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 at runtime using the
SET
statement, which enables you to modify operation of the server
without having to stop and restart it. Setting the global value of
a system variable requires the
SUPER privilege. For some system
variables, setting the session value also requires the
SUPER privilege; if so, it is
indicated in the variable description. You can also use 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 includes a table that lists all system variables and following the table provides a description of each one. Variables with no version indicated are present in all MySQL 5.5 releases. For more information about manipulation of system variables, see Section 5.1.6, “Using System Variables”.
Table 5.2 System Variable Summary
For additional system variable information, see these sections:
Section 5.1.6, “Using System Variables”, discusses the syntax for setting and displaying system variable values.
Section 5.1.6.2, “Dynamic System Variables”, lists the variables that can be set at runtime.
Information on tuning system variables can be found in Section 5.1.1, “Configuring the Server”.
Section 14.17, “InnoDB Startup Options and System Variables”, lists
InnoDB system variables.
Section 18.3.3.8.2, “NDB Cluster System Variables”, lists system variables which are specific to MySQL Cluster.
For information on server system variables specific to replication, see Section 17.1.3, “Replication and Binary Logging Options and Variables”.
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, before
MySQL 5.5.10, 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. As of MySQL 5.5.10, boolean variables can be set at
startup to the values ON,
TRUE, OFF, and
FALSE (not case sensitive). See
Section 4.2.5, “Program Option Modifiers”.
Some system variables control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to a system variable that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible that the server will adjust a value upward. For example, if you assign a value of 0 to a variable for which the minimal value is 1024, the server will set the value to 1024.
Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.
Some system variables take file name values. Unless otherwise
specified, the default file location is the data directory if the
value is a relative path name. To specify the location explicitly,
use an absolute path name. Suppose that the data directory is
/var/mysql/data. If a file-valued variable is
given as a relative path name, it will be located under
/var/mysql/data. If the value is an absolute
path name, its location is as given by the path name.
authentication_windows_log_level
| Introduced | 5.5.16 | ||
| Command-Line Format | --authentication_windows_log_level | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 4 | ||
This variable is available only if the
authentication_windows Windows
authentication plugin is enabled and debugging code is
enabled. See Section 6.5.1.4, “The Windows Native Authentication Plugin”.
This variable sets the logging level for the Windows authentication plugin. The following table shows the permitted values.
| Value | Description |
|---|---|
| 0 | No logging |
| 1 | Log only error messages |
| 2 | Log level 1 messages and warning messages |
| 3 | Log level 2 messages and information notes |
| 4 | Log level 3 messages and debug messages |
This variable was added in MySQL 5.5.16.
authentication_windows_use_principal_name
| Introduced | 5.5.16 | ||
| Command-Line Format | --authentication_windows_use_principal_name | ||
| Permitted Values | Type | boolean | |
| Default | ON | ||
This variable is available only if the
authentication_windows Windows
authentication plugin is enabled. See
Section 6.5.1.4, “The Windows Native Authentication Plugin”.
A client that authenticates using the
InitSecurityContext() function should
provide a string identifying the service to which it connects
(targetName). MySQL uses the
principal name (UPN) of the account under which the server is
running. The UPN has the form
user_id@computer_name
This variable controls whether the server sends the UPN in the
initial challenge. By default, the variable is enabled. For
security reasons, it can be disabled to avoid sending the
server's account name to a client in clear text. If the
variable is disabled, the server always sends a
0x00 byte in the first challenge, the
client does not specify targetName,
and as a result, NTLM authentication is used.
If the server fails to obtain its UPN (which will happen primarily in environments that do not support Kerberos authentication), the UPN is not sent by the server and NTLM authentication is used.
This variable was added in MySQL 5.5.16.
| Command-Line Format | --autocommit[=#] (>= 5.5.8) | ||
| System Variable (<= 5.5.2) | Name | autocommit | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | autocommit | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | ON | ||
The autocommit mode. If set to 1, all changes to a table take
effect immediately. If set to 0, you must use
COMMIT to accept a transaction
or ROLLBACK
to cancel it. If autocommit
is 0 and you change it to 1, MySQL performs an automatic
COMMIT of any open transaction.
Another way to begin a transaction is to use a
START
TRANSACTION or
BEGIN
statement. See Section 13.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.
