppose that our master and slave are of the same
capacity, and we have N slaves and 1 master. Then we have for each
server (master or slave):
reads = 1200 - 2 * writes
(from bencmarks)
reads = 9* writes / (N + 1)
(reads split, but writes go
to all servers)
9*writes/(N+1) + 2 * writes = 1200
writes = 1200/(2 + 9/(N+1)
So if N = 0, which means we have no replication, our system can handle 1200/11, about 109 writes per second (which means we will have 9 times as many reads due to the nature of our application).
If N = 1, we can get up to 184 writes per second.
If N = 8, we get up to 400.
If N = 17, 480 writes.
Eventually as N approaches infinity (and our budget negative infinity), we can get very close to 600 writes per second, increasing system throughput about 5.5 times. However, with only 8 servers, we increased it almost 4 times already.
Note that our computations assumed infinite network bandwidth, and neglected several other factors that could turn out to be significant on your system. In many cases, you may not be able to make a computation similar to the one above that will accurately predict what will happen on your system if you add N replication slaves. However, answering the following questions should help you decided whether and how much, if at all, the replication will improve the performance of your system:
Q: How can I use replication to provide redundancy/high availability?
A: With the currently available features, you would have to set up a master and a slave (or several slaves), and write a script that will monitor the master to see if it is up, and instruct your applications and the slaves of the master change in case of failure. Some suggestions:
CHANGE MASTER TO
command.
nsupdate
to dynamically update your DNS.
log-bin
option and without
log-slave-updates
. This way the slave will be ready to become a
master as soon as you issue STOP SLAVE
; RESET MASTER
, and
CHANGE MASTER TO
on the other slaves. It will also help you catch
spurious updates that may happen because of misconfiguration of the
slave (ideally, you want to configure access rights so that no client
can update the slave, except for the slave thread) combined with the
bugs in your client programs (they should never update the slave
directly).
We are currently working on integrating an automatic master election system into MySQL, but until it is ready, you will have to create your own monitoring tools.
If you have followed the instructions, and your replication setup is not working, first eliminate the user error factor by checking the following:
SHOW MASTER STATUS
.
If it is, Position
will be non-zero. If not, verify that you have
given the master log-bin
option and have set server-id
.
SHOW SLAVE STATUS
. The answer is found
in Slave_running
column. If not, verify slave options and check the
error log for messages.
SHOW PROCESSLIST
, find the thread with system user
value in
User
column and none
in the Host
column, and check the
State
column. If it says connecting to master
, verify the
privileges for the replication user on the master, master host name, your
DNS setup, whether the master is actually running, whether it is reachable
from the slave, and if all that seems ok, read the error logs.
SLAVE START
SET SQL_SLAVE_SKIP_COUNTER=1; SLAVE START;
to skip a query that
does not use auto_increment, or last_insert_id or
SET SQL_SLAVE_SKIP_COUNTER=2; SLAVE START;
otherwise. The reason
auto_increment/last_insert_id queries are different is that they take
two events in the binary log of the master.
grep -i slave /path/to/your-log.err
on the slave. There is no
generic pattern to search for on the master, as the only errors it logs
are general system errors - if it can, it will send the error to the slave
when things go wrong.
When you have determined that there is no user error involved, and replication
still either does not work at all or is unstable, it is time to start working
on a bug report. We need to get as much info as possible from you to be able
to track down the bug. Please do spend some time and effort preparing a good
bug report. Ideally, we would like to have a test case in the format found in
mysql-test/t/rpl*
directory of the source tree. If you submit a test
case like that, you can expect a patch within a day or two in most cases,
although, of course, you mileage may vary depending on a number of factors.
Second best option is a just program with easily configurable connection arguments for the master and the slave that will demonstrate the problem on our systems. You can write one in Perl or in C, depending on which language you know better.
If you have one of the above ways to demonstrate the bug, use
mysqlbug
to prepare a bug report and send it to
bugs@lists.mysql.com. If you have a phantom - a problem that
does occur but you cannot duplicate "at will":
log-slave-updates
and log-bin
- this will keep
a log of all updates on the slave.
SHOW MASTER STATUS
on the master at the time
you have discovered the problem
SHOW SLAVE STATUS
on the master at the time
you have discovered the problem
mysqlbinlog
to examine the binary logs. The following should
be helpful
to find the trouble query, for example:
mysqlbinlog -j pos_from_slave_status /path/to/log_from_slave_status | head
Once you have collected the evidence on the phantom problem, try hard to isolate it into a separate test case first. Then report the problem to bugs@lists.mysql.com with as much info as possible.
