Table of Contents
Access denied ErrorsThis chapter covers topics that deal with administering a MySQL installation:
Configuring the server
Managing user accounts
Performing backups
The server log files
The query cache
The MySQL server, mysqld, is the main program that does most of the work in a MySQL installation. The server is accompanied by several related scripts that perform setup operations when you install MySQL or that assist you in starting and stopping the server. This section provides an overview of the server and related programs. The following sections provide more detailed information about each of these programs.
Each MySQL program takes many different options. Most programs
provide a --help option that you can use to get a
description of the program's different options. For example, try
mysqld --help.
You can override default option values for MySQL programs by specifying options on the command line or in an option file. Section 4.3, “Specifying Program Options”.
The following list briefly describes the MySQL server and server-related programs:
The SQL daemon (that is, the MySQL server). To use client programs, mysqld must be running, because clients gain access to databases by connecting to the server. See Section 5.2, “mysqld — The MySQL Server”.
A server startup script. mysqld_safe attempts to start mysqld. See Section 5.3.1, “mysqld_safe — MySQL Server Startup Script”.
A server startup script. This script is used on systems that use System V-style run directories containing scripts that start system services for particular run levels. It invokes mysqld_safe to start the MySQL server. See Section 5.3.2, “mysql.server — MySQL Server Startup Script”.
A server startup script that can start or stop multiple
servers installed on the system. See
Section 5.3.3, “mysqld_multi — Manage Multiple MySQL Servers”. As of MySQL 5.0.3 (Unix-like
systems) or 5.0.13 (Windows), an alternative to
mysqld_multi is
mysqlmanager, the MySQL Instance Manager.
See Section 5.4, “mysqlmanager — The MySQL Instance Manager”.
The MySQL Instance Manager, a program for monitoring and managing MySQL servers. See Section 5.4, “mysqlmanager — The MySQL Instance Manager”.
There are several other programs that are related to MySQL installation or upgrading:
This program is used during the MySQL build/installation process. It compiles error message files from the error source files. See Section 5.5.1, “comp_err — Compile MySQL Error Message File”.
This program makes a binary release of a compiled MySQL. This
could be sent by FTP to
/pub/mysql/upload/ on
ftp.mysql.com for the convenience of other
MySQL users.
This program is used on Windows. It packages a MySQL distribution for installation after the source distribution has been built. See Section 5.5.2, “make_win_bin_dist — Package MySQL Distribution as ZIP Archive”.
This program is used after a MySQL upgrade operation. It updates the grant tables with any changes that have been made in newer versions of MySQL. See Section 5.5.4, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.
Note: As of MySQL 5.0.19, this program has been superseded by mysql_upgrade.
This script creates the MySQL database and initializes the grant tables with default privileges. It is usually executed only once, when first installing MySQL on a system. See Section 2.4.15.2, “Unix Post-Installation Procedures”, and Section 5.5.5, “mysql_install_db — MySQL Data Directory Initialization Script”.
This program enables you to improve the security of your MySQL installation. SQL. See Section 5.5.6, “mysql_secure_installation — Improve MySQL Installation Security”.
This program loads the time zone tables in the
mysql database using the contents of the
host system zoneinfo database (the set
of files describing time zones). SQL. See
Section 5.5.7, “mysql_tzinfo_to_sql — Load the Time Zone Tables”.
This program is used after a MySQL upgrade operation. It checks tables for incompatibilities and repairs them if necessary, and updates the grant tables with any changes that have been made in newer versions of MySQL. See Section 5.5.8, “mysql_upgrade — Check Tables for MySQL Upgrade”.
This program is used on Unix or Unix-like systems to create a MySQL source distribution that can be compiled on Windows. See Section 2.4.14.6.4, “Creating a Windows Source Package from the Latest Development Source”, and Section 5.5.3, “make_win_src_distribution — Create Source Distribution for Windows”.
mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:
Startup options that the server supports
Server system variables
Server status variables
How to set the server SQL mode
The server shutdown process
Note: Not all storage engines
are supported by all MySQL server binaries and configurations.
To find out how to determine which storage engines are
supported by your MySQL server installation, see
Section 13.5.4.10, “SHOW ENGINES Syntax”.
The following table provides a list of all the command line
options, server and status variables applicable within
mysqld.
The table lists command line options (Cmd-line), options valid in configuration files (Option file), server system variables (Server Var), and status variables (Status var) in one unified list, with notification of where each option/variable is valid. If a server option set on the command line or in an option file differs from the name of the corresponding server system or status variable, the variable name is noted immediately below the corresponding option. For status variables, the scope of the variable is shown (Scope) as either global, session, or both. Please see the corresponding sections for details on setting and using the options and variables. Where appropriate, a direct link to further information on the item as available.
This table is part of an ongoing process to expand and simplify the information provided on these elements. Further improvements to the table, and corresponding descriptions will be applied over the coming months.
When you start the mysqld server, you can specify program options using any of the methods described in Section 4.3, “Specifying Program Options”. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See Section 4.3.2, “Using Option Files”.
mysqld reads options from the
[mysqld] and [server]
groups. mysqld_safe reads options from the
[mysqld], [server],
[mysqld_safe], and
[safe_mysqld] groups.
mysql.server reads options from the
[mysqld] and
[mysql.server] groups.
An embedded MySQL server usually reads options from the
[server], [embedded],
and
[
groups, where xxxxx_SERVER]xxxxx is the name of
the application into which the server is embedded.
mysqld accepts many command options. For a brief summary, execute mysqld --help. To see the full list, use mysqld --verbose --help.
The following list shows some of the most common server options. Additional options are described in other sections:
Options that affect security: See Section 5.6.3, “Security-Related mysqld Options”.
SSL-related options: See Section 5.8.7.3, “SSL Command Options”.
Binary log control options: See Section 5.11.3, “The Binary Log”.
Replication-related options: See Section 6.8, “Replication Startup Options”.
Options specific to particular storage engines: See
Section 14.1.1, “MyISAM Startup Options”,
Section 14.5.3, “BDB Startup Options”,
Section 14.2.4, “InnoDB Startup Options and System Variables”, and
Section 15.6.5.1, “MySQL Cluster-Related Command Options for mysqld”.
You can also set the values of server system variables by using variable names as options, as described later in this section.
Display a short help message and exit. Use both the
--verbose and --help
options to see the full message.
This option is used internally by the MySQL test suite for replication testing and debugging.
This option controls whether user-defined functions that
have only an xxx symbol for the main
function can be loaded. By default, the option is off and
only UDFs that have at least one auxiliary symbol can be
loaded; this prevents attempts at loading functions from
shared object files other than those containing legitimate
UDFs. This option was added in version 5.0.3. See
Section 24.2.4.6, “User-Defined Function Security Precautions”.
Use standard (ANSI) SQL syntax instead of MySQL syntax.
For more precise control over the server SQL mode, use the
--sql-mode option instead. See
Section 1.9.3, “Running MySQL in ANSI Mode”, and
Section 5.2.6, “SQL Modes”.
The path to the MySQL installation directory. All paths are usually resolved relative to this directory.
Allow large result sets by saving all temporary sets in files. This option prevents most “table full” errors, but also slows down queries for which in-memory tables would suffice. Since MySQL 3.23.2, the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk tables where necessary.
The IP address to bind to.
This option is used by the mysql_install_db script to create the MySQL privilege tables without having to start a full MySQL server.
This option is unavailable if MySQL was configured with
the --disable-grant-options option. See
Section 2.4.14.2, “Typical configure Options”.
The directory where character sets are installed. See Section 5.10.1, “The Character Set Used for Data and Sorting”.
--character-set-client-handshake
Don't ignore character set information sent by the client.
To ignore client information and use the default server
character set, use
--skip-character-set-client-handshake;
this makes MySQL behave like MySQL 4.0.
--character-set-filesystem=
charset_name
The filesystem character set. This option sets the
character_set_filesystem system
variable. It was added in MySQL 5.0.19.
--character-set-server=,
charset_name-C
charset_name
Use charset_name as the default
server character set. See
Section 5.10.1, “The Character Set Used for Data and Sorting”. If you use this option
to specify a non-default character set, you should also
use --collation-server to specify the
collation.
Put the mysqld server in a closed
environment during startup by using the
chroot() system call. This is a
recommended security measure. Note that use of this option
somewhat limits LOAD DATA INFILE and
SELECT ... INTO OUTFILE.
--collation-server=
collation_name
Use collation_name as the
default server collation. See
Section 5.10.1, “The Character Set Used for Data and Sorting”.
(Windows only.) Write error log messages to
stderr and stdout
even if --log-error is specified.
mysqld does not close the console
window if this option is used.
Write a core file if mysqld dies. For
some systems, you must also specify the
--core-file-size option to
mysqld_safe. See
Section 5.3.1, “mysqld_safe — MySQL Server Startup Script”. Note that on some systems,
such as Solaris, you do not get a core file if you are
also using the --user option.
The path to the data directory.
--debug[=,
debug_options]-#
[
debug_options]
If MySQL is configured with --with-debug,
you can use this option to get a trace file of what
mysqld is doing. The
debug_options string often is
'd:t:o,.
The default is file_name''d:t:i:o,mysqld.trace'.
See
MySQL
Internals: Porting.
As of MySQL 5.0.25, using --with-debug to
configure MySQL with debugging support enables you to use
the --debug="d,parser_debug" option when
you start the server. This causes the Bison parser that is
used to process SQL statements to dump a parser trace to
the server's standard error output. Typically, this output
is written to the error log.
--default-character-set=
(DEPRECATED)
charset_name
Use charset_name as the default
character set. This option is deprecated in favor of
--character-set-server. See
Section 5.10.1, “The Character Set Used for Data and Sorting”.
--default-collation=
collation_name
Use collation_name as the
default collation. This option is deprecated in favor of
--collation-server. See
Section 5.10.1, “The Character Set Used for Data and Sorting”.
Set the default storage engine (table type) for tables. See Chapter 14, Storage Engines and Table Types.
This option is a synonym for
--default-storage-engine.
Set the default server time zone. This option sets the
global time_zone system variable. If
this option is not given, the default time zone is the
same as the system time zone (given by the value of the
system_time_zone system variable.
--delay-key-write[={OFF|ON|ALL}]
Specify how to use delayed key writes. Delayed key writing
causes key buffers not to be flushed between writes for
MyISAM tables. OFF
disables delayed key writes. ON enables
delayed key writes for those tables that were created with
the DELAY_KEY_WRITE option.
ALL delays key writes for all
MyISAM tables. See
Section 7.5.2, “Tuning Server Parameters”, and
Section 14.1.1, “MyISAM Startup Options”.
Note: If you set this
variable to ALL, you should not use
MyISAM tables from within another
program (such as another MySQL server or
myisamchk) when the tables are in use.
Doing so leads to index corruption.
Read the default DES keys from this file. These keys are
used by the DES_ENCRYPT() and
DES_DECRYPT() functions.
--disconnect-slave-event-count
This option is used internally by the MySQL test suite for replication testing and debugging.
Enable support for named pipes. This option can be used only with the mysqld-nt and mysqld-debug servers that support named-pipe connections.
--exit-info[=,
flags]-T [
flags]
This is a bit mask of different flags that you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!
Enable external locking (system locking), which is
disabled by default as of MySQL 4.0. Note that if you use
this option on a system on which lockd
does not fully work (such as Linux), it is easy for
mysqld to deadlock. This option
previously was named --enable-locking.
For more information about external locking, including conditions under which it can and cannot be used, see Section 7.3.4, “External Locking”.
Flush (synchronize) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section B.1.4.2, “What to Do If MySQL Keeps Crashing”.
Print a symbolic stack trace on failure.
Install an interrupt handler for SIGINT
(needed to stop mysqld with
^C to set breakpoints) and disable
stack tracing and core file handling. See
MySQL
Internals: Porting.
Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.
This option is unavailable if MySQL was configured with
the --disable-grant-options option. See
Section 2.4.14.2, “Typical configure Options”.
Adds consistency guarantees between the content of
InnoDB tables and the binary log. See
Section 5.11.3, “The Binary Log”. This option was removed in
MySQL 5.0.3, having been made obsolete by the introduction
of XA transaction support.
--innodb-
xxx
The InnoDB options are listed in
Section 14.2.4, “InnoDB Startup Options and System Variables”.
--language=
lang_name,
-L lang_name
Return client error messages in the given language.
lang_name can be given as the
language name or as the full pathname to the directory
where the language files are installed. See
Section 5.10.2, “Setting the Error Message Language”.
Some hardware/operating system architectures support memory pages greater than the default (usually 4KB). The actual implementation of this support depends on the underlying hardware and OS. Applications that perform a lot of memory accesses may obtain performance improvements by using large pages due to reduced Translation Lookaside Buffer (TLB) misses.
Currently, MySQL supports only the Linux implementation of large pages support (which is called HugeTLB in Linux). We have plans to extend this support to FreeBSD, Solaris and possibly other platforms.
Before large pages can be used on Linux, it is necessary
to configure the HugeTLB memory pool. For reference,
consult the hugetlbpage.txt file in
the Linux kernel source.
This option is disabled by default. It was added in MySQL 5.0.3.
--log[=,
file_name]-l [
file_name]
Log connections and SQL statements received from clients
to this file. See Section 5.11.2, “The General Query Log”. If you omit
the filename, MySQL uses
host_name.log
Enable binary logging. The server logs all statements that change data to the binary log, which is used for backup and replication. See Section 5.11.3, “The Binary Log”.
The option value, if given, is the basename for the log
sequence. The server creates binary log files in sequence
by adding a numeric suffix to the basename. It is
recommended that you specify a basename (see
Section B.1.8.1, “Open Issues in MySQL”, for the reason). Otherwise,
MySQL uses
host_name-bin
The index file for binary log filenames. See
Section 5.11.3, “The Binary Log”. If you omit the filename,
and if you didn't specify one with
--log-bin, MySQL uses
host_name-bin.index
--log-bin-trust-function-creators[={0|1}]
With no argument or an argument of 1, this option sets the
log_bin_trust_function_creators system
variable to 1. With an argument of 0, this option sets the
system variable to 0.
log_bin_trust_function_creators affects
how MySQL enforces restrictions on stored function
creation. See Section 17.4, “Binary Logging of Stored Routines and Triggers”.
This option was added in MySQL 5.0.16.