By default, client connections begin with
autocommit set to 1. To cause
clients to begin with a default of 0, set the global
autocommit value by starting
the server with the
--autocommit=0 option. To set
the variable using an option file, include these lines:
[mysqld] autocommit=0
Before MySQL 5.5.8, the global
autocommit value cannot be
set at startup. As a workaround, set the
init_connect system variable:
SET GLOBAL init_connect='SET autocommit=0';
The init_connect 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'
The content of init_connect
is not executed for users that have the
SUPER privilege (unlike the
effect of setting the global
autocommit value at startup).
| System Variable | Name | automatic_sp_privileges | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | TRUE | ||
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 privilege is
required to drop the routine.) The server also automatically
drops those privileges from the creator when the routine is
dropped. If
automatic_sp_privileges is 0,
the server does not automatically add or drop these
privileges.
The creator of a routine is the account used to execute the
CREATE statement for it. This might not be
the same as the account named as the
DEFINER in the routine definition.
See also Section 20.2.2, “Stored Routines and MySQL Privileges”.
| System Variable | Name | back_log | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 50 | ||
| Min Value | 1 | ||
| Max Value | 65535 | ||
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.
| Command-Line Format | --basedir=dir_name | ||
| System Variable | Name | basedir | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | directory name | |
The MySQL installation base directory. This variable can be
set with the --basedir option.
Relative path names for other variables usually are resolved
relative to the base directory.
| Command-Line Format | --big-tables | ||
| System Variable (<= 5.5.2) | Name | big_tables | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | big_tables | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
If set to 1, all temporary tables are stored on disk rather
than in memory. This is a little slower, but the error
The table does not occur for
tbl_name is
fullSELECT operations that require
a large temporary table. The default value for a new
connection is 0 (use in-memory temporary tables). Normally,
you should never need to set this variable, because in-memory
tables are automatically converted to disk-based tables as
required.
This variable was formerly named
sql_big_tables.
| Command-Line Format | --bulk_insert_buffer_size=# | ||
| System Variable | Name | bulk_insert_buffer_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 0 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709551615 | ||
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 nonempty 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.
| System Variable | Name | character_set_client | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
The character set for statements that arrive from the client.
The session value of this variable is set using the character
set requested by the client when the client connects to the
server. (Many clients support a
--default-character-set option to enable this
character set to be specified explicitly. See also
Section 10.1.4, “Connection Character Sets and Collations”.) The global value of the
variable is used to set the session value in cases when the
client-requested value is unknown or not available, or the
server is configured to ignore client requests:
The client is from a version of MySQL older than MySQL 4.1, and thus does not request a character set.
The client requests a character set not known to the
server. For example, a Japanese-enabled client requests
sjis when connecting to a server not
configured with sjis support.
mysqld was started with the
--skip-character-set-client-handshake
option, which causes it to ignore client character set
configuration. This reproduces MySQL 4.0 behavior and is
useful should you wish to upgrade the server without
upgrading all the clients.
ucs2, utf16, and
utf32 cannot be used as a client character
set, which means that they also do not work for
SET NAMES or
SET CHARACTER SET.
| System Variable | Name | character_set_connection | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
The character set used for literals that do not have a character set introducer and for number-to-string conversion. For information about introducers, see Section 10.1.3.8, “Character Set Introducers”.
| System Variable | Name | character_set_database | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Footnote | This option is dynamic, but only the server should set this information. You should not set the value of this variable manually. | ||
| Permitted Values | Type | string | |
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.
| Command-Line Format | --character-set-filesystem=name | ||
| System Variable | Name | character_set_filesystem | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
| Default | binary | ||
The file system character set. This variable is used to
interpret string literals that refer to file names, such as in
the LOAD DATA
INFILE and
SELECT ... INTO
OUTFILE statements and the
LOAD_FILE() function. Such file
names 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 multibyte file names are
permitted, a different value may be more appropriate. For
example, if the system represents file names using UTF-8, set
character_set_filesystem to
'utf8'.
| System Variable | Name | character_set_results | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
The character set used for returning query results such as result sets or error messages to the client.
| Command-Line Format | --character-set-server | ||
| System Variable | Name | character_set_server | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
| Default | latin1 | ||
The server's default character set.
| System Variable | Name | character_set_system | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
| Default | utf8 | ||
The character set used by the server for storing identifiers.