Optimization is a complicated task because it ultimately requires understanding of the whole system. While it may be possible to do some local optimizations with small knowledge of your system or application, the more optimal you want your system to become the more you will have to know about it.
This chapter will try to explain and give some examples of different ways to optimize MySQL. Remember, however, that there are always some (increasingly harder) additional ways to make the system even faster.
The most important part for getting a system fast is of course the basic design. You also need to know what kinds of things your system will be doing, and what your bottlenecks are.
The most common bottlenecks are:
Because MySQL uses extremely fast table locking (multiple readers / single writers) the biggest remaining problem is a mix of a steady stream of inserts and slow selects on the same table.
We believe that for a huge number of systems the extremely fast performance in other cases make this choice a win. This case is usually also possible to solve by having multiple copies of the table, but it takes more effort and hardware.
We are also working on some extensions to solve this problem for some common application niches.
Because all SQL servers implement different parts of SQL, it takes work to write portable SQL applications. For very simple selects/inserts it is very easy, but the more you need the harder it gets. If you want an application that is fast with many databases it becomes even harder!
To make a complex application portable you need to choose a number of SQL servers that it should work with.
You can use the MySQL crash-me program/web-page http://www.mysql.com/information/crash-me.php to find functions, types, and limits you can use with a selection of database servers. Crash-me now tests far from everything possible, but it is still comprehensive with about 450 things tested.
For example, you shouldn't have column names longer than 18 characters if you want to be able to use Informix or DB2.
Both the MySQL benchmarks and crash-me programs are very database-independent. By taking a look at how we have handled this, you can get a feeling for what you have to do to write your application database-independent. The benchmarks themselves can be found in the `sql-bench' directory in the MySQL source distribution. They are written in Perl with DBI database interface (which solves the access part of the problem).
See http://www.mysql.com/information/benchmarks.html for the results from this benchmark.
As you can see in these results, all databases have some weak points. That is, they have different design compromises that lead to different behavior.
If you strive for database independence, you need to get a good feeling for each SQL server's bottlenecks. MySQL is VERY fast in retrieving and updating things, but will have a problem in mixing slow readers/writers on the same table. Oracle, on the other hand, has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transaction databases in general are not very good at generating summary tables from log tables, as in this case row locking is almost useless.
To get your application really database-independent, you need to define an easy extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ classes interface to the databases.
If you use some specific feature for some database (like the
REPLACE
command in MySQL), you should code a method for
the other SQL servers to implement the same feature (but slower). With
MySQL you can use the /*! */
syntax to add
MySQL-specific keywords to a query. The code inside
/**/
will be treated as a comment (ignored) by most other SQL
servers.
If REAL high performance is more important than exactness, as in some Web applications, a possibility is to create an application layer that caches all results to give you even higher performance. By letting old results 'expire' after a while, you can keep the cache reasonably fresh. This is quite nice in case of extremely high load, in which case you can dynamically increase the cache and set the expire timeout higher until things get back to normal.
In this case the table creation information should contain information of the initial size of the cache and how often the table should normally be refreshed.
During MySQL initial development, the features of MySQL were made to fit our largest customer. They handle data warehousing for a couple of the biggest retailers in Sweden.
From all stores, we get weekly summaries of all bonus card transactions, and we are expected to provide useful information for the store owners to help them find how their advertisement campaigns are affecting their customers.
The data is quite huge (about 7 million summary transactions per month), and we have data for 4-10 years that we need to present to the users. We got weekly requests from the customers that they want to get 'instant' access to new reports from this data.
We solved this by storing all information per month in compressed 'transaction' tables. We have a set of simple macros (script) that generates summary tables grouped by different criteria (product group, customer id, store ...) from the transaction tables. The reports are Web pages that are dynamically generated by a small Perl script that parses a Web page, executes the SQL statements in it, and inserts the results. We would have used PHP or mod_perl instead but they were not available at that time.
For graphical data we wrote a simple tool in C
that can produce
GIFs based on the result of a SQL query (with some processing of the
result). This is also dynamically executed from the Perl script that
parses the HTML
files.