--log-bin-trust-routine-creators[={0|1}]
This is the old name for
--log-bin-trust-function-creators. Before
MySQL 5.0.16, it also applies to stored procedures, not
just stored functions and sets the
log_bin_trust_routine_creators system
variable. As of 5.0.16, this option is deprecated. It is
recognized for backward compatibility but its use results
in a warning.
This option was added in MySQL 5.0.6.
Log errors and startup messages to this file. See
Section 5.11.1, “The Error Log”. If you omit the filename,
MySQL uses
host_name.err.err.
Log all MyISAM changes to this file
(used only when debugging MyISAM).
--log-long-format
(DEPRECATED)
Log extra information to the update log, binary update
log, and slow query log, if they have been activated. For
example, the username and timestamp are logged for all
queries. This option is deprecated, as it now represents
the default logging behavior. (See the description for
--log-short-format.) The
--log-queries-not-using-indexes option is
available for the purpose of logging queries that do not
use indexes to the slow query log.
--log-queries-not-using-indexes
If you are using this option with
--log-slow-queries, queries that do not
use indexes are logged to the slow query log. See
Section 5.11.4, “The Slow Query Log”.
Log less information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are not logged for queries.
Log slow administrative statements such as
OPTIMIZE TABLE, ANALYZE
TABLE, and ALTER TABLE to the
slow query log.
--log-slow-queries[=
file_name]
Log all queries that have taken more than
long_query_time seconds to execute to
this file. See Section 5.11.4, “The Slow Query Log”. See the
descriptions of the --log-long-format and
--log-short-format options for details.
The name of the memory-mapped transaction coordinator log
file (for XA transactions that affect multiple storage
engines when the binary log is disabled). The default name
is tc.log. The file is created under
the data directory if not given as a full pathname.
Currently, this option is unused. Added in MySQL 5.0.3.
The size in bytes of the memory-mapped transaction coordinator log. The default size is 24KB. Added in MySQL 5.0.3.
--log-warnings[=,
level]-W [
level]
Print out warnings such as Aborted
connection... to the error log. Enabling this
option is recommended, for example, if you use replication
(you get more information about what is happening, such as
messages about network failures and reconnections). This
option is enabled (1) by default, and the default
level value if omitted is 1. To
disable this option, use
--log-warnings=0. Aborted connections are
not logged to the error log unless the value is greater
than 1. See Section B.1.2.10, “Communication Errors and Aborted Connections”.
Give table-modifying operations
(INSERT, REPLACE,
DELETE, UPDATE)
lower priority than selects. This can also be done via
{INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY
... to lower the priority of only one query, or
by SET LOW_PRIORITY_UPDATES=1 to change
the priority in one thread. This affects only storage
engines that use only table-level locking
(MyISAM, MEMORY,
MERGE). See
Section 7.3.2, “Table Locking Issues”.
This option is used internally by the MySQL test suite for replication testing and debugging.
Lock the mysqld process in memory. This
works on systems such as Solaris that support the
mlockall() system call. This might help
if you have a problem where the operating system is
causing mysqld to swap on disk. Note
that use of this option requires that you run the server
as root, which is normally not a good
idea for security reasons. See
Section 5.6.5, “How to Run MySQL as a Normal User”.
--myisam-recover[=
option[,option]...]]
Set the MyISAM storage engine recovery
mode. The option value is any combination of the values of
DEFAULT, BACKUP,
FORCE, or QUICK. If
you specify multiple values, separate them by commas. You
can also use a value of "" to disable
this option. If this option is used, each time
mysqld opens a
MyISAM table, it checks whether the
table is marked as crashed or wasn't closed properly. (The
last option works only if you are running with external
locking disabled.) If this is the case,
mysqld runs a check on the table. If
the table was corrupted, mysqld
attempts to repair it.
The following options affect how the repair works:
| Option | Description |
DEFAULT | The same as not giving any option to --myisam-recover. |
BACKUP | If the data file was changed during recovery, save a backup of the
|
FORCE | Run recovery even if we would lose more than one row from the
.MYD file. |
QUICK | Don't check the rows in the table if there aren't any delete blocks. |
Before the server automatically repairs a table, it writes
a note about the repair to the error log. If you want to
be able to recover from most problems without user
intervention, you should use the options
BACKUP,FORCE. This forces a repair of a
table even if some rows would be deleted, but it keeps the
old data file as a backup so that you can later examine
what happened.
--ndb-connectstring=
connect_string
When using the NDB storage engine, it
is possible to point out the management server that
distributes the cluster configuration by setting the
connect string option. See
Section 15.4.4.2, “The Cluster Connectstring”, for syntax.
If the binary includes support for the NDB
Cluster storage engine, this option enables the
engine, which is disabled by default. See
Chapter 15, MySQL Cluster.
Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 5.7.9, “Password Hashing as of MySQL 4.1”.
Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See MySQL Internals: Porting.
Change the number of file descriptors available to
mysqld. If this option is not set or is
set to 0, mysqld uses the value to
reserve file descriptors with
setrlimit(). If the value is 0,
mysqld reserves
max_connections×5 or
max_connections +
table_open_cache×2 files (whichever is
larger). You should try increasing this value if
mysqld gives you the error Too
many open files.
The pathname of the process ID file. This file is used by other programs such as mysqld_safe to determine the server's process ID.
The port number to use when listening for TCP/IP
connections. The port number must be 1024 or higher unless
the server is started by the root
system user.
On some systems, when the server is stopped, the TCP/IP port might not become available immediately. If the server is restarted quickly afterward, its attempt to reopen the port can fail. This option indicates how many seconds the server should wait for the TCP/IP port to become free if it cannot be opened. The default is not to wait. This option was added in MySQL 5.0.19.
Skip some optimization stages.
If this option is enabled, a user cannot create new MySQL
users by using the GRANT statement, if
the user doesn't have the INSERT
privilege for the mysql.user table or
any column in the table.
Disallow authentication by clients that attempt to use accounts that have old (pre-4.1) passwords.
Enable shared-memory connections by local clients. This option is available only on Windows.
--shared-memory-base-name=
name
The name of shared memory to use for shared-memory
connections. This option is available only on Windows. The
default name is MYSQL. The name is case
sensitive.
Disable the BDB storage engine. This
saves memory and might speed up some operations. Do not
use this option if you require BDB
tables.
Turn off the ability to select and insert at the same time
on MyISAM tables. (This is to be used
only if you think you have found a bug in this feature.)
See Section 7.3.3, “Concurrent Inserts”.
Do not use external locking (system locking). For more information about external locking, including conditions under which it can and cannot be used, see Section 7.3.4, “External Locking”.
External locking has been disabled by default since MySQL 4.0.
This option causes the server not to use the privilege
system at all, which gives anyone with access to the
server unrestricted access to all
databases. You can cause a running server to
start using the grant tables again by executing
mysqladmin flush-privileges or
mysqladmin reload command from a system
shell, or by issuing a MySQL FLUSH
PRIVILEGES statement after connecting to the
server. This option also suppresses loading of
user-defined functions (UDFs).
This option is unavailable if MySQL was configured with
the --disable-grant-options option. See
Section 2.4.14.2, “Typical configure Options”.
Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See Section 7.5.7, “How MySQL Uses DNS”.
Disable the InnoDB storage engine. This
saves memory and disk space and might speed up some
operations. Do not use this option if you require
InnoDB tables.
Disable the MERGE storage engine. This
option was added in MySQL 5.0.24. It can be used if the
following behavior is undesirable: If a user has access to
MyISAM table
t, that user can create a
MERGE table
m that accesses
t. However, if the user's
privileges on t are
subsequently revoked, the user can continue to access
t by doing so through
m.
Do not resolve hostnames when checking client connections.
Use only IP numbers. If you use this option, all
Host column values in the grant tables
must be IP numbers or localhost. See
Section 7.5.7, “How MySQL Uses DNS”.
Disable the NDB Cluster storage engine.
This is the default for binaries that were built with
NDB Cluster storage engine support; the
server allocates memory and other resources for this
storage engine only if the --ndbcluster
option is given explicitly. See
Section 15.4.3, “Quick Test Setup of MySQL Cluster”, for an example of
usage.
Don't listen for TCP/IP connections at all. All interaction with mysqld must be made via named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See Section 7.5.7, “How MySQL Uses DNS”.
This option is used internally by the MySQL test suite for replication testing and debugging.
Options that begin with --ssl specify
whether to allow clients to connect via SSL and indicate
where to find SSL keys and certificates. See
Section 5.8.7.3, “SSL Command Options”.
Instructs the MySQL server not to run as a service.
--symbolic-links,
--skip-symbolic-links
Enable or disable symbolic link support. This option has different effects on Windows and Unix:
On Windows, enabling symbolic links allows you to
establish a symbolic link to a database directory by
creating a
db_name.sym
On Unix, enabling symbolic links means that you can
link a MyISAM index file or data
file to another directory with the INDEX
DIRECTORY or DATA
DIRECTORY options of the CREATE
TABLE statement. If you delete or rename the
table, the files that its symbolic links point to also
are deleted or renamed. See
Section 7.6.1.2, “Using Symbolic Links for Tables on Unix”.
If MySQL is configured with
--with-debug=full, all MySQL programs
check for memory overruns during each memory allocation
and memory freeing operation. This checking is very slow,
so for the server you can avoid it when you don't need it
by using the --skip-safemalloc option.
With this option, the SHOW DATABASES
statement is allowed only to users who have the
SHOW DATABASES privilege, and the
statement displays all database names. Without this
option, SHOW DATABASES is allowed to
all users, but displays each database name only if the
user has the SHOW DATABASES privilege
or some privilege for the database. Note that
any global privilege is considered a
privilege for the database.
Don't write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See MySQL Internals: Porting.
Disable using thread priorities for faster response time.
On Unix, this option specifies the Unix socket file to use
when listening for local connections. The default value is
/tmp/mysql.sock. On Windows, the
option specifies the pipe name to use when listening for
local connections that use a named pipe. The default value
is MySQL (not case sensitive).
--sql-mode=
value[,value[,value...]]
Set the SQL mode. See Section 5.2.6, “SQL Modes”.
As of MySQL 5.0.13, SYSDATE() by
default returns the time at which it executes, not the
time at which the statement in which it occurs begins
executing. This differs from the behavior of
NOW(). This option causes
SYSDATE() to be an alias for
NOW(). For information about the
implications for binary logging and replication, see the
description for SYSDATE() in
Section 12.6, “Date and Time Functions” and for
SET TIMESTAMP in
Section 13.5.3, “SET Syntax”.
This option was added in MySQL 5.0.20.
--tc-heuristic-recover={COMMIT|ROLLBACK}
The type of decision to use in the heuristic recovery process. Currently, this option is unused. Added in MySQL 5.0.3.
This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to “leak” memory, because it is being allocated to the directory entry cache rather than to the disk cache.
Sets the default transaction isolation level. The
level value can be
READ-UNCOMMITTED,
READ-COMMITTED,
REPEATABLE-READ, or
SERIALIZABLE. See
Section 13.4.6, “SET TRANSACTION Syntax”.
The path of the directory to use for creating temporary
files. It might be useful if your default
/tmp directory resides on a partition
that is too small to hold temporary tables. This option
accepts several paths that are used in round-robin
fashion. Paths should be separated by colon characters
(‘:’) on Unix and semicolon
characters (‘;’) on
Windows, NetWare, and OS/2. If the MySQL server is acting
as a replication slave, you should not set
--tmpdir to point to a directory on a
memory-based filesystem or to a directory that is cleared
when the server host restarts. For more information about
the storage location of temporary files, see
Section B.1.4.4, “Where MySQL Stores Temporary Files”. A replication slave
needs some of its temporary files to survive a machine
restart so that it can replicate temporary tables or
LOAD DATA INFILE operations. If files
in the temporary file directory are lost when the server
restarts, replication fails.
--user={,
user_name|user_id}-u
{
user_name|user_id}
Run the mysqld server as the user
having the name user_name or
the numeric user ID user_id.
(“User” in this context refers to a system
login account, not a MySQL user listed in the grant
tables.)
This option is mandatory when
starting mysqld as
root. The server changes its user ID
during its startup sequence, causing it to run as that
particular user rather than as root.
See Section 5.6.1, “General Security Guidelines”.
To avoid a possible security hole where a user adds a
--user=root option to a
my.cnf file (thus causing the server
to run as root),
mysqld uses only the first
--user option specified and produces a
warning if there are multiple --user
options. Options in /etc/my.cnf and
$MYSQL_HOME/my.cnf are processed
before command-line options, so it is recommended that you
put a --user option in
/etc/my.cnf and specify a value other
than root. The option in
/etc/my.cnf is found before any other
--user options, which ensures that the
server runs as a user other than root,
and that a warning results if any other
--user option is found.
Display version information and exit.
You can assign a value to a server system variable by using an
option of the form
--.
For example, var_name=value--key_buffer_size=32M sets the
key_buffer_size variable to a value of
32MB.
Note that when you assign a value to a variable, MySQL might automatically correct the value to stay within a given range, or adjust the value to the closest allowable value if only certain values are allowed.
If you want to restrict the maximum value to which a variable
can be set at runtime with SET, you can
define this by using the
--maximum-
command-line option.
var_name=value
It is also possible to set variables by using
--set-variable=
or var_name=value-O
syntax. This syntax is deprecated.
var_name=value
You can change the values of most system variables for a
running server with the SET statement. See
Section 13.5.3, “SET Syntax”.
Section 5.2.3, “System Variables”, provides a full description for all variables, and additional information for setting them at server startup and runtime. Section 7.5.2, “Tuning Server Parameters”, includes information on optimizing the server by tuning system variables.
The mysql server maintains many system
variables that indicate how it is configured. Each system
variable has a default value. System variables can be set at
server startup using options on the command line or in an
option file. Most of them can be changed dynamically while the
server is running by means of the SET
statement, which enables you to modify operation of the server
without having to stop and restart it. You can refer to system
variable values in expressions.
There are several ways to see the names and values of system variables:
To see the values that a server will use based on its compiled-in defaults and any option files that it reads, use this command:
mysqld --verbose --help
To see the values that a server will use based on its compiled-in defaults, ignoring the settings in any option files, use this command:
mysqld --no-defaults --verbose --help
To see the current values used by a running server, use
the SHOW VARIABLES statement.