The value is always utf8.
| Command-Line Format | --character-sets-dir=dir_name | ||
| System Variable | Name | character_sets_dir | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | directory name | |
The directory where character sets are installed.
| System Variable | Name | collation_connection | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
The collation of the connection character set.
| System Variable | Name | collation_database | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Footnote | This option is dynamic, but only the server should set this information. You should not set the value of this variable manually. | ||
| Permitted Values | Type | string | |
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.
| Command-Line Format | --collation-server | ||
| System Variable | Name | collation_server | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
| Default | latin1_swedish_ci | ||
The server's default collation.
| Command-Line Format | --completion_type=# | ||
| System Variable | Name | completion_type | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (<= 5.5.2) | Type | integer | |
| Default | 0 | ||
| Valid Values | 0 | ||
1 | |||
2 | |||
| Permitted Values (>= 5.5.3) | Type | enumeration | |
| Default | NO_CHAIN | ||
| Valid Values | NO_CHAIN | ||
CHAIN | |||
RELEASE | |||
0 | |||
1 | |||
2 | |||
The transaction completion type. This variable can take the values shown in the following table. As of MySQL 5.5.3, the variable can be assigned using either the name values or corresponding integer values. Before 5.5.3, only the integer values can be used.
| Value | Description |
|---|---|
NO_CHAIN (or 0) | COMMIT and
ROLLBACK
are unaffected. This is the default value. |
CHAIN (or 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.) |
RELEASE (or 2) | COMMIT and
ROLLBACK
are equivalent to COMMIT RELEASE
and ROLLBACK RELEASE, respectively.
(The server disconnects after terminating the
transaction.) |
completion_type affects
transactions that begin with
START
TRANSACTION or
BEGIN and
end with COMMIT or
ROLLBACK. It
does not apply to implicit commits resulting from execution of
the statements listed in Section 13.3.3, “Statements That Cause an Implicit Commit”. It
also does not apply for
XA
COMMIT,
XA
ROLLBACK, or when
autocommit=1.
| Command-Line Format | --concurrent_insert[=#] | ||
| System Variable | Name | concurrent_insert | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (<= 5.5.2) | Type | integer | |
| Default | 1 | ||
| Valid Values | 0 | ||
1 | |||
2 | |||
| Permitted Values (>= 5.5.3) | Type | enumeration | |
| Default | AUTO | ||
| Valid Values | NEVER | ||
AUTO | |||
ALWAYS | |||
0 | |||
1 | |||
2 | |||
If AUTO (the default), MySQL permits
INSERT and
SELECT statements to run
concurrently for MyISAM tables that have no
free blocks in the middle of the data file. If you start
mysqld with
--skip-new,
this variable is set to NEVER.
This variable can take the values shown in the following table. As of MySQL 5.5.3, the variable can be assigned using either the name values or corresponding integer values. Before 5.5.3, only the integer values can be used.
| Value | Description |
|---|---|
NEVER (or 0) | Disables concurrent inserts |
AUTO (or 1) | (Default) Enables concurrent insert for MyISAM tables
that do not have holes |
ALWAYS (or 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 8.11.3, “Concurrent Inserts”.
| Command-Line Format | --connect_timeout=# | ||
| System Variable | Name | connect_timeout | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 10 | ||
| Min Value | 2 | ||
| Max Value | 31536000 | ||
The number of seconds that the mysqld
server waits for a connect packet before responding with
Bad handshake. The default value is 10
seconds.
Increasing the
connect_timeout value might
help if clients frequently encounter errors of the form
Lost connection to MySQL server at
'.
XXX', system error:
errno
| Command-Line Format | --datadir=dir_name | ||
| System Variable | Name | datadir | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | directory name | |
The MySQL data directory. This variable can be set with the
--datadir option.
This variable is unused.