In most cases a new report can simply be done by copying an existing script and modifying the SQL query in it. In some cases, we will need to add more fields to an existing summary table or generate a new one, but this is also quite simple, as we keep all transactions tables on disk. (Currently we have at least 50G of transactions tables and 200G of other customer data.)
We also let our customers access the summary tables directly with ODBC so that the advanced users can themselves experiment with the data.
We haven't had any problems handling this with quite modest Sun Ultra SPARCstation (2x200 Mhz). We recently upgraded one of our servers to a 2 CPU 400 Mhz UltraSPARC, and we are now planning to start handling transactions on the product level, which would mean a ten-fold increase of data. We think we can keep up with this by just adding more disk to our systems.
We are also experimenting with Intel-Linux to be able to get more CPU power cheaper. Now that we have the binary portable database format (new in Version 3.23), we will start to use this for some parts of the application.
Our initial feelings are that Linux will perform much better on low-to-medium load and Solaris will perform better when you start to get a high load because of extreme disk IO, but we don't yet have anything conclusive about this. After some discussion with a Linux Kernel developer, this might be a side effect of Linux giving so much resources to the batch job that the interactive performance gets very low. This makes the machine feel very slow and unresponsive while big batches are going. Hopefully this will be better handled in future Linux Kernels.
This should contain a technical description of the MySQL
benchmark suite (and crash-me
), but that description is not
written yet. Currently, you can get a good idea of the benchmark by
looking at the code and results in the `sql-bench' directory in any
MySQL source distributions.
This benchmark suite is meant to be a benchmark that will tell any user what things a given SQL implementation performs well or poorly at.
Note that this benchmark is single threaded, so it measures the minimum time for the operations. We plan to in the future add a lot of multi-threaded tests to the benchmark suite.
For example, (run on the same NT 4.0 machine):
Seconds | Seconds | |
mysql | 367 | 249 |
mysql_odbc | 464 | |
db2_odbc | 1206 | |
informix_odbc | 121126 | |
ms-sql_odbc | 1634 | |
oracle_odbc | 20800 | |
solid_odbc | 877 | |
sybase_odbc | 17614 |
Seconds | Seconds | |
mysql | 381 | 206 |
mysql_odbc | 619 | |
db2_odbc | 3460 | |
informix_odbc | 2692 | |
ms-sql_odbc | 4012 | |
oracle_odbc | 11291 | |
solid_odbc | 1801 | |
sybase_odbc | 4802 |
In the above test MySQL was run with a 8M index cache.
We have gather some more benchmark results at http://www.mysql.com/information/benchmarks.html.
Note that Oracle is not included because they asked to be removed. All Oracle benchmarks have to be passed by Oracle! We believe that makes Oracle benchmarks VERY biased because the above benchmarks are supposed to show what a standard installation can do for a single client.
To run the benchmark suite, you have to download a MySQL source distribution, install the perl DBI driver, the perl DBD driver for the database you want to test and then do:
cd sql-bench perl run-all-tests --server=#
where # is one of supported servers. You can get a list of all options
and supported servers by doing run-all-tests --help
.
crash-me
tries to determine what features a database supports and
what its capabilities and limitations are by actually running
queries. For example, it determines:
VARCHAR
column can be
We can find the result from crash-me on a lot of different databases at http://www.mysql.com/information/crash-me.php.
You should definitely benchmark your application and database to find out where the bottlenecks are. By fixing it (or by replacing the bottleneck with a 'dummy module') you can then easily identify the next bottleneck (and so on). Even if the overall performance for your application is sufficient, you should at least make a plan for each bottleneck, and decide how to solve it if someday you really need the extra performance.
For an example of portable benchmark programs, look at the MySQL benchmark suite. See section 5.1.4 The MySQL Benchmark Suite. You can take any program from this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you.
It is very common that some problems only occur when the system is very heavily loaded. We have had many customers who contact us when they have a (tested) system in production and have encountered load problems. In every one of these cases so far, it has been problems with basic design (table scans are NOT good at high load) or OS/Library issues. Most of this would be a LOT easier to fix if the systems were not already in production.
To avoid problems like this, you should put some effort into benchmarking your whole application under the worst possible load! You can use Super Smack for this, and it is available at: http://www.mysql.com/Downloads/super-smack/super-smack-1.0.tar.gz. As the name suggests, it can bring your system down to its knees if you ask it, so make sure to use it only on your development systems.