This section provides a description of each system variable. Variables with no version indicated are present in all MySQL 5.0 releases. For historical information concerning their implementation, please see MySQL 3.23, 4.0, 4.1 Reference Manual.
For additional system variable information, see these sections:
Section 5.2.4, “Using System Variables”, discusses the syntax for setting and displaying system variable values.
Section 5.2.4.2, “Dynamic System Variables”, lists the variables that can be set at runtime.
Information on tuning system variables can be found in Section 7.5.2, “Tuning Server Parameters”.
Section 14.2.4, “InnoDB Startup Options and System Variables”, lists
InnoDB system variables.
Note: Some of the following variable
descriptions refer to “enabling” or
“disabling” a variable. These variables can be
enabled with the SET statement by setting
them to ON or 1, or
disabled by setting them to OFF or
0. However, to set such a variable on the
command line or in an option file, you must set it to
1 or 0; setting it to
ON or OFF will not work.
For example, on the command line,
--delay_key_write=1 works but
--delay_key_write=ON does not.
Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.
auto_increment_increment and
auto_increment_offset are intended for
use with master-to-master replication, and can be used to
control the operation of AUTO_INCREMENT
columns. Both variables can be set globally or locally,
and each can assume an integer value between 1 and 65,535
inclusive. Setting the value of either of these two
variables to 0 causes its value to be set to 1 instead.
Attempting to set the value of either of these two
variables to an integer greater than 65,535 or less than 0
causes its value to be set to 65,535 instead. Attempting
to set the value of
auto_increment_increment or
auto_increment_offset to a non-integer
value gives rise to an error, and the actual value of the
variable remains unchanged.
These two variables affect
AUTO_INCREMENT column behavior as
follows:
auto_increment_increment controls
the interval between successive column values. For
example:
mysql>SHOW VARIABLES LIKE 'auto_inc%';+--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 1 | | auto_increment_offset | 1 | +--------------------------+-------+ 2 rows in set (0.00 sec) mysql>CREATE TABLE autoinc1->(col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);Query OK, 0 rows affected (0.04 sec) mysql>SET @@auto_increment_increment=10;Query OK, 0 rows affected (0.00 sec) mysql>SHOW VARIABLES LIKE 'auto_inc%';+--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 10 | | auto_increment_offset | 1 | +--------------------------+-------+ 2 rows in set (0.01 sec) mysql>INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);Query OK, 4 rows affected (0.00 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql>SELECT col FROM autoinc1;+-----+ | col | +-----+ | 1 | | 11 | | 21 | | 31 | +-----+ 4 rows in set (0.00 sec)
(Note how SHOW VARIABLES is used
here to obtain the current values for these
variables.)
auto_increment_offset determines
the starting point for the
AUTO_INCREMENT column value.
Consider the following, assuming that these statements
are executed during the same session as the example
given in the description for
auto_increment_increment:
mysql>SET @@auto_increment_offset=5;Query OK, 0 rows affected (0.00 sec) mysql>SHOW VARIABLES LIKE 'auto_inc%';+--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 10 | | auto_increment_offset | 5 | +--------------------------+-------+ 2 rows in set (0.00 sec) mysql>CREATE TABLE autoinc2->(col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);Query OK, 0 rows affected (0.06 sec) mysql>INSERT INTO autoinc2 VALUES (NULL), (NULL), (NULL), (NULL);Query OK, 4 rows affected (0.00 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql>SELECT col FROM autoinc2;+-----+ | col | +-----+ | 5 | | 15 | | 25 | | 35 | +-----+ 4 rows in set (0.02 sec)
If the value of
auto_increment_offset is greater
than that of
auto_increment_increment, the value
of auto_increment_offset is
ignored.
Should one or both of these variables be changed and then
new rows inserted into a table containing an
AUTO_INCREMENT column, the results may
seem counterintuitive because the series of
AUTO_INCREMENT values is calculated
without regard to any values already present in the
column, and the next value inserted is the least value in
the series that is greater than the maximum existing value
in the AUTO_INCREMENT column. In other
words, the series is calculated like so:
auto_increment_offset +
N ×
auto_increment_increment
where N is a positive integer
value in the series [1, 2, 3, ...]. For example:
mysql>SHOW VARIABLES LIKE 'auto_inc%';+--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 10 | | auto_increment_offset | 5 | +--------------------------+-------+ 2 rows in set (0.00 sec) mysql>SELECT col FROM autoinc1;+-----+ | col | +-----+ | 1 | | 11 | | 21 | | 31 | +-----+ 4 rows in set (0.00 sec) mysql>INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);Query OK, 4 rows affected (0.00 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql>SELECT col FROM autoinc1;+-----+ | col | +-----+ | 1 | | 11 | | 21 | | 31 | | 35 | | 45 | | 55 | | 65 | +-----+ 8 rows in set (0.00 sec)
The values shown for
auto_increment_increment and
auto_increment_offset generate the
series 5 + N × 10, that
is, [5, 15, 25, 35, 45, ...]. The greatest value present
in the col column prior to the
INSERT is 31, and the next available
value in the AUTO_INCREMENT series is
35, so the inserted values for col
begin at that point and the results are as shown for the
SELECT query.
It is important to remember that it is not possible to
confine the effects of these two variables to a single
table, and thus they do not take the place of the
sequences offered by some other database management
systems; these variables control the behavior of all
AUTO_INCREMENT columns in
all tables on the MySQL server. If
one of these variables is set globally, its effects
persist until the global value is changed or overridden by
setting them locally, or until mysqld
is restarted. If set locally, the new value affects
AUTO_INCREMENT columns for all tables
into which new rows are inserted by the current user for
the duration of the session, unless the values are changed
during that session.
The auto_increment_increment variable
was added in MySQL 5.0.2. Its default value is 1. See
Section 6.13, “Auto-Increment in Multiple-Master Replication”.
This variable was introduced in MySQL 5.0.2. Its default
value is 1. For particulars, see the description for
auto_increment_increment.
When this variable has a value of 1 (the default), the
server automatically grants the EXECUTE
and ALTER ROUTINE privileges to the
creator of a stored routine, if the user cannot already
execute and alter or drop the routine. (The ALTER
ROUTINE privileges is required to drop the
routine.) The server also automatically drops those
privileges when the creator drops the routine. If
automatic_sp_privileges is 0, the
server does not automatically add and drop these
privileges. This variable was added in MySQL 5.0.3.
The number of outstanding connection requests MySQL can
have. This comes into play when the main MySQL thread gets
very many connection requests in a very short time. It
then takes some time (although very little) for the main
thread to check the connection and start a new thread. The
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() system call
should have more details. Check your OS documentation for
the maximum value for this variable.
back_log cannot be set higher than your
operating system limit.
basedir
The MySQL installation base directory. This variable can
be set with the --basedir option.
The size of the buffer that is allocated for caching
indexes and rows for BDB tables. If you
don't use BDB tables, you should start
mysqld with --skip-bdb
to not allocate memory for this cache.
The base directory for BDB tables. This
should be assigned the same value as the
datadir variable.
The size of the buffer that is allocated for caching
indexes and rows for BDB tables. If you
don't use BDB tables, you should set
this to 0 or start mysqld with
--skip-bdb to not allocate memory for
this cache.
The directory where the BDB storage
engine writes its log files. This variable can be set with
the --bdb-logdir option.
The maximum number of locks that can be active for a
BDB table (10,000 by default). You
should increase this value if errors such as the following
occur when you perform long transactions or when
mysqld has to examine many rows to
calculate a query:
bdb: Lock table is out of available locks Got error 12 from ...
This is ON if you are using
--bdb-shared-data to start Berkeley DB in
multi-process mode. (Do not use
DB_PRIVATE when initializing Berkeley
DB.)
The BDB temporary file directory.
The size of the cache to hold the SQL statements for the
binary log during a transaction. A binary log cache is
allocated for each client if the server supports any
transactional storage engines and if the server has the
binary log enabled (--log-bin option). If
you often use large, multiple-statement transactions, you
can increase this cache size to get more performance. The
Binlog_cache_use and
Binlog_cache_disk_use status variables
can be useful for tuning the size of this variable. See
Section 5.11.3, “The Binary Log”.
MySQL Enterprise
For recommendations on the optimum setting for
binlog_cache_size subscribe to the
MySQL Network Monitoring and Advisory Service. For more
information see
http://www.mysql.com/products/enterprise/advisors.html.
MyISAM uses a special tree-like cache
to make bulk inserts faster for INSERT ...
SELECT, INSERT ... VALUES (...), (...),
..., and LOAD DATA INFILE
when adding data to non-empty tables. This variable limits
the size of the cache tree in bytes per thread. Setting it
to 0 disables this optimization. The default value is 8MB.
The character set for statements that arrive from the client.
The character set used for literals that do not have a character set introducer and for number-to-string conversion.
The character set used by the default database. The server
sets this variable whenever the default database changes.
If there is no default database, the variable has the same
value as character_set_server.
The filesystem character set. This variable is used to
interpret string literals that refer to filenames, such as
in the LOAD DATA INFILE and
SELECT ... INTO OUTFILE statements and
the LOAD_FILE() function. Such
filenames are converted from
character_set_client to
character_set_filesystem before the
file opening attempt occurs. The default value is
binary, which means that no conversion
occurs. For systems on which multi-byte filenames are
allowed, a different value may be more appropriate. For
example, if the system represents filenames using UTF-8,
set character_set_filesystem to
'utf8'. This variable was added in
MySQL 5.0.19.
The character set used for returning query results to the client.
The server's default character set.
The character set used by the server for storing
identifiers. The value is always utf8.
The directory where character sets are installed.
The collation of the connection character set.
The collation used by the default database. The server
sets this variable whenever the default database changes.
If there is no default database, the variable has the same
value as collation_server.
The server's default collation.
The transaction completion type:
If the value is 0 (the default),
COMMIT and
ROLLBACK are unaffected.
If the value is 1, COMMIT and
ROLLBACK are equivalent to
COMMIT AND CHAIN and
ROLLBACK AND CHAIN, respectively.
(A new transaction starts immediately with the same
isolation level as the just-terminated transaction.)
If the value is 2, COMMIT and
ROLLBACK are equivalent to
COMMIT RELEASE and
ROLLBACK RELEASE, respectively.
(The server disconnects after terminating the
transaction.)
This variable was added in MySQL 5.0.3
If 1 (the default), MySQL allows INSERT
and SELECT statements to run
concurrently for MyISAM tables that
have no free blocks in the middle of the data file. You
can turn this option off by starting
mysqld with --safe or
--skip-new.
In MySQL 5.0.6, this variable was changed to take three integer values:
| Value | Description |
| 0 | Off |
| 1 | (Default) Enables concurrent insert for MyISAM tables
that don't have holes |
| 2 | Enables concurrent inserts for all MyISAM tables,
even those that have holes. For a table with a
hole, new rows are inserted at the end of the
table if it is in use by another thread.
Otherwise, MySQL acquires a normal write lock and
inserts the row into the hole. |
See also Section 7.3.3, “Concurrent Inserts”.
The number of seconds that the mysqld
server waits for a connect packet before responding with
Bad handshake.
datadir
The MySQL data directory. This variable can be set with
the --datadir option.
This variable is not implemented.
This variable is not implemented.
The default mode value to use for the
WEEK() function. See
Section 12.6, “Date and Time Functions”.
This option applies only to MyISAM
tables. It can have one of the following values to affect
handling of the DELAY_KEY_WRITE table
option that can be used in CREATE TABLE
statements.
| Option | Description |
OFF | DELAY_KEY_WRITE is ignored. |
ON | MySQL honors any DELAY_KEY_WRITE option specified in
CREATE TABLE statements. This
is the default value. |
ALL | All new opened tables are treated as if they were created with the
DELAY_KEY_WRITE option enabled. |
If DELAY_KEY_WRITE is enabled for a
table, the key buffer is not flushed for the table on
every index update, but only when the table is closed.
This speeds up writes on keys a lot, but if you use this
feature, you should add automatic checking of all
MyISAM tables by starting the server
with the --myisam-recover option (for
example, --myisam-recover=BACKUP,FORCE).
See Section 5.2.2, “Command Options”, and
Section 14.1.1, “MyISAM Startup Options”.
Note that if you enable external locking with
--external-locking, there is no
protection against index corruption for tables that use
delayed key writes.
After inserting delayed_insert_limit
delayed rows, the INSERT DELAYED
handler thread checks whether there are any
SELECT statements pending. If so, it
allows them to execute before continuing to insert delayed
rows.
How many seconds an INSERT DELAYED
handler thread should wait for INSERT
statements before terminating.
This is a per-table limit on the number of rows to queue
when handling INSERT DELAYED
statements. If the queue becomes full, any client that
issues an INSERT DELAYED statement
waits until there is room in the queue again.
This variable indicates the number of digits of precision
by which to increase the result of division operations
performed with the / operator. The
default value is 4. The minimum and maximum values are 0
and 30, respectively. The following example illustrates
the effect of increasing the default value.
mysql>SELECT 1/7;+--------+ | 1/7 | +--------+ | 0.1429 | +--------+ mysql>SET div_precision_increment = 12;mysql>SELECT 1/7;+----------------+ | 1/7 | +----------------+ | 0.142857142857 | +----------------+
This variable was added in MySQL 5.0.6.
This variable applies to NDB. By default it is 0
(OFF): If you execute a query such as
SELECT * FROM t WHERE mycol = 42, where
mycol is a non-indexed column, the
query is executed as a full table scan on every NDB node.
Each node sends every row to the MySQL server, which
applies the WHERE condition. If
engine_condition_pushdown is set to 1
(ON), the condition is “pushed
down” to the storage engine and sent to the NDB
nodes. Each node uses the condition to perform the scan,
and only sends back to the MySQL server the rows that
match the condition.
This variable was added in MySQL 5.0.3. Before that, the
default NDB behavior is the same as for
a value of OFF.
The number of days for automatic binary log removal. The default is 0, which means “no automatic removal.” Possible removals happen at startup and at binary log rotation.
flush
If ON, the server flushes
(synchronizes) all changes to disk after each SQL
statement. Normally, MySQL does a write of all changes to
disk only after each SQL statement and lets the operating
system handle the synchronizing to disk. See
Section B.1.4.2, “What to Do If MySQL Keeps Crashing”. This variable is set to
ON if you start
mysqld with the
--flush option.