This variable is unused.
| Command-Line Format | --debug[=debug_options] | ||
| System Variable | Name | debug | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (Unix) | Type | string | |
| Default | d:t:i:o,/tmp/mysqld.trace | ||
| Permitted Values (Windows) | Type | string | |
| Default | d:t:i:O,\mysqld.trace | ||
This variable indicates the current debugging settings. It is
available only for servers built with debugging support. The
initial value comes from the value of instances of the
--debug option given at server
startup. The global and session values may be set at runtime;
the SUPER privilege is
required, even for the session value.
Assigning a value that begins with + or
- cause the value to added to or subtracted
from the current value:
mysql>SET debug = 'T';mysql>SELECT @@debug;+---------+ | @@debug | +---------+ | T | +---------+ mysql>SET debug = '+P';mysql>SELECT @@debug;+---------+ | @@debug | +---------+ | P:T | +---------+ mysql>SET debug = '-P';mysql>SELECT @@debug;+---------+ | @@debug | +---------+ | T | +---------+
For more information, see Section 24.5.3, “The DBUG Package”.
| System Variable (<= 5.5.2) | Name | debug_sync | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | debug_sync | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
This variable is the user interface to the Debug Sync
facility. Use of Debug Sync requires that MySQL be configured
with the -DENABLE_DEBUG_SYNC=1
CMake option (see
Section 2.9.4, “MySQL Source-Configuration Options”). If Debug Sync
is not compiled in, this system variable is not available.
The global variable value is read only and indicates whether
the facility is enabled. By default, Debug Sync is disabled
and the value of debug_sync
is OFF. If the server is started with
--debug-sync-timeout=,
where NN is a timeout value greater
than 0, Debug Sync is enabled and the value of
debug_sync is ON -
current signal followed by the signal name. Also,
N becomes the default timeout for
individual synchronization points.
The session value can be read by any user and will have the
same value as the global variable. The session value can be
set by users that have the
SUPER privilege to control
synchronization points.
For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.
| Command-Line Format | --default-storage-engine=name | ||
| System Variable (<= 5.5.2) | Name | storage_engine | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | default_storage_engine | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (<= 5.5.4) | Type | enumeration | |
| Default | MyISAM | ||
| Permitted Values (>= 5.5.5) | Type | enumeration | |
| Default | InnoDB | ||
The default storage engine. To set the storage engine at
server startup, use the
--default-storage-engine
option. See Section 5.1.4, “Server Command Options”.
To see which storage engines are available and enabled, use
the SHOW ENGINES statement or
query the INFORMATION_SCHEMA
ENGINES table.
If you disable the default storage engine at server startup, you must set the default engine to a different engine or the server will not start.
This variable was added in MySQL 5.5.3 to be used in
preference to storage_engine,
which is now deprecated.
| Command-Line Format | --default_week_format=# | ||
| System Variable | Name | default_week_format | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 7 | ||
The default mode value to use for the
WEEK() function. See
Section 12.7, “Date and Time Functions”.
| Command-Line Format | --delay-key-write[=name] | ||
| System Variable | Name | delay_key_write | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | enumeration | |
| Default | ON | ||
| Valid Values | ON | ||
OFF | |||
ALL | |||
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-options option
(for example,
--myisam-recover-options=BACKUP,FORCE).
See Section 5.1.4, “Server Command Options”, and
Section 15.3.1, “MyISAM Startup Options”.
If you enable external locking with
--external-locking, there is
no protection against index corruption for tables that use
delayed key writes.
| Command-Line Format | --delayed_insert_limit=# | ||
| System Variable | Name | delayed_insert_limit | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 100 | ||
| Min Value | 1 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 100 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 100 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709551615 | ||
After inserting
delayed_insert_limit delayed
rows, the INSERT DELAYED
handler thread checks whether there are any
SELECT statements pending. If
so, it permits them to execute before continuing to insert
delayed rows.
| Command-Line Format | --delayed_insert_timeout=# | ||
| System Variable | Name | delayed_insert_timeout | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 300 | ||
How many seconds an INSERT
DELAYED handler thread should wait for
INSERT statements before
terminating.
| Command-Line Format | --delayed_queue_size=# | ||
| System Variable | Name | delayed_queue_size | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 1000 | ||
| Min Value | 1 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 1000 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 1000 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709551615 | ||
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.
| Command-Line Format | --div_precision_increment=# | ||
| System Variable | Name | div_precision_increment | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 4 | ||
| Min Value | 0 | ||
| Max Value | 30 | ||
This variable indicates the number of digits by which to
increase the scale of 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 | +----------------+
| Deprecated | 5.5.3, by optimizer_switch | ||
| Command-Line Format | --engine-condition-pushdown | ||
| System Variable | Name | engine_condition_pushdown | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | ON | ||
The engine condition pushdown optimization enables processing for certain comparisons to be “pushed down” to the storage engine level for more efficient execution. For more information, see Section 8.2.1.5, “Engine Condition Pushdown Optimization”.