SELECT
s and Other QueriesFirst, one thing that affects all queries: The more complex permission system setup you have, the more overhead you get.
If you do not have any GRANT
statements done, MySQL will
optimize the permission checking somewhat. So if you have a very high
volume it may be worth the time to avoid grants. Otherwise more
permission check results in a larger overhead.
If your problem is with some explicit MySQL function, you can always time this in the MySQL client:
mysql> select benchmark(1000000,1+1); +------------------------+ | benchmark(1000000,1+1) | +------------------------+ | 0 | +------------------------+ 1 row in set (0.32 sec)
The above shows that MySQL can execute 1,000,000 +
expressions in 0.32 seconds on a PentiumII 400MHz
.
All MySQL functions should be very optimized, but there may be
some exceptions, and the benchmark(loop_count,expression)
is a
great tool to find out if this is a problem with your query.
EXPLAIN
Syntax (Get Information About a SELECT
)EXPLAIN tbl_name or EXPLAIN SELECT select_options
EXPLAIN tbl_name
is a synonym for DESCRIBE tbl_name
or
SHOW COLUMNS FROM tbl_name
.
When you precede a SELECT
statement with the keyword EXPLAIN
,
MySQL explains how it would process the SELECT
, providing
information about how tables are joined and in which order.
With the help of EXPLAIN
, you can see when you must add indexes
to tables to get a faster SELECT
that uses indexes to find the
records. You can also see if the optimizer joins the tables in an optimal
order. To force the optimizer to use a specific join order for a
SELECT
statement, add a STRAIGHT_JOIN
clause.
For non-simple joins, EXPLAIN
returns a row of information for each
table used in the SELECT
statement. The tables are listed in the order
they would be read. MySQL resolves all joins using a single-sweep
multi-join method. This means that MySQL reads a row from the first
table, then finds a matching row in the second table, then in the third table
and so on. When all tables are processed, it outputs the selected columns and
backtracks through the table list until a table is found for which there are
more matching rows. The next row is read from this table and the process
continues with the next table.
Output from EXPLAIN
includes the following columns:
table
type
possible_keys
possible_keys
column indicates which indexes MySQL
could use to find the rows in this table. Note that this column is
totally independent of the order of the tables. That means that some of
the keys in possible_keys may not be usable in practice with the
generated table order.
If this column is empty, there are no relevant indexes. In this case,
you may be able to improve the performance of your query by examining
the WHERE
clause to see if it refers to some column or columns
that would be suitable for indexing. If so, create an appropriate index
and check the query with EXPLAIN
again. See section 6.5.4 ALTER TABLE
Syntax.
To see what indexes a table has, use SHOW INDEX FROM tbl_name
.
key
key
column indicates the key that MySQL actually
decided to use. The key is NULL
if no index was chosen. If
MySQL chooses the wrong index, you can probably force
MySQL to use another index by using myisamchk --analyze
,
See section 4.4.6.1 myisamchk
Invocation Syntax, or by using USE INDEX/IGNORE INDEX
.
See section 6.4.1.1 JOIN
Syntax.
key_len
key_len
column indicates the length of the key that
MySQL decided to use. The length is NULL
if the
key
is NULL
. Note that this tells us how many parts of a
multi-part key MySQL will actually use.
ref
ref
column shows which columns or constants are used with the
key
to select rows from the table.
rows
rows
column indicates the number of rows MySQL
believes it must examine to execute the query.
Extra
Distinct
Not exists
LEFT JOIN
optimization on the
query and will not examine more rows in this table for the previous row
combination after it finds one row that matches the LEFT JOIN
criteria.
Here is an example for this:
SELECT * FROM t1 LEFT JOIN t2 ON t1.id=t2.id WHERE t2.id IS NULL;Assume that
t2.id
is defined with NOT NULL
. In this case
MySQL will scan t1
and look up the rows in t2
through t1.id
. If MySQL finds a matching row in
t2
, it knows that t2.id
can never be NULL
, and will
not scan through the rest of the rows in t2
that has the same
id
. In other words, for each row in t1
, MySQL
only needs to do a single lookup in t2
, independent of how many
matching rows there are in t2
.
range checked for each record (index map: #)
Using filesort
join type
and storing the sort key + pointer to
the row for all rows that match the WHERE
. Then the keys are
sorted. Finally the rows are retrieved in sorted order.
Using index
Using temporary
ORDER BY
on a different column set than you did a GROUP
BY
on.