If this is set to a non-zero value, all tables are closed
every flush_time seconds to free up
resources and synchronize unflushed data to disk. We
recommend that this option be used only on systems with
minimal resources.
The list of operators supported by boolean full-text
searches performed using IN BOOLEAN
MODE. See Section 12.8.1, “Boolean Full-Text Searches”.
The default variable value is
'+ -><()~*:""&|'. The
rules for changing the value are as follows:
Operator function is determined by position within the string.
The replacement value must be 14 characters.
Each character must be an ASCII non-alphanumeric character.
Either the first or second character must be a space.
No duplicates are allowed except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.
Positions 10, 13, and 14 (which by default are set to
‘:’,
‘&’, and
‘|’) are reserved for
future extensions.
The maximum length of the word to be included in a
FULLTEXT index.
Note:
FULLTEXT indexes must be rebuilt after
changing this variable. Use REPAIR TABLE
.
tbl_name QUICK
The minimum length of the word to be included in a
FULLTEXT index.
Note:
FULLTEXT indexes must be rebuilt after
changing this variable. Use REPAIR TABLE
.
tbl_name QUICK
The number of top matches to use for full-text searches
performed using WITH QUERY EXPANSION.
The file from which to read the list of stopwords for
full-text searches. All the words from the file are used;
comments are not honored. By default,
a built-in list of stopwords is used (as defined in the
myisam/ft_static.c file). Setting
this variable to the empty string ('')
disables stopword filtering.
Note:
FULLTEXT indexes must be rebuilt after
changing this variable or the contents of the stopword
file. Use REPAIR TABLE
.
tbl_name QUICK
The maximum allowed result length for the
GROUP_CONCAT() function. The default is
1024.
YES if mysqld
supports ARCHIVE tables,
NO if not.
YES if mysqld
supports BDB tables.
DISABLED if --skip-bdb
is used.
YES if mysqld
supports BLACKHOLE tables,
NO if not.
YES if the zlib
compression library is available to the server,
NO if not. If not, the
COMPRESS() and
UNCOMPRESS() functions cannot be used.
YES if the crypt()
system call is available to the server,
NO if not. If not, the
ENCRYPT() function cannot be used.
YES if mysqld
supports CSV tables,
NO if not.
YES if mysqld
supports EXAMPLE tables,
NO if not.
YES if mysqld
supports FEDERATED tables,
NO if not. This variable was added in
MySQL 5.0.3.
YES if the server supports spatial data
types, NO if not.
YES if mysqld
supports InnoDB tables.
DISABLED if
--skip-innodb is used.
In MySQL 5.0, this variable appears only for
reasons of backward compatibility. It is always
NO because ISAM
tables are no longer supported.
YES if mysqld
supports MERGE tables.
DISABLED if
--skip-merge is used. This variable was
added in MySQL 5.0.24.
YES if mysqld
supports NDB Cluster tables.
DISABLED if
--skip-ndbcluster is used.
YES if mysqld
supports SSL connections, NO if not.
YES if mysqld
supports the query cache, NO if not.
In MySQL 5.0, this variable appears only for
reasons of backward compatibility. It is always
NO because RAID
tables are no longer supported.
YES if RTREE indexes
are available, NO if not. (These are
used for spatial indexes in MyISAM
tables.)
YES if symbolic link support is
enabled, NO if not. This is required on
Unix for support of the DATA DIRECTORY
and INDEX DIRECTORY table options, and
on Windows for support of data directory symlinks.
The server sets this variable to the server hostname at startup. This variable was added in MySQL 5.0.38.
A string to be executed by the server for each client that
connects. The string consists of one or more SQL
statements. To specify multiple statements, separate them
by semicolon characters. For example, each client begins
by default with autocommit mode enabled. There is no
global system variable to specify that autocommit should
be disabled by default, but
init_connect can be used to achieve the
same effect:
SET GLOBAL init_connect='SET AUTOCOMMIT=0';
This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:
[mysqld] init_connect='SET AUTOCOMMIT=0'
Note that the content of init_connect
is not executed for users that have the
SUPER privilege. This is done so that
an erroneous value for init_connect
does not prevent all clients from connecting. For example,
the value might contain a statement that has a syntax
error, thus causing client connections to fail. Not
executing init_connect for users that
have the SUPER privilege enables them
to open a connection and fix the
init_connect value.
The name of the file specified with the
--init-file option when you start the
server. This should be a file containing SQL statements
that you want the server to execute when it starts. Each
statement must be on a single line and should not include
comments.
Note that the --init-file option is
unavailable if MySQL was configured with the
--disable-grant-options option. See
Section 2.4.14.2, “Typical configure Options”.
This variable is similar to
init_connect, but is a string to be
executed by a slave server each time the SQL thread
starts. The format of the string is the same as for the
init_connect variable.
innodb_
xxx
InnoDB system variables are listed in
Section 14.2.4, “InnoDB Startup Options and System Variables”.
The number of seconds the server waits for activity on an
interactive connection before closing it. An interactive
client is defined as a client that uses the
CLIENT_INTERACTIVE option to
mysql_real_connect(). See also
wait_timeout.
The size of the buffer that is used for joins that do not
use indexes and thus perform full table scans. Normally,
the best way to get fast joins is to add indexes. Increase
the value of join_buffer_size to get a
faster full join when adding indexes is not possible. One
join buffer is allocated for each full join between two
tables. For a complex join between several tables for
which indexes are not used, multiple join buffers might be
necessary.
Index blocks for MyISAM tables are
buffered and are shared by all threads.
key_buffer_size is the size of the
buffer used for index blocks. The key buffer is also known
as the key cache.
The maximum allowable setting for
key_buffer_size is 4GB. The effective
maximum size might be less, depending on your available
physical RAM and per-process RAM limits imposed by your
operating system or hardware platform.
Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache. Consider also the memory requirements of other storage engines.
For even more speed when writing many rows at the same
time, use LOCK TABLES. See
Section 7.2.17, “Speed of INSERT Statements”.
You can check the performance of the key buffer by issuing
a SHOW STATUS statement and examining
the Key_read_requests,
Key_reads,
Key_write_requests, and
Key_writes status variables. (See
Section 13.5.4, “SHOW Syntax”.) The
Key_reads/Key_read_requests ratio
should normally be less than 0.01. The
Key_writes/Key_write_requests ratio is
usually near 1 if you are using mostly updates and
deletes, but might be much smaller if you tend to do
updates that affect many rows at the same time or if you
are using the DELAY_KEY_WRITE table
option.
The fraction of the key buffer in use can be determined
using key_buffer_size in conjunction
with the Key_blocks_unused status
variable and the buffer block size, which is available
from the key_cache_block_size system
variable:
1 - ((Key_blocks_unused × key_cache_block_size) / key_buffer_size)
This value is an approximation because some space in the key buffer may be allocated internally for administrative structures.
It is possible to create multiple
MyISAM key caches. The size limit of
4GB applies to each cache individually, not as a group.
See Section 7.4.6, “The MyISAM Key Cache”.
This value controls the demotion of buffers from the hot
sub-chain of a key cache to the warm sub-chain. Lower
values cause demotion to happen more quickly. The minimum
value is 100. The default value is 300. See
Section 7.4.6, “The MyISAM Key Cache”.
The size in bytes of blocks in the key cache. The default
value is 1024. See Section 7.4.6, “The MyISAM Key Cache”.
The division point between the hot and warm sub-chains of
the key cache buffer chain. The value is the percentage of
the buffer chain to use for the warm sub-chain. Allowable
values range from 1 to 100. The default value is 100. See
Section 7.4.6, “The MyISAM Key Cache”.
language
The language used for error messages.
Whether mysqld was compiled with options for large file support.
large_pages
Whether large page support is enabled. This variable was added in MySQL 5.0.3.
This variable specifies the locale that controls the
language used to display day and month names and
abbreviations. This variable affects the output from the
DATE_FORMAT(),
DAYNAME() and
MONTHNAME() functions. Locale names are
POSIX-style values such as 'ja_JP' or
'pt_BR'. The default value is
'en_US' regardless of your system's
locale setting. For further information, see
Section 5.10.9, “MySQL Server Locale Support”. This variable was added
in MySQL 5.0.25.
The type of license the server has.
Whether LOCAL is supported for
LOAD DATA INFILE statements. See
Section 5.6.4, “Security Issues with LOAD DATA LOCAL”.
Whether mysqld was locked in memory
with --memlock.
log
Whether logging of all statements to the general query log is enabled. See Section 5.11.2, “The General Query Log”.
Whether the binary log is enabled. See Section 5.11.3, “The Binary Log”.
log_bin_trust_function_creators
This variable applies when binary logging is enabled. It
controls whether stored function creators can be trusted
not to create stored functions that will cause unsafe
events to be written to the binary log. If set to 0 (the
default), users are not allowed to create or alter stored
functions unless they have the SUPER
privilege in addition to the CREATE
ROUTINE or ALTER ROUTINE
privilege. A setting of 0 also enforces the restriction
that a function must be declared with the
DETERMINISTIC characteristic, or with
the READS SQL DATA or NO
SQL characteristic. If the variable is set to 1,
MySQL does not enforce these restrictions on stored
function creation. See
Section 17.4, “Binary Logging of Stored Routines and Triggers”.
This variable was added in MySQL 5.0.16.
log_bin_trust_routine_creators
This is the old name for
log_bin_trust_function_creators. Before
MySQL 5.0.16, it also applies to stored procedures, not
just stored functions. As of 5.0.16, this variable is
deprecated. It is recognized for backward compatibility
but its use results in a warning.
This variable was added in MySQL 5.0.6.
The location of the error log.
Whether queries that do not use indexes are logged to the slow query log. See Section 5.11.4, “The Slow Query Log”. This variable was added in MySQL 5.0.23.
Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this variable to have any effect. See Section 6.8, “Replication Startup Options”.
Whether slow queries should be logged. “Slow”
is determined by the value of the
long_query_time variable. See
Section 5.11.4, “The Slow Query Log”.
Whether to produce additional warning messages. It is enabled (1) by default and can be disabled by setting it to 0. Aborted connections are not logged to the error log unless the value is greater than 1.
If a query takes longer than this many seconds, the server
increments the Slow_queries status
variable. If you are using the
--log-slow-queries option, the query is
logged to the slow query log file. This value is measured
in real time, not CPU time, so a query that is under the
threshold on a lightly loaded system might be above the
threshold on a heavily loaded one. The minimum value is 1.
The default is 10. See Section 5.11.4, “The Slow Query Log”.
If set to 1, all
INSERT, UPDATE,
DELETE, and LOCK TABLE
WRITE statements wait until there is no pending
SELECT or LOCK TABLE
READ on the affected table. This affects only
storage engines that use only table-level locking
(MyISAM, MEMORY,
MERGE). This variable previously was
named sql_low_priority_updates.
This variable describes the case sensitivity of filenames
on the filesystem where the data directory is located.
OFF means filenames are case sensitive,
ON means they are not case sensitive.
If set to 1, table names are stored in lowercase on disk and table name comparisons are not case sensitive. If set to 2 table names are stored as given but compared in lowercase. This option also applies to database names and table aliases. See Section 9.2.2, “Identifier Case Sensitivity”.
If you are using InnoDB tables, you
should set this variable to 1 on all platforms to force
names to be converted to lowercase.
You should not set this variable to 0
if you are running MySQL on a system that does not have
case-sensitive filenames (such as Windows or Mac OS X). If
this variable is not set at startup and the filesystem on
which the data directory is located does not have
case-sensitive filenames, MySQL automatically sets
lower_case_table_names to 2.
The maximum size of one packet or any generated/intermediate string.
The packet message buffer is initialized to
net_buffer_length bytes, but can grow
up to max_allowed_packet bytes when
needed. This value by default is small, to catch large
(possibly incorrect) packets.
You must increase this value if you are using large
BLOB columns or long strings. It should
be as big as the largest BLOB you want
to use. The protocol limit for
max_allowed_packet is 1GB.
If a multiple-statement transaction requires more than
this many bytes of memory, the server generates a
Multi-statement transaction required more than
'max_binlog_cache_size' bytes of storage error.
The minimum value is 4096, the maximum and default values
are 4GB.
If a write to the binary log causes the current log file size to exceed the value of this variable, the server rotates the binary logs (closes the current file and opens the next one). You cannot set this variable to more than 1GB or to less than 4096 bytes. The default value is 1GB.
A transaction is written in one chunk to the binary log,
so it is never split between several binary logs.
Therefore, if you have big transactions, you might see
binary logs larger than
max_binlog_size.
If max_relay_log_size is 0, the value
of max_binlog_size applies to relay
logs as well.
If there are more than this number of interrupted
connections from a host, that host is blocked from further
connections. You can unblock blocked hosts with the
FLUSH HOSTS statement.
The number of simultaneous client connections allowed. By
default, this is 100. See
Section B.1.2.6, “Too many connections”, for more
information.
MySQL Enterprise
For notification that the maximum number of connections
is getting dangerously high and for advice on setting
the optimum value for max_connections
subscribe to the MySQL Network Monitoring and Advisory
Service. For more information see
http://www.mysql.com/products/enterprise/advisors.html.
Increasing this value increases the number of file descriptors that mysqld requires. See Section 7.4.8, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.
Do not start more than this number of threads to handle
INSERT DELAYED statements. If you try
to insert data into a new table after all INSERT
DELAYED threads are in use, the row is inserted
as if the DELAYED attribute wasn't
specified. If you set this to 0, MySQL never creates a
thread to handle DELAYED rows; in
effect, this disables DELAYED entirely.
The maximum number of error, warning, and note messages to
be stored for display by the SHOW
ERRORS and SHOW WARNINGS
statements.
This variable sets the maximum size to which
MEMORY tables are allowed to grow. The
value of the variable is used to calculate
MEMORY table
MAX_ROWS values. Setting this variable
has no effect on any existing MEMORY
table, unless the table is re-created with a statement
such as CREATE TABLE or altered with
ALTER TABLE or TRUNCATE
TABLE.
MySQL Enterprise
Subscribers to the MySQL Network Monitoring and Advisory
Service receive recommendations for the optimum setting
for max_heap_table_size. For more
information see
http://www.mysql.com/products/enterprise/advisors.html.
This variable is a synonym for
max_delayed_threads.