Engine condition pushdown is used only by the
NDBCLUSTER storage engine.
Enabling this optimization on a MySQL Server acting as a MySQL
Cluster SQL node causes WHERE conditions on
unindexed columns to be evaluated on the cluster's data nodes
and only the rows that match to be sent back to the SQL node
that issued the query. This greatly reduces the amount of
cluster data that must be sent over the network, increasing
the efficiency with which results are returned.
The engine_condition_pushdown
variable controls whether engine condition pushdown is
enabled. By default, this variable is ON
(1). Setting it to OFF (0) disables
pushdown.
This variable is deprecated as of MySQL 5.5.3 and is removed
in MySQL 5.6. Use the
engine_condition_pushdown flag of the
optimizer_switch variable
instead. See Section 8.9.2, “Controlling Switchable Optimizations”.
The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.18, “SHOW ERRORS Syntax”.
| Command-Line Format | --event-scheduler[=value] | ||
| System Variable | Name | event_scheduler | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | enumeration | |
| Default | OFF | ||
| Valid Values | ON | ||
OFF | |||
DISABLED | |||
This variable indicates the status of the Event Scheduler;
possible values are ON,
OFF, and DISABLED, with
the default being OFF. This variable and
its effects on the Event Scheduler's operation are discussed
in greater detail in the
Overview section
of the Events chapter.
| Command-Line Format | --expire_logs_days=# | ||
| System Variable | Name | expire_logs_days | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 99 | ||
The number of days for automatic binary log file removal. The default is 0, which means “no automatic removal.” Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.4, “MySQL Server Logs”.
To remove binary log files manually, use the
PURGE BINARY LOGS statement.
See Section 13.4.1.1, “PURGE BINARY LOGS Syntax”.
| Introduced | 5.5.7 | ||
| System Variable | Name | external_user | |
| Variable Scope | Session | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The external user name used during the authentication process,
as set by the plugin used to authenticate the client. With
native (built-in) MySQL authentication, or if the plugin does
not set the value, this variable is NULL.
See Section 6.3.7, “Proxy Users”.
This variable was added in MySQL 5.5.7.
| Command-Line Format | --flush | ||
| System Variable | Name | flush | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
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.5.3.3, “What to Do If MySQL Keeps Crashing”. This
variable is set to ON if you start
mysqld with the
--flush option.
| Command-Line Format | --flush_time=# | ||
| System Variable | Name | flush_time | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Permitted Values (Windows) | Type | integer | |
| Default | 1800 | ||
| Min Value | 0 | ||
If this is set to a nonzero value, all tables are closed every
flush_time seconds to free up
resources and synchronize unflushed data to disk. This option
is best used only on systems with minimal resources.
If set to 1 (the default), foreign key constraints for
InnoDB tables are checked. If set to 0,
foreign key constraints are ignored, with a couple of
exceptions. When re-creating a table that was dropped, an
error is returned if the table definition does not conform to
the foreign key constraints referencing the table. Likewise,
an ALTER TABLE operation
returns an error if a foreign key definition is incorrectly
formed. For more information, see
Section 13.1.17.5, “Using FOREIGN KEY Constraints”.
Disabling foreign key checking can be useful for reloading
InnoDB tables in an order different from
that required by their parent/child relationships. See
Section 14.11.7, “InnoDB and FOREIGN KEY Constraints”.
Setting foreign_key_checks to
0 also affects data definition statements:
DROP
SCHEMA drops a schema even if it contains tables
that have foreign keys that are referred to by tables outside
the schema, and DROP TABLE
drops tables that have foreign keys that are referred to by
other tables.
Setting foreign_key_checks
to 1 does not trigger a scan of the existing table data.