Where used
WHERE
clause will be used to restrict which rows will be
matched against the next table or sent to the client. If you don't have
this information and the table is of type ALL
or index
,
you may have something wrong in your query (if you don't intend to
fetch/examine all rows from the table).
Using filesort
and Using temporary
.
The different join types are listed below, ordered from best to worst type:
system
const
join type.
const
const
tables are very fast as they are read only once!
eq_ref
const
types. It is used when all parts of an index are used by
the join and the index is UNIQUE
or a PRIMARY KEY
.
ref
ref
is used if the join
uses only a leftmost prefix of the key, or if the key is not UNIQUE
or a PRIMARY KEY
(in other words, if the join cannot select a single
row based on the key value). If the key that is used matches only a few rows,
this join type is good.
range
key
column indicates which index is used.
The key_len
contains the longest key part that was used.
The ref
column will be NULL for this type.
index
ALL
, except that only the index tree is
scanned. This is usually faster than ALL
, as the index file is usually
smaller than the data file.
ALL
const
, and usually very bad in all other
cases. You normally can avoid ALL
by adding more indexes, so that
the row can be retrieved based on constant values or column values from
earlier tables.
You can get a good indication of how good a join is by multiplying all values
in the rows
column of the EXPLAIN
output. This should tell you
roughly how many rows MySQL must examine to execute the query. This
number is also used when you restrict queries with the max_join_size
variable.
See section 5.5.2 Tuning Server Parameters.
The following example shows how a JOIN
can be optimized progressively
using the information provided by EXPLAIN
.
Suppose you have the SELECT
statement shown below, that you examine
using EXPLAIN
:
EXPLAIN SELECT tt.TicketNumber, tt.TimeIn, tt.ProjectReference, tt.EstimatedShipDate, tt.ActualShipDate, tt.ClientID, tt.ServiceCodes, tt.RepetitiveID, tt.CurrentProcess, tt.CurrentDPPerson, tt.RecordVolume, tt.DPPrinted, et.COUNTRY, et_1.COUNTRY, do.CUSTNAME FROM tt, et, et AS et_1, do WHERE tt.SubmitTime IS NULL AND tt.ActualPC = et.EMPLOYID AND tt.AssignedPC = et_1.EMPLOYID AND tt.ClientID = do.CUSTNMBR;
For this example, assume that:
Table | Column | Column type |
tt | ActualPC | CHAR(10)
|
tt | AssignedPC | CHAR(10)
|
tt | ClientID | CHAR(10)
|
et | EMPLOYID | CHAR(15)
|
do | CUSTNMBR | CHAR(15)
|
Table | Index |
tt | ActualPC
|
tt | AssignedPC
|
tt | ClientID
|
et | EMPLOYID (primary key)
|
do | CUSTNMBR (primary key)
|
tt.ActualPC
values aren't evenly distributed.
Initially, before any optimizations have been performed, the EXPLAIN
statement produces the following information:
table type possible_keys key key_len ref rows Extra et ALL PRIMARY NULL NULL NULL 74 do ALL PRIMARY NULL NULL NULL 2135 et_1 ALL PRIMARY NULL NULL NULL 74 tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 range checked for each record (key map: 35)
Because type
is ALL
for each table, this output indicates that
MySQL is doing a full join for all tables! This will take quite a
long time, as the product of the number of rows in each table must be
examined! For the case at hand, this is 74 * 2135 * 74 * 3872 =
45,268,558,720
rows. If the tables were bigger, you can only imagine how
long it would take.
One problem here is that MySQL can't (yet) use indexes on columns
efficiently if they are declared differently. In this context,
VARCHAR
and CHAR
are the same unless they are declared as
different lengths. Because tt.ActualPC
is declared as CHAR(10)
and et.EMPLOYID
is declared as CHAR(15)
, there is a length
mismatch.
To fix this disparity between column lengths, use ALTER TABLE
to
lengthen ActualPC
from 10 characters to 15 characters:
mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15);
Now tt.ActualPC
and et.EMPLOYID
are both VARCHAR(15)
.
Executing the EXPLAIN
statement again produces this result:
table type possible_keys key key_len ref rows Extra tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 where used do ALL PRIMARY NULL NULL NULL 2135 range checked for each record (key map: 1) et_1 ALL PRIMARY NULL NULL NULL 74 range checked for each record (key map: 1) et eq_ref PRIMARY PRIMARY 15 tt.ActualPC 1
This is not perfect, but is much better (the product of the rows
values is now less by a factor of 74). This version is executed in a couple
of seconds.