Do not allow SELECT statements that
probably need to examine more than
max_join_size rows (for single-table
statements) or row combinations (for multiple-table
statements) or that are likely to do more than
max_join_size disk seeks. By setting
this value, you can catch SELECT
statements where keys are not used properly and that would
probably take a long time. Set it if your users tend to
perform joins that lack a WHERE clause,
that take a long time, or that return millions of rows.
Setting this variable to a value other than
DEFAULT resets the value of
SQL_BIG_SELECTS to
0. If you set the
SQL_BIG_SELECTS value again, the
max_join_size variable is ignored.
If a query result is in the query cache, no result size check is performed, because the result has previously been computed and it does not burden the server to send it to the client.
This variable previously was named
sql_max_join_size.
The cutoff on the size of index values that determines
which filesort algorithm to use. See
Section 7.2.11, “ORDER BY Optimization”.
This variable limits the total number of prepared statements in the server. It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. The default value is 16,382. The allowable range of values is from 0 to 1 million. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. This variable was added in MySQL 5.0.21.
If a write by a replication slave to its relay log causes
the current log file size to exceed the value of this
variable, the slave rotates the relay logs (closes the
current file and opens the next one). If
max_relay_log_size is 0, the server
uses max_binlog_size for both the
binary log and the relay log. If
max_relay_log_size is greater than 0,
it constrains the size of the relay log, which enables you
to have different sizes for the two logs. You must set
max_relay_log_size to between 4096
bytes and 1GB (inclusive), or to 0. The default value is
0. See
Section 6.3, “Replication Implementation Details”.
Limit the assumed maximum number of seeks when looking up
rows based on a key. The MySQL optimizer assumes that no
more than this number of key seeks are required when
searching for matching rows in a table by scanning an
index, regardless of the actual cardinality of the index
(see Section 13.5.4.13, “SHOW INDEX Syntax”). By setting this to a
low value (say, 100), you can force MySQL to prefer
indexes instead of table scans.
The number of bytes to use when sorting
BLOB or TEXT values.
Only the first max_sort_length bytes of
each value are used; the rest are ignored.
The number of times that a stored procedure may call itself. The default value for this option is 0, which completely disallows recursion in stored procedures. The maximum value is 255.
This variable can be set globally and per session.
The maximum number of temporary tables a client can keep open at the same time. (This option does not yet do anything.)
The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means “no limit.”
Before MySQL 5.0.3, this variable has only global scope.
Beginning with MySQL 5.0.3, it also has a read-only
session scope. The session variable has the same value as
the global variable unless the current account has a
non-zero MAX_USER_CONNECTIONS resource
limit. In that case, the session value reflects the
account limit.
After this many write locks, allow some pending read lock requests to be processed in between.
The maximum number of ranges to send to a table handler at once during range selects. The default value is 256. Sending multiple ranges to a handler at once can improve the performance of certain selects dramatically. This especially true for the NDB Cluster table handler, which needs to send the range requests to all nodes. Sending a batch of those requests at once reduces the communication costs significantly. This variable was added in MySQL 5.0.3.
The block size to be used for MyISAM
index pages.
The default pointer size in bytes, to be used by
CREATE TABLE for
MyISAM tables when no
MAX_ROWS option is specified. This
variable cannot be less than 2 or larger than 7. The
default value is 6 (4 before MySQL 5.0.6). This variable
was added in MySQL 4.1.2. See
Section B.1.2.11, “The table is full”.
myisam_max_extra_sort_file_size
(DEPRECATED)
If the temporary file used for fast
MyISAM index creation would be larger
than using the key cache by the amount specified here,
prefer the key cache method. This is mainly used to force
long character keys in large tables to use the slower key
cache method to create the index. The value is given in
bytes.
Note: This variable was removed in MySQL 5.0.6.
The maximum size of the temporary file that MySQL is
allowed to use while re-creating a
MyISAM index (during REPAIR
TABLE, ALTER TABLE, or
LOAD DATA INFILE). If the file size
would be larger than this value, the index is created
using the key cache instead, which is slower. The value is
given in bytes.
The default value is 2GB. If MyISAM
index files exceed this size and disk space is available,
increasing the value may help performance.
The value of the --myisam-recover option.
See Section 5.2.2, “Command Options”.
If this value is greater than 1, MyISAM
table indexes are created in parallel (each index in its
own thread) during the Repair by
sorting process. The default value is 1.
Note: Multi-threaded repair is still beta-quality code.
The size of the buffer that is allocated when sorting
MyISAM indexes during a REPAIR
TABLE or when creating indexes with
CREATE INDEX or ALTER
TABLE.
How the server treats NULL values when
collecting statistics about the distribution of index
values for MyISAM tables. This variable
has two possible values, nulls_equal
and nulls_unequal. For
nulls_equal, all
NULL index values are considered equal
and form a single value group that has a size equal to the
number of NULL values. For
nulls_unequal, NULL
values are considered unequal, and each
NULL forms a distinct value group of
size 1.
The method that is used for generating table statistics
influences how the optimizer chooses indexes for query
execution, as described in
Section 7.4.7, “MyISAM Index Statistics Collection”.
This variable was added in MySQL 5.0.14. For older
versions, the statistics collection method is equivalent
to nulls_equal.
Specifies the maximum number of ranges to send to a
storage engine during range selects. The default value is
256. Sending multiple ranges to an engine is a feature
that can improve the performance of certain selects
dramatically, particularly for
NDBCLUSTER. This engine needs to send
the range requests to all nodes, and sending many of those
requests at once reduces the communication costs
significantly. This variable was added in MySQL 5.0.3.
(Windows only.) Indicates whether the server supports connections over named pipes.
Determines the probability of gaps in an autoincremented
column. Set to 1 to minimize this. Set
to a high value for optimization — makes inserts
faster, but decreases the likelihood that consecutive
autoincrement numbers will be used in a batch of inserts.
Default value: 32. Mimimum value:
1.
The number of milliseconds to wait before checking the
NDB query cache. Setting this to
0 (the default and minimum value) means
that the NDB query cache will be
checked for validation on every query.
The recommended maximum value for this variable is
1000, which means that the query cache
is checked once per second. A larger value means the
NDB query cache is less often checked
and invalidated due to updates on a different
mysqld. It is generally not desirable
to set this to a value greater than
2000.
Forces sending of buffers to NDB
immediately, without waiting for other threads. Defaults
to ON.
Sets the granularity of the statistics by determining the
number of starting and ending keys to store in the
statistics memory cache. Zero means no caching takes
place; in this case, the data nodes are always queried
directly. Default value: 32.
Use NDB index statistics in query
optimization. Defaults to ON.
How often to query data nodes instead of the statistics
cache. For example, a value of 20 (the
default) means to direct every
20th query to the data nodes.
ndb_report_thresh_binlog_epoch_slip
This is a threshold on the number of epochs to be behind
before reporting binlog status. For example, a value of
3 (the default) means that if the
difference between which epoch has been received from the
storage nodes and which epoch has been applied to the
binlog is 3 or more, a status message will be sent to the
cluster log.
ndb_report_thresh_binlog_mem_usage
This is a threshold on the percentage of free memory
remaining before reporting binlog status. For example, a
value of 10 (the default) means that if
the amount of available memory for receiving binlog data
from the data nodes falls below 10%, a status message will
be sent to the cluster log.
Forces NDB to use a count of records
during SELECT COUNT(*) query planning
to speed up this type of query. The default value is
ON. For faster queries overall, disable
this feature by setting the value of
ndb_use_exact_count to
OFF.
You can disable NDB transaction support
by setting this variable's values to
OFF (not recommended). The default is
ON.
Each client thread is associated with a connection buffer
and result buffer. Both begin with a size given by
net_buffer_length but are dynamically
enlarged up to max_allowed_packet bytes
as needed. The result buffer shrinks to
net_buffer_length after each SQL
statement.
This variable should not normally be changed, but if you
have very little memory, you can set it to the expected
length of statements sent by clients. If statements exceed
this length, the connection buffer is automatically
enlarged. The maximum value to which
net_buffer_length can be set is 1MB.
The number of seconds to wait for more data from a
connection before aborting the read. This timeout applies
only to TCP/IP connections, not to connections made via
Unix socket files, named pipes, or shared memory. When the
server is reading from the client,
net_read_timeout is the timeout value
controlling when to abort. When the server is writing to
the client, net_write_timeout is the
timeout value controlling when to abort. See also
slave_net_timeout.
If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads.
The number of seconds to wait for a block to be written to
a connection before aborting the write. This timeout
applies only to TCP/IP connections, not to connections
made via Unix socket files, named pipes, or shared memory.
See also net_read_timeout.
This variable was used in MySQL 4.0 to turn on some 4.1
behaviors, and is retained for backward compatibility. In
MySQL 5.0, its value is always
OFF.
Whether the server should use pre-4.1-style passwords for
MySQL user accounts. See Section B.1.2.3, “Client does not support authentication protocol”.
This is not a variable, but it can be used when setting
some variables. It is described in
Section 13.5.3, “SET Syntax”.
The number of files that the operating system allows
mysqld to open. This is the real value
allowed by the system and might be different from the
value you gave using the
--open-files-limit option to
mysqld or
mysqld_safe. The value is 0 on systems
where MySQL can't change the number of open files.
Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans. This variable was added in MySQL 5.0.1.
The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to the maximum number of tables used in a query plus 2, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for performing searches. This variable was added in MySQL 5.0.1.
The pathname of the process ID (PID) file. This variable
can be set with the --pid-file option.
port
The number of the port on which the server listens for
TCP/IP connections. This variable can be set with the
--port option.
The size of the buffer that is allocated when preloading indexes.
The current number of prepared statements. (The maximum
number of statements is given by the
max_prepared_stmt_count system
variable.) This variable was added in MySQL 5.0.21. In
MySQL 5.0.32, it was converted to the global
Prepared_stmt_count status variable.
The version of the client/server protocol used by the MySQL server.
The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit.
Don't cache results that are larger than this number of bytes. The default value is 1MB.
The minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096 (4KB). Tuning information for this variable is given in Section 5.13.3, “Query Cache Configuration”.
The amount of memory allocated for caching query results.
The default value is 0, which disables the query cache.
The allowable values are multiples of 1024; other values
are rounded down to the nearest multiple. Note that
query_cache_size bytes of memory are
allocated even if query_cache_type is
set to 0. See Section 5.13.3, “Query Cache Configuration”,
for more information.
Set the query cache type. Setting the
GLOBAL value sets the type for all
clients that connect thereafter. Individual clients can
set the SESSION value to affect their
own use of the query cache. Possible values are shown in
the following table:
| Option | Description |
0 or OFF | Don't cache results in or retrieve results from the query cache. Note
that this does not deallocate the query cache
buffer. To do that, you should set
query_cache_size to 0. |
1 or ON | Cache all query results except for those that begin with SELECT
SQL_NO_CACHE. |
2 or DEMAND | Cache results only for queries that begin with SELECT
SQL_CACHE. |
This variable defaults to ON.
Normally, when one client acquires a
WRITE lock on a
MyISAM table, other clients are not
blocked from issuing statements that read from the table
if the query results are present in the query cache.
Setting this variable to 1 causes acquisition of a
WRITE lock for a table to invalidate
any queries in the query cache that refer to the table.
This forces other clients that attempt to access the table
to wait while the lock is in effect.
The size of the persistent buffer used for statement
parsing and execution. This buffer is not freed between
statements. If you are running complex queries, a larger
query_prealloc_size value might be
helpful in improving performance, because it can reduce
the need for the server to perform memory allocation
during query execution operations.
The size of blocks that are allocated when doing range optimization.
Each thread that does a sequential scan allocates a buffer of this size (in bytes) for each table it scans. If you do many sequential scans, you might want to increase this value, which defaults to 131072.
read_buffer_size and
read_rnd_buffer_size are not specific
to any storage engine and apply in a general manner for
optimization. See Section 7.5.5, “How MySQL Uses Memory”, for
example.
When this variable is set to ON, the
server allows no updates except from users that have the
SUPER privilege or (on a slave server)
from updates performed by slave threads. On a slave
server, this can be useful to ensure that the slave
accepts updates only from its master server and not from
clients. As of MySQL 5.0.16, this variable does not apply
to TEMPORARY tables.
read_only exists only as a
GLOBAL variable, so changes to its
value require the SUPER privilege.
Changes to read_only on a master server
are not replicated to slave servers. The value can be set
on a slave server independent of the setting on the
master.
When reading rows in sorted order following a key-sorting
operation, the rows are read through this buffer to avoid
disk seeks. Setting the variable to a large value can
improve ORDER BY performance by a lot.
However, this is a buffer allocated for each client, so
you should not set the global variable to a large value.
Instead, change the session variable only from within
those clients that need to run large queries.
read_buffer_size and
read_rnd_buffer_size are not specific
to any storage engine and apply in a general manner for
optimization. See Section 7.5.5, “How MySQL Uses Memory”, for
example.
Disables or enables automatic purging of relay log files
as soon as they are not needed any more. The default value
is 1 (ON).
This variable is unused.
If the MySQL server has been started with the
--secure-auth option, it blocks
connections from all accounts that have passwords stored
in the old (pre-4.1) format. In that case, the value of
this variable is ON, otherwise it is
OFF.
You should enable this option if you want to prevent all use of passwords employing the old format (and hence insecure communication over the network).
Server startup fails with an error if this option is
enabled and the privilege tables are in pre-4.1 format.
See Section B.1.2.3, “Client does not support authentication protocol”.
The server ID. This value is set by the
--server-id option. It is used for
replication to enable master and slave servers to identify
themselves uniquely.
(Windows only.) Whether the server allows shared-memory connections.
(Windows only.) The name of shared memory to use for
shared-memory connections. This is useful when running
multiple MySQL instances on a single physical machine. The
default name is MYSQL. The name is case
sensitive.
This is OFF if
mysqld uses external locking,
ON if external locking is disabled.
skip_networking
This is ON if the server allows only
local (non-TCP/IP) connections. On Unix, local connections
use a Unix socket file. On Windows, local connections use
a named pipe or shared memory. On NetWare, only TCP/IP
connections are supported, so do not set this variable to
ON. This variable can be set to
ON with the
--skip-networking option.
This prevents people from using the SHOW
DATABASES statement if they do not have the
SHOW DATABASES privilege. This can
improve security if you have concerns about users being
able to see databases belonging to other users. Its effect
depends on the SHOW DATABASES
privilege: If the variable value is ON,
the SHOW DATABASES statement is allowed
only to users who have the SHOW
DATABASES privilege, and the statement displays
all database names. If the value is
OFF, SHOW DATABASES
is allowed to all users, but displays the names of only
those databases for which the user has the SHOW
DATABASES or other privilege.