Therefore, rows added to the table while
foreign_key_checks=0 will
not be verified for consistency.
With foreign_key_checks=0,
dropping an index required by a foreign key constraint
places the table in an inconsistent state and causes the
foreign key check that occurs at table load to fail. To
avoid this problem, remove the foreign key constraint before
dropping the index (Bug #70260).
| Command-Line Format | --ft_boolean_syntax=name | ||
| System Variable | Name | ft_boolean_syntax | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
| Default | + -><()~*:""&| | ||
The list of operators supported by boolean full-text searches
performed using IN BOOLEAN MODE. See
Section 12.9.2, “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 nonalphanumeric character.
Either the first or second character must be a space.
No duplicates are permitted 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.
| Command-Line Format | --ft_max_word_len=# | ||
| System Variable | Name | ft_max_word_len | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Min Value | 10 | ||
The maximum length of the word to be included in a
FULLTEXT index.
FULLTEXT indexes must be rebuilt after
changing this variable. Use REPAIR TABLE
.
tbl_name QUICK
| Command-Line Format | --ft_min_word_len=# | ||
| System Variable | Name | ft_min_word_len | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 4 | ||
| Min Value | 1 | ||
The minimum length of the word to be included in a
FULLTEXT index.
FULLTEXT indexes must be rebuilt after
changing this variable. Use REPAIR TABLE
.
tbl_name QUICK
| Command-Line Format | --ft_query_expansion_limit=# | ||
| System Variable | Name | ft_query_expansion_limit | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 20 | ||
| Min Value | 0 | ||
| Max Value | 1000 | ||
The number of top matches to use for full-text searches
performed using WITH QUERY EXPANSION.
| Command-Line Format | --ft_stopword_file=file_name | ||
| System Variable | Name | ft_stopword_file | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | file name | |
The file from which to read the list of stopwords for
full-text searches. The server looks for the file in the data
directory unless an absolute path name is given to specify a
different directory. 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
storage/myisam/ft_static.c file). Setting
this variable to the empty string ('')
disables stopword filtering. See also
Section 12.9.4, “Full-Text Stopwords”.
FULLTEXT indexes must be rebuilt after
changing this variable or the contents of the stopword file.
Use REPAIR TABLE
.
tbl_name QUICK
| Command-Line Format | --general-log | ||
| System Variable | Name | general_log | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
Whether the general query log is enabled. The value can be 0
(or OFF) to disable the log or 1 (or
ON) to enable the log. The default value
depends on whether the
--general_log option is given.
The destination for log output is controlled by the
log_output system variable;
if that value is NONE, no log entries are
written even if the log is enabled.
| Command-Line Format | --general-log-file=file_name | ||
| System Variable | Name | general_log_file | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | file name | |
| Default | host_name.log | ||
The name of the general query log file. The default value is
host_name.log--general_log_file option.
| Command-Line Format | --group_concat_max_len=# | ||
| System Variable | Name | group_concat_max_len | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 1024 | ||
| Min Value | 4 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 1024 | ||
| Min Value | 4 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 1024 | ||
| Min Value | 4 | ||
| Max Value | 18446744073709551615 | ||
The maximum permitted result length in bytes for the
GROUP_CONCAT() function. The
default is 1024.
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.
This variable is deprecated and is removed in MySQL 5.6. Use
SHOW ENGINES instead.
YES if mysqld supports
dynamic loading of plugins, NO if not. If
the value is NO, you cannot use options
such as --plugin-load to load
plugins at server startup, or the INSTALL
PLUGIN statement to load plugins at runtime.
YES if the server supports spatial data
types, NO if not.
YES if mysqld supports
InnoDB tables. DISABLED
if
--skip-innodb
is used.
This variable is deprecated and is removed in MySQL 5.6. Use
SHOW ENGINES instead.
This variable is an alias for
have_ssl.
YES if mysqld supports
partitioning.
This variable is deprecated and is removed in MySQL 5.6. Use
SHOW PLUGINS instead. For more
information, see Chapter 19, Partitioning.
YES if statement profiling capability is
present, NO if not. If present, the
profiling system variable controls whether
this capability is enabled or disabled. See
Section 13.7.5.32, “SHOW PROFILES Syntax”.