A second alteration can be made to eliminate the column length mismatches
for the tt.AssignedPC = et_1.EMPLOYID
and tt.ClientID =
do.CUSTNMBR
comparisons:
mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15), MODIFY ClientID VARCHAR(15);
Now EXPLAIN
produces the output shown below:
table type possible_keys key key_len ref rows Extra et ALL PRIMARY NULL NULL NULL 74 tt ref AssignedPC,ClientID,ActualPC ActualPC 15 et.EMPLOYID 52 where used et_1 eq_ref PRIMARY PRIMARY 15 tt.AssignedPC 1 do eq_ref PRIMARY PRIMARY 15 tt.ClientID 1
This is almost as good as it can get.
The remaining problem is that, by default, MySQL assumes that values
in the tt.ActualPC
column are evenly distributed, and that isn't the
case for the tt
table. Fortunately, it is easy to tell MySQL
about this:
shell> myisamchk --analyze PATH_TO_MYSQL_DATABASE/tt shell> mysqladmin refresh
Now the join is perfect, and EXPLAIN
produces this result:
table type possible_keys key key_len ref rows Extra tt ALL AssignedPC,ClientID,ActualPC NULL NULL NULL 3872 where used et eq_ref PRIMARY PRIMARY 15 tt.ActualPC 1 et_1 eq_ref PRIMARY PRIMARY 15 tt.AssignedPC 1 do eq_ref PRIMARY PRIMARY 15 tt.ClientID 1
Note that the rows
column in the output from EXPLAIN
is an
educated guess from the MySQL join optimizer. To optimize a
query, you should check if the numbers are even close to the truth. If not,
you may get better performance by using STRAIGHT_JOIN
in your
SELECT
statement and trying to list the tables in a different order in
the FROM
clause.
In most cases you can estimate the performance by counting disk seeks.
For small tables, you can usually find the row in 1 disk seek (as the
index is probably cached). For bigger tables, you can estimate that
(using B++ tree indexes) you will need: log(row_count) /
log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) +
1
seeks to find a row.
In MySQL an index block is usually 1024 bytes and the data
pointer is usually 4 bytes. A 500,000 row table with an
index length of 3 (medium integer) gives you:
log(500,000)/log(1024/3*2/(3+4)) + 1
= 4 seeks.
As the above index would require about 500,000 * 7 * 3/2 = 5.2M, (assuming that the index buffers are filled to 2/3, which is typical) you will probably have much of the index in memory and you will probably only need 1-2 calls to read data from the OS to find the row.
For writes, however, you will need 4 seek requests (as above) to find where to place the new index and normally 2 seeks to update the index and write the row.
Note that the above doesn't mean that your application will slowly degenerate by N log N! As long as everything is cached by the OS or SQL server things will only go marginally slower while the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by N log N). To avoid this, increase the index cache as the data grows. See section 5.5.2 Tuning Server Parameters.
SELECT
Queries
In general, when you want to make a slow SELECT ... WHERE
faster, the
first thing to check is whether or not you can add an index. See section 5.4.3 How MySQL Uses Indexes. All references between different tables
should usually be done with indexes. You can use the EXPLAIN
command
to determine which indexes are used for a SELECT
.
See section 5.2.1 EXPLAIN
Syntax (Get Information About a SELECT
).
Some general tips:
myisamchk
--analyze
on a table after it has been loaded with relevant data. This
updates a value for each index part that indicates the average number of
rows that have the same value. (For unique indexes, this is always 1,
of course.). MySQL will use this to decide which index to
choose when you connect two tables with 'a non-constant expression'.
You can check the result from the analyze
run by doing SHOW
INDEX FROM table_name
and examining the Cardinality
column.
myisamchk
--sort-index --sort-records=1
(if you want to sort on index 1). If you
have a unique index from which you want to read all records in order
according to that index, this is a good way to make that faster. Note,
however, that this sorting isn't written optimally and will take a long
time for a large table!
WHERE
Clauses
The WHERE
optimizations are put in the SELECT
part here because
they are mostly used with SELECT
, but the same optimizations apply for
WHERE
in DELETE
and UPDATE
statements.