Whether to use compression of the slave/master protocol if both the slave and the master support it.
The name of the directory where the slave creates
temporary files for replicating LOAD DATA
INFILE statements.
The number of seconds to wait for more data from a master/slave connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory.
slave_skip_errors
The replication errors that the slave should skip (ignore).
If a replication slave SQL thread fails to execute a
transaction because of an InnoDB
deadlock or exceeded InnoDB's
innodb_lock_wait_timeout or
NDBCluster's
TransactionDeadlockDetectionTimeout or
TransactionInactiveTimeout, it
automatically retries
slave_transaction_retries times before
stopping with an error. The default priot to MySQL 4.0.3
is 0. You must explicitly set the value greater than 0 to
enable the “retry” behavior, which is
probably a good idea. In MySQL 5.0.3 or newer, the default
is 10.
If creating a thread takes longer than this many seconds,
the server increments the
Slow_launch_threads status variable.
socket
On Unix platforms, this variable is the name of the socket
file that is used for local client connections. The
default is /tmp/mysql.sock. (For some
distribution formats, the directory might be different,
such as /var/lib/mysql for RPMs.)
On Windows, this variable is the name of the named pipe
that is used for local client connections. The default
value is MySQL (not case sensitive).
Each thread that needs to do a sort allocates a buffer of
this size. Increase this value for faster ORDER
BY or GROUP BY operations.
See Section B.1.4.4, “Where MySQL Stores Temporary Files”.
The current server SQL mode, which can be set dynamically. See Section 5.2.6, “SQL Modes”.
The number of events from the master that a slave server
should skip. See
Section 13.6.2.6, “SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax”.
The path to a file with a list of trusted SSL CAs. This variable was added in MySQL 5.0.23.
The path to a directory that contains trusted SSL CA certificates in PEM format. This variable was added in MySQL 5.0.23.
The name of the SSL certificate file to use for establishing a secure connection. This variable was added in MySQL 5.0.23.
A list of allowable ciphers to use for SSL encryption. The
cipher list has the same format as the openssl
ciphers command. This variable was added in
MySQL 5.0.23.
The name of the SSL key file to use for establishing a secure connection. This variable was added in MySQL 5.0.23.
The default storage engine (table type). To set the
storage engine at server startup, use the
--default-storage-engine option. See
Section 5.2.2, “Command Options”.
If the value of this variable is positive, the MySQL
server synchronizes its binary log to disk (using
fdatasync()) after every
sync_binlog writes to the binary log.
Note that there is one write to the binary log per
statement if autocommit is enabled, and one write per
transaction otherwise. The default value is 0, which does
no synchronizing to disk. A value of 1 is the safest
choice, because in the event of a crash you lose at most
one statement or transaction from the binary log. However,
it is also the slowest choice (unless the disk has a
battery-backed cache, which makes synchronization very
fast).
If the value of sync_binlog is 0 (the
default), no extra flushing is done. The server relies on
the operating system to flush the file contents
occasionaly as for any other file.
If this variable is set to 1, when any non-temporary table
is created its .frm file is
synchronized to disk (using
fdatasync()). This is slower but safer
in case of a crash. The default is 1.
The server system time zone. When the server begins
executing, it inherits a time zone setting from the
machine defaults, possibly modified by the environment of
the account used for running the server or the startup
script. The value is used to set
system_time_zone. Typically the time
zone is specified by the TZ environment
variable. It also can be specified using the
--timezone option of the
mysqld_safe script.
The system_time_zone variable differs
from time_zone. Although they might
have the same value, the latter variable is used to
initialize the time zone for each client that connects.
See Section 5.10.8, “MySQL Server Time Zone Support”.
The number of open tables for all threads. Increasing this
value increases the number of file descriptors that
mysqld requires. You can check whether
you need to increase the table cache by checking the
Opened_tables status variable. See
Section 5.2.5, “Status Variables”. If the value of
Opened_tables is large and you don't do
FLUSH TABLES often (which just forces
all tables to be closed and reopened), then you should
increase the value of the table_cache
variable. For more information about the table cache, see
Section 7.4.8, “How MySQL Opens and Closes Tables”.
Specifies a wait timeout for table-level locks, in
seconds. The default timeout is 50 seconds. The timeout is
active only if the connection has open cursors. This
variable can also be set globally at runtime (you need the
SUPER privilege to do this). It's
available as of MySQL 5.0.10.
This variable is a synonym for
storage_engine. In MySQL
5.0, storage_engine is the
preferred name.
How many threads the server should cache for reuse. When a
client disconnects, the client's threads are put in the
cache if there are fewer than
thread_cache_size threads there.
Requests for threads are satisfied by reusing threads
taken from the cache if possible, 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 provide a notable
performance improvement if you have a good thread
implementation.) By examining the difference between the
Connections and
Threads_created status variables, you
can see how efficient the thread cache is. For details,
see Section 5.2.5, “Status Variables”.
On Solaris, mysqld calls
thr_setconcurrency() with this value.
This function enables applications to give the threads
system a hint about the desired number of threads that
should be run at the same time.
The stack size for each thread. Many of the limits
detected by the crash-me test are
dependent on this value. The default is large enough for
normal operation. See Section 7.1.4, “The MySQL Benchmark Suite”.
The default is 192KB.
This variable is not implemented.
The current time zone. This variable is used to initialize
the time zone for each client that connects. By default,
the initial value of this is 'SYSTEM'
(which means, “use the value of
system_time_zone”). The value
can be specified explicitly at server startup with the
--default-time-zone option. See
Section 5.10.8, “MySQL Server Time Zone Support”.
This variable controls whether InnoDB
mutexes are timed. If this variable is set to 0 or
OFF (the default), mutex timing is
disabled. If the variable is set to 1 or
ON, mutex timing is enabled. With
timing enabled, the os_wait_times value
in the output from SHOW ENGINE INNODB
MUTEX indicates the amount of time (in ms) spent
in operating system waits. Otherwise, the value is 0. This
variable was added in MySQL 5.0.3.
The maximum size of in-memory temporary tables. (The
actual limit is determined as the smaller of
max_heap_table_size and
tmp_table_size.) If an in-memory
temporary table exceeds the limit, MySQL automatically
converts it to an on-disk MyISAM table.
Increase the value of tmp_table_size
(and max_heap_table_size if necessary)
if you do many advanced GROUP BY
queries and you have lots of memory.
tmpdir
The directory used for temporary files and temporary
tables. This variable can be set to a list of several
paths that are used in round-robin fashion. Paths should
be separated by colon characters
(‘:’) on Unix and semicolon
characters (‘;’) on
Windows, NetWare, and OS/2.
The multiple-directory feature can be used to spread the
load between several physical disks. If the MySQL server
is acting as a replication slave, you should not set
tmpdir to point to a directory on a
memory-based filesystem or to a directory that is cleared
when the server host restarts. A replication slave needs
some of its temporary files to survive a machine restart
so that it can replicate temporary tables or LOAD
DATA INFILE operations. If files in the
temporary file directory are lost when the server
restarts, replication fails. However, if you are using
MySQL 4.0.0 or later, you can set the slave's temporary
directory using the slave_load_tmpdir
variable. In that case, the slave won't use the general
tmpdir value and you can set
tmpdir to a non-permanent location.
The amount in bytes by which to increase a per-transaction
memory pool which needs memory. See the description of
transaction_prealloc_size.
There is a per-transaction memory pool from which various
transaction-related allocations take memory. The initial
size of the pool in bytes is
transaction_prealloc_size. For every
allocation that cannot be satisfied from the pool because
it has insufficient memory available, the pool is
increased by
transaction_alloc_block_size bytes.
When the transaction ends, the pool is truncated to
transaction_prealloc_size bytes.
By making transaction_prealloc_size
sufficiently large to contain all statements within a
single transaction, you can avoid many
malloc() calls.
The default transaction isolation level. Defaults to
REPEATABLE-READ.
This variable is set by the SET TRANSACTION
ISOLATION LEVEL statement. See
Section 13.4.6, “SET TRANSACTION Syntax”. If you set
tx_isolation directly to an isolation
level name that contains a space, the name should be
enclosed within quotes, with the space replaced by a dash.
For example:
SET tx_isolation = 'READ-COMMITTED';
This variable controls whether updates to a view can be
made when the view does not contain all columns of the
primary key defined in the underlying table, if the update
statement contains a LIMIT clause.
(Such updates often are generated by GUI tools.) An update
is an UPDATE or
DELETE statement. Primary key here
means a PRIMARY KEY, or a
UNIQUE index in which no column can
contain NULL.
The variable can have two values:
1 or YES: Issue
a warning only (not an error message). This is the
default value.
0 or NO:
Prohibit the update.
This variable was added in MySQL 5.0.2.
version
The version number for the server.
Starting with MySQL 5.0.24, the version number will also
indicate whether the server is a standard release
(Community) or Enterprise release (for example,
5.0.28-enterprise-gpl-nt).
The BDB storage engine version.
The configure script has a
--with-comment option that allows a
comment to be specified when building MySQL. This variable
contains the value of that comment.
For precompiled binaries, this variable will hold the
server version and license information. Starting with
MySQL 5.0.24, version_comment will
include the full server type and license. For community
users this will appear as MySQL Community Edition
- Standard (GPL). For Enterprise users, the
version might be displayed as MySQL Enterprise
Server (GPL). The corresponding license for your
MySQL binary is shown in parentheses. For server compiled
from source, the default value will be the same as that
for Community releases.
The type of machine or architecture on which MySQL was built.
The type of operating system on which MySQL was built.
The number of seconds the server waits for activity on a non-interactive connection before closing it. This timeout applies only to TCP/IP and Unix socket file connections, not to connections made via named pipes, or shared memory.
On thread startup, the session
wait_timeout value is initialized from
the global wait_timeout value or from
the global interactive_timeout value,
depending on the type of client (as defined by the
CLIENT_INTERACTIVE connect option to
mysql_real_connect()). See also
interactive_timeout.
MySQL Enterprise Expert use of server system variables is part of the service offered by the MySQL Network Monitoring and Advisory Service. To subscribe see http://www.mysql.com/products/enterprise/advisors.html.
The mysql server maintains many system
variables that indicate how it is configured.
Section 5.2.3, “System Variables”, describes the
meaning of these variables. Each system variable has a default
value. System variables can be set at server startup using
options on the command line or in an option file. Most of them
can be changed dynamically while the server is running by
means of the SET statement, which enables
you to modify operation of the server without having to stop
and restart it. You can refer to system variable values in
expressions.
The server maintains two kinds of system variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:
When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified on the command line or in an option file. (See Section 4.3, “Specifying Program Options”.)
The server also maintains a set of session variables for
each client that connects. The client's session variables
are initialized at connect time using the current values
of the corresponding global variables. For example, the
client's SQL mode is controlled by the session
sql_mode value, which is initialized
when the client connects to the value of the global
sql_mode value.
System variable values can be set globally at server startup
by using options on the command line or in an option file.
When you use a startup option to set a variable that takes a
numeric value, the value can be given with a suffix of
K, M, or
G (either uppercase or lowercase) to
indicate a multiplier of 1024,
10242 or
10243; that is, units of kilobytes,
megabytes, or gigabytes, respectively. Thus, the following
command starts the server with a query cache size of 16
megabytes and a maximum packet size of one gigabyte:
mysqld --query_cache_size=16M --max_allowed_packet=1G
Within an option file, those variables are set like this:
[mysqld] query_cache_size=16M max_allowed_packet=1G
The lettercase of suffix letters does not matter;
16M and 16m are
equivalent, as are 1G and
1g.
If you want to restrict the maximum value to which a system
variable can be set at runtime with the SET
statement, you can specify this maximum by using an option of
the form
--maximum-
at server startup. For example, to prevent the value of
var_name=valuequery_cache_size from being increased to
more than 32MB at runtime, use the option
--maximum-query_cache_size=32M.
Many system variables are dynamic and can be changed while the
server runs by using the SET statement. For
a list, see Section 5.2.4.2, “Dynamic System Variables”. To
change a system variable with SET, refer to
it as var_name, optionally preceded
by a modifier:
To indicate explicitly that a variable is a global
variable, precede its name by GLOBAL or
@@global.. The SUPER
privilege is required to set global variables.
To indicate explicitly that a variable is a session
variable, precede its name by SESSION,
@@session., or @@.
Setting a session variable requires no special privilege,
but a client can change only its own session variables,
not those of any other client.
LOCAL and @@local.
are synonyms for SESSION and
@@session..
If no modifier is present, SET changes
the session variable.
A SET statement can contain multiple
variable assignments, separated by commas. If you set several
system variables, the most recent GLOBAL or
SESSION modifier in the statement is used
for following variables that have no modifier specified.
Examples:
SET sort_buffer_size=10000; SET @@local.sort_buffer_size=10000; SET GLOBAL sort_buffer_size=1000000, SESSION sort_buffer_size=1000000; SET @@sort_buffer_size=1000000; SET @@global.sort_buffer_size=1000000, @@local.sort_buffer_size=1000000;
When you assign a value to a system variable with
SET, you cannot use suffix letters in the
value (as can be done with startup options). However, the
value can take the form of an expression:
SET sort_buffer_size = 10 * 1024 * 1024;
The @@
syntax for system variables is supported for compatibility
with some other database systems.
var_name
If you change a session system variable, the value remains in effect until your session ends or until you change the variable to a different value. The change is not visible to other clients.
If you change a global system variable, the value is
remembered and used for new connections until the server
restarts. (To make a global system variable setting permanent,
you should set it in an option file.) The change is visible to
any client that accesses that global variable. However, the
change affects the corresponding session variable only for
clients that connect after the change. The global variable
change does not affect the session variable for any client
that is currently connected (not even that of the client that
issues the SET GLOBAL statement).
To prevent incorrect usage, MySQL produces an error if you use
SET GLOBAL with a variable that can only be
used with SET SESSION or if you do not
specify GLOBAL (or
@@global.) when setting a global variable.