YES if mysqld supports
the query cache, NO if not.
YES if RTREE indexes are
available, NO if not. (These are used for
spatial indexes in MyISAM tables.)
YES if mysqld supports
SSL connections, NO if not.
DISABLED indicates that the server was
compiled with SSL support, but was not started with the
appropriate
--ssl- options.
For more information, see
Section 6.4.2, “Building MySQL with Support for Secure Connections”.
xxx
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. If the server
is started with the
--skip-symbolic-links
option, the value is DISABLED.
| System Variable | Name | hostname | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The server sets this variable to the server host name at startup.
This variable is a synonym for the
last_insert_id variable. It
exists for compatibility with other database systems. You can
read its value with SELECT @@identity, and
set it using SET identity.
| Command-Line Format | --init-connect=name | ||
| System Variable | Name | init_connect | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
A string to be executed by the server for each client that
connects. The string consists of one or more SQL statements,
separated by semicolon characters. For example, each client
session begins by default with autocommit mode enabled. For
older servers (before MySQL 5.5.8), there is no global
autocommit system variable to
specify that autocommit should be disabled by default, but as
a workaround init_connect can
be used to achieve the same effect:
SET GLOBAL init_connect='SET autocommit=0';
The init_connect 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'
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 server discards any result sets produced by statements in
the value of of init_connect.
| Command-Line Format | --init-file=file_name | ||
| System Variable | Name | init_file | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | file name | |
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. For more information, see the description of
--init-file.
innodb_
xxx
InnoDB system variables are
listed in Section 14.17, “InnoDB Startup Options and System Variables”. These variables
control many aspects of storage, memory use, and I/O patterns
for InnoDB tables, and are especially
important now that InnoDB is the default
storage engine.
The value to be used by the following
INSERT or
ALTER TABLE statement when
inserting an AUTO_INCREMENT value. This is
mainly used with the binary log.
| Command-Line Format | --interactive_timeout=# | ||
| System Variable | Name | interactive_timeout | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 28800 | ||
| Min Value | 1 | ||
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.
| Command-Line Format | --join_buffer_size=# | ||
| System Variable | Name | join_buffer_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (Windows, >= 5.5.3) | Type | integer | |
| Default | 131072 | ||
| Min Value | 128 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Windows, 32-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 131072 | ||
| Min Value | 8200 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Windows, 64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 131072 | ||
| Min Value | 8228 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Other, 32-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 131072 | ||
| Min Value | 8200 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Other, 32-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 131072 | ||
| Min Value | 128 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Other, 64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 131072 | ||
| Min Value | 8228 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (Other, 64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 131072 | ||
| Min Value | 128 | ||
| Max Value | 18446744073709547520 | ||
The minimum size of the buffer that is used for plain index
scans, range index scans, and 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.
There is no gain from setting the buffer larger than required to hold each matching row, and all joins allocate at least the minimum size, so use caution in setting this variable to a large value globally. It is better to keep the global setting small and change to a larger setting only in sessions that are doing large joins. Memory allocation time can cause substantial performance drops if the global size is larger than needed by most queries that use it.
The maximum permissible setting for
join_buffer_size is
4GB−1. Larger values are permitted for 64-bit platforms
(except 64-bit Windows, for which large values are truncated
to 4GB−1 with a warning).
Prior to MySQL 5.5.3, the minimum allowed value for this variable was 8200 for 32-bit platforms and 8228 for 64-bit platforms. In MySQL 5.5.3 and later, the minimum is 128 for all platforms.
For additional information about join buffering, see Section 8.2.1.8, “Nested-Loop Join Algorithms”.
| Command-Line Format | --keep_files_on_create=# | ||
| System Variable | Name | keep_files_on_create | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
If a MyISAM table is created with no
DATA DIRECTORY option, the
.MYD file is created in the database
directory. By default, if MyISAM finds an
existing .MYD file in this case, it
overwrites it. The same applies to .MYI
files for tables created with no INDEX
DIRECTORY option. To suppress this behavior, set the
keep_files_on_create variable
to ON (1), in which case
MyISAM will not overwrite existing files
and returns an error instead. The default value is
OFF (0).