Also note that this section is incomplete. MySQL does many optimizations, and we have not had time to document them all.
Some of the optimizations performed by MySQL are listed below:
((a AND b) AND c OR (((a AND b) AND (c AND d)))) -> (a AND b AND c) OR (a AND b AND c AND d)
(a<b AND b=c) AND a=5 -> b>5 AND b=c AND a=5
(B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6) -> B=5 OR B=6
COUNT(*)
on a single table without a WHERE
is retrieved
directly from the table information. This is also done for any NOT NULL
expression when used with only one table.
SELECT
statements are impossible and returns no rows.
HAVING
is merged with WHERE
if you don't use GROUP BY
or group functions (COUNT()
, MIN()
...).
WHERE
is constructed to get a fast
WHERE
evaluation for each sub-join and also to skip records as
soon as possible.
WHERE
clause on a UNIQUE
index, or a PRIMARY KEY
, where all index parts are used with constant
expressions and the index parts are defined as NOT NULL
.
mysql> SELECT * FROM t WHERE primary_key=1; mysql> SELECT * FROM t1,t2 WHERE t1.primary_key=1 AND t2.primary_key=t1.id;
ORDER BY
and in GROUP
BY
come from the same table, then this table is preferred first when
joining.
ORDER BY
clause and a different GROUP BY
clause, or if the ORDER BY
or GROUP BY
contains columns
from tables other than the first table in the join queue, a temporary
table is created.
SQL_SMALL_RESULT
, MySQL will use an in-memory
temporary table.
HAVING
clause
are skipped.
Some examples of queries that are very fast:
mysql> SELECT COUNT(*) FROM tbl_name; mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name; mysql> SELECT MAX(key_part2) FROM tbl_name WHERE key_part_1=constant; mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... LIMIT 10; mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;
The following queries are resolved using only the index tree (assuming the indexed columns are numeric):
mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val; mysql> SELECT COUNT(*) FROM tbl_name WHERE key_part1=val1 AND key_part2=val2; mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;
The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass:
mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... ; mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... ;
DISTINCT
DISTINCT
is converted to a GROUP BY
on all columns,
DISTINCT
combined with ORDER BY
will in many cases also
need a temporary table.
When combining LIMIT #
with DISTINCT
, MySQL will stop
as soon as it finds #
unique rows.
If you don't use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match.
SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;
In the case, assuming t1 is used before t2 (check with EXPLAIN
), then
MySQL will stop reading from t2 (for that particular row in t1)
when the first row in t2 is found.
LEFT JOIN
and RIGHT JOIN
A LEFT JOIN B
in MySQL is implemented as follows:
B
is set to be dependent on table A
and all tables
that A
is dependent on.
A
is set to be dependent on all tables (except B
)
that are used in the LEFT JOIN
condition.
LEFT JOIN
conditions are moved to the WHERE
clause.
WHERE
optimizations are done.
A
that matches the WHERE
clause, but there
wasn't any row in B
that matched the LEFT JOIN
condition,
then an extra B
row is generated with all columns set to NULL
.
LEFT JOIN
to find rows that don't exist in some
table and you have the following test: column_name IS NULL
in the
WHERE
part, where column_name is a column that is declared as
NOT NULL
, then MySQL will stop searching after more rows
(for a particular key combination) after it has found one row that
matches the LEFT JOIN
condition.
RIGHT JOIN
is implemented analogously as LEFT JOIN
.
The table read order forced by LEFT JOIN
and STRAIGHT JOIN
will help the join optimizer (which calculates in which order tables
should be joined) to do its work much more quickly, as there are fewer
table permutations to check.
Note that the above means that if you do a query of type:
SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key
MySQL will do a full scan on b
as the LEFT
JOIN
will force it to be read before d
.
The fix in this case is to change the query to:
SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key
LIMIT
In some cases MySQL will handle the query differently when you are
using LIMIT #
and not using HAVING
:
LIMIT
, MySQL
will use indexes in some cases when it normally would prefer to do a
full table scan.
LIMIT #
with ORDER BY
, MySQL will end the
sorting as soon as it has found the first #
lines instead of sorting
the whole table.
LIMIT #
with DISTINCT
, MySQL will stop
as soon as it finds #
unique rows.
GROUP BY
can be resolved by reading the key in order
(or do a sort on the key) and then calculate summaries until the
key value changes. In this case LIMIT #
will not calculate any
unnecessary GROUP BY
's.