To set a SESSION variable to the
GLOBAL value or a GLOBAL
value to the compiled-in MySQL default value, use the
DEFAULT keyword. For example, the following
two statements are identical in setting the session value of
max_join_size to the global value:
SET max_join_size=DEFAULT; SET @@session.max_join_size=@@global.max_join_size;
Not all system variables can be set to
DEFAULT. In such cases, use of
DEFAULT results in an error.
You can refer to the values of specific global or sesson
system variables in expressions by using one of the
@@-modifiers. For example, you can retrieve
values in a SELECT statement like this:
SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;
When you refer to a system variable in an expression as
@@ (that
is, when you do not specify var_name@@global. or
@@session.), MySQL returns the session
value if it exists and the global value otherwise. (This
differs from SET
@@, which always
refers to the session value.)
var_name =
value
Note: Some system variables can be
enabled with the SET statement by setting
them to ON or 1, or
disabled by setting them to OFF or
0. However, to set such a variable on the
command line or in an option file, you must set it to
1 or 0; setting it to
ON or OFF will not work.
For example, on the command line,
--delay_key_write=1 works but
--delay_key_write=ON does not.
To display system variable names and values, use the
SHOW VARIABLES statement:
mysql> SHOW VARIABLES;
+--------+--------------------------------------------------------------+
| Variable_name | Value |
+--------+--------------------------------------------------------------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
| automatic_sp_privileges | ON |
| back_log | 50 |
| basedir | / |
| bdb_cache_size | 8388600 |
| bdb_home | /var/lib/mysql/ |
| bdb_log_buffer_size | 32768 |
| bdb_logdir | |
| bdb_max_lock | 10000 |
| bdb_shared_data | OFF |
| bdb_tmpdir | /tmp/ |
| binlog_cache_size | 32768 |
| bulk_insert_buffer_size | 8388608 |
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_database | latin1 |
| character_set_results | latin1 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
| collation_connection | latin1_swedish_ci |
| collation_database | latin1_swedish_ci |
| collation_server | latin1_swedish_ci |
...
| innodb_additional_mem_pool_size | 1048576 |
| innodb_autoextend_increment | 8 |
| innodb_buffer_pool_awe_mem_mb | 0 |
| innodb_buffer_pool_size | 8388608 |
| innodb_checksums | ON |
| innodb_commit_concurrency | 0 |
| innodb_concurrency_tickets | 500 |
| innodb_data_file_path | ibdata1:10M:autoextend |
| innodb_data_home_dir | |
...
| version | 5.0.19 |
| version_comment | MySQL Community Edition - (GPL) |
| version_compile_machine | i686 |
| version_compile_os | pc-linux-gnu |
| wait_timeout | 28800 |
+--------+--------------------------------------------------------------+
With a LIKE clause, the statement displays
only those variables that match the pattern. To obtain a
specific variable name, use a LIKE clause
as shown:
SHOW VARIABLES LIKE 'max_join_size'; SHOW SESSION VARIABLES LIKE 'max_join_size';
To get a list of variables whose name match a pattern, use the
‘%’ wildcard character in a
LIKE clause:
SHOW VARIABLES LIKE '%size%'; SHOW GLOBAL VARIABLES LIKE '%size%';
Wildcard characters can be used in any position within the
pattern to be matched. Strictly speaking, because
‘_’ is a wildcard that matches
any single character, you should escape it as
‘\_’ to match it literally. In
practice, this is rarely necessary.
For SHOW VARIABLES, if you specify neither
GLOBAL nor SESSION,
MySQL returns SESSION values.
The reason for requiring the GLOBAL keyword
when setting GLOBAL-only variables but not
when retrieving them is to prevent problems in the future. If
we were to remove a SESSION variable that
has the same name as a GLOBAL variable, a
client with the SUPER privilege might
accidentally change the GLOBAL variable
rather than just the SESSION variable for
its own connection. If we add a SESSION
variable with the same name as a GLOBAL
variable, a client that intends to change the
GLOBAL variable might find only its own
SESSION variable changed.
A structured variable differs from a regular system variable in two respects:
Its value is a structure with components that specify server parameters considered to be closely related.
There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.
MySQL 5.0 supports one structured variable type, which specifies parameters governing the operation of key caches. A key cache structured variable has these components:
key_buffer_size
key_cache_block_size
key_cache_division_limit
key_cache_age_threshold
This section describes the syntax for referring to
structured variables. Key cache variables are used for
syntax examples, but specific details about how key caches
operate are found elsewhere, in
Section 7.4.6, “The MyISAM Key Cache”.
To refer to a component of a structured variable instance,
you can use a compound name in
instance_name.component_name
format. Examples:
hot_cache.key_buffer_size hot_cache.key_cache_block_size cold_cache.key_cache_block_size
For each structured system variable, an instance with the
name of default is always predefined. If
you refer to a component of a structured variable without
any instance name, the default instance
is used. Thus, default.key_buffer_size
and key_buffer_size both refer to the
same system variable.
Structured variable instances and components follow these naming rules:
For a given type of structured variable, each instance
must have a name that is unique
within variables of that type.
However, instance names need not be unique
across structured variable types.
For example, each structured variable has an instance
named default, so
default is not unique across variable
types.
The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.
If a structured variable instance name is not legal as
an unquoted identifier, refer to it as a quoted
identifier using backticks. For example,
hot-cache is not legal, but
`hot-cache` is.
global, session,
and local are not legal instance
names. This avoids a conflict with notation such as
@@global.
for referring to non-structured system variables.
var_name
Currently, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules will assume greater significance if some other type of structured variable is created in the future.
With one exception, you can refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option:
shell> mysqld --hot_cache.key_buffer_size=64K
In an option file, use this syntax:
[mysqld] hot_cache.key_buffer_size=64K
If you start the server with this option, it creates a key
cache named hot_cache with a size of 64KB
in addition to the default key cache that has a default size
of 8MB.
Suppose that you start the server as follows:
shell>mysqld --key_buffer_size=256K \--extra_cache.key_buffer_size=128K \--extra_cache.key_cache_block_size=2048
In this case, the server sets the size of the default key
cache to 256KB. (You could also have written
--default.key_buffer_size=256K.) In
addition, the server creates a second key cache named
extra_cache that has a size of 128KB,
with the size of block buffers for caching table index
blocks set to 2048 bytes.
The following example starts the server with three different key caches having sizes in a 3:1:1 ratio:
shell>mysqld --key_buffer_size=6M \--hot_cache.key_buffer_size=2M \--cold_cache.key_buffer_size=2M
Structured variable values may be set and retrieved at
runtime as well. For example, to set a key cache named
hot_cache to a size of 10MB, use either
of these statements:
mysql>SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;mysql>SET @@global.hot_cache.key_buffer_size = 10*1024*1024;
To retrieve the cache size, do this:
mysql> SELECT @@global.hot_cache.key_buffer_size;
However, the following statement does not work. The variable
is not interpreted as a compound name, but as a simple
string for a LIKE pattern-matching
operation:
mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';
This is the exception to being able to use structured variable names anywhere a simple variable name may occur.
Many server system variables are dynamic and can be set at
runtime using SET GLOBAL or SET
SESSION. You can also obtain their values using
SELECT. See
Section 5.2.4, “Using System Variables”.
The following table shows the full list of all dynamic
system variables. The last column indicates for each
variable whether GLOBAL or
SESSION (or both) apply. The table also
lists session options that can be set with the
SET statement.
Section 13.5.3, “SET Syntax”, discusses these options.
Variables that have a type of “string” take a
string value. Variables that have a type of
“numeric” take a numeric value. Variables that
have a type of “boolean” can be set to 0, 1,
ON or OFF. (If you set
them on the command line or in an option file, use the
numeric values.) Variables that are marked as
“enumeration” normally should be set to one of
the available values for the variable, but can also be set
to the number that corresponds to the desired enumeration
value. For enumerated system variables, the first
enumeration value corresponds to 0. This differs from
ENUM columns, for which the first
enumeration value corresponds to 1.
| Variable Name | Value Type | Type |
autocommit | boolean | SESSION |
automatic_sp_privileges | boolean | GLOBAL |
big_tables | boolean | SESSION |
binlog_cache_size | numeric | GLOBAL |
bulk_insert_buffer_size | numeric | GLOBAL | SESSION |
character_set_client | string | GLOBAL | SESSION |
character_set_connection | string | GLOBAL | SESSION
|
character_set_filesystem | string | GLOBAL | SESSION |
character_set_results | string | GLOBAL | SESSION |
character_set_server | string | GLOBAL | SESSION |
collation_connection | string | GLOBAL | SESSION |
collation_server | string | GLOBAL | SESSION |
completion_type | numeric | GLOBAL | SESSION |
concurrent_insert | numeric | GLOBAL |
connect_timeout | numeric | GLOBAL |
default_week_format | numeric | GLOBAL | SESSION |
delay_key_write | OFF | ON | ALL | GLOBAL |
delayed_insert_limit | numeric | GLOBAL |
delayed_insert_timeout | numeric | GLOBAL |
delayed_queue_size | numeric | GLOBAL |
div_precision_increment | numeric | GLOBAL | SESSION |
engine_condition_pushdown | boolean | GLOBAL | SESSION |
error_count | numeric | SESSION |
expire_logs_days | numeric | GLOBAL |
flush | boolean | GLOBAL |
flush_time | numeric | GLOBAL |
foreign_key_checks | boolean | SESSION |
ft_boolean_syntax | string | GLOBAL |
group_concat_max_len | numeric | GLOBAL | SESSION |
identity | numeric | SESSION |
innodb_autoextend_increment | numeric | GLOBAL |
innodb_commit_concurrency | numeric | GLOBAL |
innodb_concurrency_tickets | numeric | GLOBAL |
innodb_max_dirty_pages_pct | numeric | GLOBAL |
innodb_max_purge_lag | numeric | GLOBAL |
innodb_support_xa | boolean | GLOBAL | SESSION |
innodb_sync_spin_loops | numeric | GLOBAL |
innodb_table_locks | boolean | GLOBAL | SESSION |
innodb_thread_concurrency | numeric | GLOBAL |
innodb_thread_sleep_delay | numeric | GLOBAL |
insert_id | numeric | SESSION |
interactive_timeout | numeric | GLOBAL | SESSION |
join_buffer_size | numeric | GLOBAL | SESSION |
key_buffer_size | numeric | GLOBAL |
last_insert_id | numeric | SESSION |
lc_time_names | string | GLOBAL | SESSION |
local_infile | boolean | GLOBAL |
log_queries_not_using_indexes | boolean | GLOBAL |
log_warnings | numeric | GLOBAL |
long_query_time | numeric | GLOBAL | SESSION |
low_priority_updates | boolean | GLOBAL | SESSION |
max_allowed_packet | numeric | GLOBAL | SESSION |
max_binlog_cache_size | numeric | GLOBAL |
max_binlog_size | numeric | GLOBAL |
max_connect_errors | numeric | GLOBAL |
max_connections | numeric | GLOBAL |
max_delayed_threads | numeric | GLOBAL |
max_error_count | numeric | GLOBAL | SESSION |
max_heap_table_size | numeric | GLOBAL | SESSION |
max_insert_delayed_threads | numeric | GLOBAL |
max_join_size | numeric | GLOBAL | SESSION |
max_prepared_stmt_count | numeric | GLOBAL |
max_relay_log_size | numeric | GLOBAL |
max_seeks_for_key | numeric | GLOBAL | SESSION |
max_sort_length | numeric | GLOBAL | SESSION |
max_tmp_tables | numeric | GLOBAL | SESSION |
max_user_connections | numeric | GLOBAL |
max_write_lock_count | numeric | GLOBAL |
multi_range_count | numeric | GLOBAL | SESSION |
myisam_data_pointer_size | numeric | GLOBAL |
log_bin_trust_function_creators | boolean | GLOBAL |
myisam_max_sort_file_size | numeric | GLOBAL | SESSION |
myisam_repair_threads | numeric | GLOBAL | SESSION |
myisam_sort_buffer_size | numeric | GLOBAL | SESSION |
myisam_stats_method | enum | GLOBAL | SESSION |
net_buffer_length | numeric | GLOBAL | SESSION |
net_read_timeout | numeric | GLOBAL | SESSION |
net_retry_count | numeric | GLOBAL | SESSION |
net_write_timeout | numeric | GLOBAL | SESSION |
old_passwords | numeric | GLOBAL | SESSION |
optimizer_prune_level | numeric | GLOBAL | SESSION |
optimizer_search_depth | numeric | GLOBAL | SESSION |
preload_buffer_size | numeric | GLOBAL | SESSION |
profiling | boolean | SESSION |
profiling_history_size | numeric | SESSION |
query_alloc_block_size | numeric | GLOBAL | SESSION |
query_cache_limit | numeric | GLOBAL |
query_cache_size | numeric | GLOBAL |
query_cache_type | enumeration | GLOBAL | SESSION |
query_cache_wlock_invalidate | boolean | GLOBAL | SESSION |
query_prealloc_size | numeric | GLOBAL | SESSION |
range_alloc_block_size | numeric | GLOBAL | SESSION |
read_buffer_size | numeric | GLOBAL | SESSION |
read_only | numeric | GLOBAL |
read_rnd_buffer_size | numeric | GLOBAL | SESSION |
rpl_recovery_rank | numeric | GLOBAL |
safe_show_database | boolean | GLOBAL |
secure_auth | boolean | GLOBAL |
server_id | numeric | GLOBAL |
slave_compressed_protocol | boolean | GLOBAL |
slave_net_timeout | numeric | GLOBAL |
slave_transaction_retries | numeric | GLOBAL |
slow_launch_time | numeric | GLOBAL |
sort_buffer_size | numeric | GLOBAL | SESSION |
sql_auto_is_null | boolean | SESSION |
sql_big_selects | boolean | SESSION |
sql_big_tables | boolean | SESSION |
sql_buffer_result | boolean | SESSION |
sql_log_bin | boolean | SESSION |
sql_log_off | boolean | SESSION |
sql_log_update | boolean | SESSION |
sql_low_priority_updates | boolean | GLOBAL | SESSION |
sql_max_join_size | numeric | GLOBAL | SESSION |
sql_mode | enumeration | GLOBAL | SESSION |
sql_notes | boolean | SESSION |
sql_quote_show_create | boolean | SESSION |
sql_safe_updates | boolean | SESSION |
sql_select_limit | numeric | SESSION |
sql_slave_skip_counter | numeric | GLOBAL |
updatable_views_with_limit | enumeration | GLOBAL | SESSION |
sql_warnings | boolean | SESSION |
sync_binlog | numeric | GLOBAL |
sync_frm | boolean | GLOBAL |
storage_engine | enumeration | GLOBAL | SESSION |
table_cache | numeric | GLOBAL |
table_type | enumeration | GLOBAL | SESSION |
thread_cache_size | numeric | GLOBAL |
time_zone | string | GLOBAL | SESSION |
timestamp | boolean | SESSION |
tmp_table_size | enumeration | GLOBAL | SESSION |
transaction_alloc_block_size | numeric | GLOBAL | SESSION |
transaction_prealloc_size | numeric | GLOBAL | SESSION |
tx_isolation | enumeration | GLOBAL | SESSION |
unique_checks | boolean | SESSION |
wait_timeout | numeric | GLOBAL | SESSION |
warning_count | numeric | SESSION |
MySQL Enterprise Improper configuration of system variables can adversely affect performance and security. The MySQL Network Monitoring and Advisory Service continually monitors system variables and provides expert advice about appropriate settings. For more information see http://www.mysql.com/products/enterprise/advisors.html.