#
rows to the client, it
will abort the query.
LIMIT 0
will always quickly return an empty set. This is useful
to check the query and to get the column types of the result columns.
LIMIT #
to calculate how much
space is needed to resolve the query.
INSERT
QueriesThe time to insert a record consists approximately of:
where the numbers are somewhat proportional to the overall time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently running query).
The size of the table slows down the insertion of indexes by N log N (B-trees).
Some ways to speed up inserts:
INSERT
statements. This is much faster (many
times in some cases) than using separate INSERT
statements.
INSERT DELAYED
statement. See section 6.4.2 INSERT
Syntax.
MyISAM
you can insert rows at the same time
SELECT
s are running if there are no deleted rows in the tables.
LOAD DATA INFILE
. This
is usually 20 times faster than using a lot of INSERT
statements.
See section 6.4.8 LOAD DATA INFILE
Syntax.
LOAD DATA INFILE
run even
faster when the table has many indexes. Use the following procedure:
CREATE TABLE
. For example, using
mysql
or Perl-DBI.
FLUSH TABLES
statement or the shell command mysqladmin
flush-tables
.
myisamchk --keys-used=0 -rq /path/to/db/tbl_name
. This will
remove all usage of all indexes from the table.
LOAD DATA INFILE
. This will not
update any indexes and will therefore be very fast.
myisampack
on it to make it smaller. See section 7.1.2.3 Compressed Table Characteristics.
myisamchk -r -q
/path/to/db/tbl_name
. This will create the index tree in memory before
writing it to disk, which is much faster because it avoids lots of disk
seeks. The resulting index tree is also perfectly balanced.
FLUSH TABLES
statement or the shell command mysqladmin
flush-tables
.
LOAD DATA INFILE
in some future
version of MySQL.
mysql> LOCK TABLES a WRITE; mysql> INSERT INTO a VALUES (1,23),(2,34),(4,33); mysql> INSERT INTO a VALUES (8,26),(6,29); mysql> UNLOCK TABLES;The main speed difference is that the index buffer is flushed to disk only once, after all
INSERT
statements have completed. Normally there would
be as many index buffer flushes as there are different INSERT
statements. Locking is not needed if you can insert all rows with a single
statement.
Locking will also lower the total time of multi-connection tests, but the
maximum wait time for some threads will go up (because they wait for
locks). For example:
thread 1 does 1000 inserts thread 2, 3, and 4 does 1 insert thread 5 does 1000 insertsIf you don't use locking, 2, 3, and 4 will finish before 1 and 5. If you use locking, 2, 3, and 4 probably will not finish before 1 or 5, but the total time should be about 40% faster. As
INSERT
, UPDATE
, and DELETE
operations are very
fast in MySQL, you will obtain better overall performance by
adding locks around everything that does more than about 5 inserts or
updates in a row. If you do very many inserts in a row, you could do a
LOCK TABLES
followed by an UNLOCK TABLES
once in a while
(about each 1000 rows) to allow other threads access to the table. This
would still result in a nice performance gain.
Of course, LOAD DATA INFILE
is much faster for loading data.
To get some more speed for both LOAD DATA INFILE
and
INSERT
, enlarge the key buffer. See section 5.5.2 Tuning Server Parameters.
UPDATE
Queries
Update queries are optimized as a SELECT
query with the additional
overhead of a write. The speed of the write is dependent on the size of
the data that is being updated and the number of indexes that are
updated. Indexes that are not changed will not be updated.
Also, another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table.
Note that, with dynamic record format, updating a record to
a longer total length may split the record. So if you do this often,
it is very important to OPTIMIZE TABLE
sometimes.
See section 4.5.1 OPTIMIZE TABLE
Syntax.
DELETE
Queries
If you want to delete all rows in the table, you should use
TRUNCATE TABLE table_name
. See section 6.4.6 TRUNCATE
Syntax.
The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. See section 5.5.2 Tuning Server Parameters.
Unsorted tips for faster systems:
thread_cache_size
variable. See section 5.5.2 Tuning Server Parameters.
EXPLAIN
command. See section 5.2.1 EXPLAIN
Syntax (Get Information About a SELECT
).
SELECT
queries on tables that are updated a
lot. This is to avoid problems with table locking.
MyISAM
tables can insert rows in a table without deleted
rows at the same time another table is reading from it. If this is important
for you, you should consid