The server maintains many status variables that provide
information about its operation. You can view these variables
and their values by using the SHOW [GLOBAL]
STATUS statement. The optional
GLOBAL keyword aggregates the values over
all connections.
mysql> SHOW GLOBAL STATUS;
+-----------------------------------+------------+
| Variable_name | Value |
+-----------------------------------+------------+
| Aborted_clients | 0 |
| Aborted_connects | 0 |
| Bytes_received | 155372598 |
| Bytes_sent | 1176560426 |
...
| Connections | 30023 |
| Created_tmp_disk_tables | 0 |
| Created_tmp_files | 3 |
| Created_tmp_tables | 2 |
...
| Threads_created | 217 |
| Threads_running | 88 |
| Uptime | 1389872 |
+-----------------------------------+------------+
Note: Before MySQL 5.0.2,
SHOW STATUS returned global status values.
Because the default as of 5.0.2 is to return session values,
this is incompatible with previous versions. To issue a
SHOW STATUS statement that will retrieve
global status values for all versions of MySQL, write it like
this:
SHOW /*!50002 GLOBAL */ STATUS;
Many status variables are reset to 0 by the FLUSH
STATUS statement.
The status variables have the following meanings. Variables with no version indicated were already present prior to MySQL 5.0. For information regarding their implementation history, see MySQL 3.23, 4.0, 4.1 Reference Manual.
The number of connections that were aborted because the client died without closing the connection properly. See Section B.1.2.10, “Communication Errors and Aborted Connections”.
The number of failed attempts to connect to the MySQL server. See Section B.1.2.10, “Communication Errors and Aborted Connections”.
The number of transactions that used the temporary binary
log cache but that exceeded the value of
binlog_cache_size and used a temporary
file to store statements from the transaction.
The number of transactions that used the temporary binary log cache.
The number of bytes received from all clients.
The number of bytes sent to all clients.
The Com_
statement counter variables indicate the number of times
each xxxxxx statement has been
executed. There is one status variable for each type of
statement. For example, Com_delete and
Com_insert count
DELETE and INSERT
statements, respectively. However, if a query result is
returned from query cache, the server increments the
Qcache_hits status variable, not
Com_select. See
Section 5.13.4, “Query Cache Status and Maintenance”.
All of the
Com_stmt_
variables are increased even if a prepared statement
argument is unknown or an error occurred during execution.
In other words, their values correspond to the number of
requests issued, not to the number of requests
successfully completed.
xxx
The
Com_stmt_
status variables were added in 5.0.8:
xxx
Com_stmt_prepare
Com_stmt_execute
Com_stmt_fetch
Com_stmt_send_long_data
Com_stmt_reset
Com_stmt_close
Those variables stand for prepared statement commands.
Their names refer to the
COM_
command set used in the network layer. In other words,
their values increase whenever prepared statement API
calls such as mysql_stmt_prepare(),
mysql_stmt_execute(), and so forth are
executed. However, xxxCom_stmt_prepare,
Com_stmt_execute and
Com_stmt_close also increase for
PREPARE, EXECUTE, or
DEALLOCATE PREPARE, respectively.
Additionally, the values of the older (available since
MySQL 4.1.3) statement counter variables
Com_prepare_sql,
Com_execute_sql, and
Com_dealloc_sql increase for the
PREPARE, EXECUTE,
and DEALLOCATE PREPARE statements.
Com_stmt_fetch stands for the total
number of network round-trips issued when fetching from
cursors.
Whether the client connection uses compression in the client/server protocol. Added in MySQL 5.0.16.
The number of connection attempts (successful or not) to the MySQL server.
The number of temporary tables on disk created automatically by the server while executing statements.
How many temporary files mysqld has created.
The number of in-memory temporary tables created
automatically by the server while executing statements. If
Created_tmp_disk_tables is large, you
may want to increase the tmp_table_size
value to cause temporary tables to be memory-based instead
of disk-based.
The number of rows written with INSERT
DELAYED for which some error occurred (probably
duplicate key).
The number of INSERT DELAYED handler
threads in use.
The number of INSERT DELAYED rows
written.
The number of executed FLUSH
statements.
The number of internal COMMIT
statements.
The number of times that rows have been deleted from tables.
The MySQL server can ask the NDB
Cluster storage engine if it knows about a table
with a given name. This is called discovery.
Handler_discover indicates the number
of times that tables have been discovered via this
mechanism.
A counter for the prepare phase of two-phase commit operations. Added in MySQL 5.0.3.
The number of times the first entry was read from an
index. If this value 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.
The number of requests to read a row based on a key. If this value is high, it is a good indication that your tables are properly indexed for your queries.
The number of requests to read the next row in key order. This value is incremented if you are querying an index column with a range constraint or if you are doing an index scan.
The number of requests to read the previous row in key
order. This read method is mainly used to optimize
ORDER BY ... DESC.
The number of requests to read a row based on a fixed position. This value is high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan entire tables or you have joins that don't use keys properly.
The number of requests to read the next row in the data file. This value is 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.
The number of requests for a storage engine to perform a rollback operation.
The number of requests for a storage engine to place a savepoint. Added in MySQL 5.0.3.
The number of requests for a storage engine to roll back to a savepoint. Added in MySQL 5.0.3.
The number of requests to update a row in a table.
The number of requests to insert a row in a table.
The number of pages containing data (dirty or clean). Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_dirty
The number of pages currently dirty. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_flushed
The number of buffer pool page-flush requests. Added in MySQL 5.0.2.
The number of free pages. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_latched
The number of latched pages in InnoDB
buffer pool. These are pages currently being read or
written or that cannot be flushed or removed for some
other reason. Added in MySQL 5.0.2.
The number of pages that are busy because they have been
allocated for administrative overhead such as row locks or
the adaptive hash index. This value can also be calculated
as Innodb_buffer_pool_pages_total
– Innodb_buffer_pool_pages_free
– Innodb_buffer_pool_pages_data.
Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_total
The total size of buffer pool, in pages. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_ahead_rnd
The number of “random” read-aheads initiated
by InnoDB. This happens when a query
scans a large portion of a table but in random order.
Added in MySQL 5.0.2.
Innodb_buffer_pool_read_ahead_seq
The number of sequential read-aheads initiated by
InnoDB. This happens when
InnoDB does a sequential full table
scan. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_requests
The number of logical read requests
InnoDB has done. Added in MySQL 5.0.2.
The number of logical reads that InnoDB
could not satisfy from the buffer pool and had to do a
single-page read. Added in MySQL 5.0.2.
Normally, writes to the InnoDB buffer
pool happen in the background. However, if it is necessary
to read or create a page and no clean pages are available,
it is also necessary to wait for pages to be flushed
first. This counter counts instances of these waits. If
the buffer pool size has been set properly, this value
should be small. Added in MySQL 5.0.2.
Innodb_buffer_pool_write_requests
The number writes done to the InnoDB
buffer pool. Added in MySQL 5.0.2.
The number of fsync() operations so
far. Added in MySQL 5.0.2.
The current number of pending fsync()
operations. Added in MySQL 5.0.2.
The current number of pending reads. Added in MySQL 5.0.2.
The current number of pending writes. Added in MySQL 5.0.2.
The amount of data read so far, in bytes. Added in MySQL 5.0.2.
The total number of data reads. Added in MySQL 5.0.2.
The total number of data writes. Added in MySQL 5.0.2.
The amount of data written so far, in bytes. Added in MySQL 5.0.2.
Innodb_dblwr_writes,
Innodb_dblwr_pages_written
The number of doublewrite operations that have been
performed and the number of pages that have been written
for this purpose. Added in MySQL 5.0.2. See
Section 14.2.14.1, “InnoDB Disk I/O”.
The number of times that the log buffer was too small and a wait was required for it to be flushed before continuing. Added in MySQL 5.0.2.
The number of log write requests. Added in MySQL 5.0.2.
The number of physical writes to the log file. Added in MySQL 5.0.2.
The number of fsync() writes done to
the log file. Added in MySQL 5.0.2.
The number of pending log file fsync()
operations. Added in MySQL 5.0.2.
The number of pending log file writes. Added in MySQL 5.0.2.
The number of bytes written to the log file. Added in MySQL 5.0.2.
The compiled-in InnoDB page size
(default 16KB). Many values are counted in pages; the page
size allows them to be easily converted to bytes. Added in
MySQL 5.0.2.
The number of pages created. Added in MySQL 5.0.2.
The number of pages read. Added in MySQL 5.0.2.
The number of pages written. Added in MySQL 5.0.2.
The number of row locks currently being waited for. Added in MySQL 5.0.3.
The total time spent in acquiring row locks, in milliseconds. Added in MySQL 5.0.3.
The average time to acquire a row lock, in milliseconds. Added in MySQL 5.0.3.
The maximum time to acquire a row lock, in milliseconds. Added in MySQL 5.0.3.
The number of times a row lock had to be waited for. Added in MySQL 5.0.3.
The number of rows deleted from InnoDB
tables. Added in MySQL 5.0.2.
The number of rows inserted into InnoDB
tables. Added in MySQL 5.0.2.
The number of rows read from InnoDB
tables. Added in MySQL 5.0.2.
The number of rows updated in InnoDB
tables. Added in MySQL 5.0.2.
The number of key blocks in the key cache that have changed but have not yet been flushed to disk.
The number of unused blocks in the key cache. You can use
this value to determine how much of the key cache is in
use; see the discussion of
key_buffer_size in
Section 5.2.3, “System Variables”.
The number of used blocks in the key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time.
The number of requests to read a key block from the cache.
The number of physical reads of a key block from disk. If
Key_reads is large, then your
key_buffer_size value is probably too
small. The cache miss rate can be calculated as
Key_reads/Key_read_requests.
The number of requests to write a key block to the cache.
The number of physical writes of a key block to disk.
The total cost of the last compiled query as computed by
the query optimizer. This is useful for comparing the cost
of different query plans for the same query. The default
value of 0 means that no query has been compiled yet. This
variable was added in MySQL 5.0.1, with a default value of
-1. In MySQL 5.0.7, the default was changed to 0; also in
version 5.0.7, the scope of
Last_query_cost was changed to session
rather than global.
Prior to MySQL 5.0.16, this variable was not updated for queries served from the query cache.
The maximum number of connections that have been in use simultaneously since the server started.
If the server is acting as a MySQL Cluster node, then the value of this variable its node ID in the cluster.
If the server is not part of a MySQL Cluster, then the value of this variable is 0.
If the server is part of a MySQL Cluster, the value of this variable is the hostname or IP address of the Cluster management server from which it gets its configuration data.
If the server is not part of a MySQL Cluster, then the value of this variable is an empty string.
Prior to MySQL 5.0.23, this variable was named
Ndb_connected_host.
If the server is part of a MySQL Cluster, the value of this variable is the number of the port through which it is connected to the Cluster management server from which it gets its configuration data.
If the server is not part of a MySQL Cluster, then the value of this variable is 0.
Prior to MySQL 5.0.23, this variable was named
Ndb_connected_port.
If the server is part of a MySQL Cluster, the value of this variable is the number of data nodes in the cluster.
If the server is not part of a MySQL Cluster, then the value of this variable is 0.
Prior to MySQL 5.0.29, this variable was named
Ndb_number_of_storage_nodes.
The number of rows waiting to be written in
INSERT DELAY queues.
The number of files that are open.
The number of streams that are open (used mainly for logging).
The number of tables that are open.
The number of tables that have been opened. If
Opened_tables is big, your
table_cache value is probably too
small.
The current number of prepared statements. (The maximum
number of statements is given by the
max_prepared_stmt_count system
variable.) This variable was added in MySQL 5.0.32.
The number of free memory blocks in the query cache.
The amount of free memory for the query cache.
The number of query cache hits.
The number of queries added to the query cache.
The number of queries that were deleted from the query cache because of low memory.
The number of non-cached queries (not cacheable, or not
cached due to the query_cache_type
setting).
The number of queries registered in the query cache.
The total number of blocks in the query cache.
The number of statements that clients have sent to the server.
The status of fail-safe replication (not yet implemented).
The number of joins that perform table scans because they do not use indexes. If this value is not 0, you should carefully check the indexes of your tables.
The number of joins that used a range search on a reference table.
The number of joins that used ranges on the first table. This is normally not a critical issue even if the value is quite large.
The number of joins without keys that check for key usage after each row. If this is not 0, you should carefully check the indexes of your tables.
The number of joins that did a full scan of the first table.
The number of temporary tables that the slave SQL thread currently has open.
This is ON if this server is a slave
that is connected to a master.
The total number of times since startup that the replication slave SQL thread has retried transactions. This variable was added in version 5.0.4.
The number of threads that have taken more than
slow_launch_time seconds to create.
The number of queries that have taken more than
long_query_time seconds. See
Section 5.11.4, “The Slow Query Log”.
The number of merge passes that the sort algorithm has had
to do. If this value is large, you should consider
increasing the value of the
sort_buffer_size system variable.
The number of sorts that were done using ranges.
The number of sorted rows.
The number of sorts that were done by scanning the table.
Ssl_
xxx
Variables used for SSL connections.
The number of times that a table lock was acquired immediately.
The number of times that 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 or tables or use replication.
For the memory-mapped implementation of the log that is used by mysqld when it acts as the