If a MyISAM table is created with a
DATA DIRECTORY or INDEX
DIRECTORY option and an existing
.MYD or .MYI file is
found, MyISAM always returns an error. It will not overwrite a
file in the specified directory.
| Command-Line Format | --key_buffer_size=# | ||
| System Variable | Name | key_buffer_size | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 8 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 8 | ||
| Max Value | OS_PER_PROCESS_LIMIT | ||
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 permissible setting for
key_buffer_size is
4GB−1 on 32-bit platforms. Larger values are permitted
for 64-bit platforms. 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. The value of this variable indicates the amount of
memory requested. Internally, the server allocates as much
memory as possible up to this amount, but the actual
allocation might be less.
You can increase the value to get better index handling for
all reads and multiple writes; on a system whose primary
function is to run MySQL using the
MyISAM storage engine, 25% of the
machine's total memory is an acceptable value for this
variable. However, you should be aware that, if you make the
value too large (for example, more than 50% of the
machine's total memory), your system might start to page
and become extremely slow. This is because MySQL relies on the
operating system to perform file system caching for data
reads, so you must leave some room for the file system cache.
You should also consider the memory requirements of any other
storage engines that you may be using in addition to
MyISAM.
For even more speed when writing many rows at the same time,
use LOCK TABLES. See
Section 8.2.2.1, “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 13.7.5, “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 is allocated internally for administrative structures. Factors that influence the amount of overhead for these structures include block size and pointer size. As block size increases, the percentage of the key buffer lost to overhead tends to decrease. Larger blocks results in a smaller number of read operations (because more keys are obtained per read), but conversely an increase in reads of keys that are not examined (if not all keys in a block are relevant to a query).
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 8.10.2, “The MyISAM Key Cache”.
| Command-Line Format | --key_cache_age_threshold=# | ||
| System Variable | Name | key_cache_age_threshold | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 300 | ||
| Min Value | 100 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 300 | ||
| Min Value | 100 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 300 | ||
| Min Value | 100 | ||
| Max Value | 18446744073709551615 | ||
This value controls the demotion of buffers from the hot sublist of a key cache to the warm sublist. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. See Section 8.10.2, “The MyISAM Key Cache”.
| Command-Line Format | --key_cache_block_size=# | ||
| System Variable | Name | key_cache_block_size | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 1024 | ||
| Min Value | 512 | ||
| Max Value | 16384 | ||
The size in bytes of blocks in the key cache. The default value is 1024. See Section 8.10.2, “The MyISAM Key Cache”.
| Command-Line Format | --key_cache_division_limit=# | ||
| System Variable | Name | key_cache_division_limit | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 100 | ||
| Min Value | 1 | ||
| Max Value | 100 | ||
The division point between the hot and warm sublists of the key cache buffer list. The value is the percentage of the buffer list to use for the warm sublist. Permissible values range from 1 to 100. The default value is 100. See Section 8.10.2, “The MyISAM Key Cache”.
| Deprecated | 5.5.0, by lc-messages-dir | ||
| Command-Line Format | --language=name | ||
| System Variable | Name | language | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | directory name | |
| Default | /usr/local/mysql/share/mysql/english/ | ||
The directory where error messages are located. See Section 10.2, “Setting the Error Message Language”.
The language system variable
is removed as of MySQL 5.5.0 (although the
--language command-line option
can still be used). Similar information is available from the
lc_messages_dir and
lc_messages variables.
| System Variable | Name | large_files_support | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
Whether mysqld was compiled with options for large file support.
| Command-Line Format | --large-pages | ||
| System Variable | Name | large_pages | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Platform Specific | Linux | ||
| Permitted Values (Linux) | Type | boolean | |
| Default | FALSE | ||
Whether large page support is enabled (via the
--large-pages option). See
Section 8.12.4.2, “Enabling Large Page Support”.
| System Variable | Name | large_page_size | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values (Linux) | Type | integer | |
| Default | 0 | ||
If large page support is enabled, this shows the size of memory pages. Large memory pages are supported only on Linux; on other platforms, the value of this variable is always 0. See Section 8.12.4.2, “Enabling Large Page Support”.
The value to be returned from
LAST_INSERT_ID(). This is
stored in the binary log when you use
LAST_INSERT_ID() in a statement
that updates a table. Setting this variable does not update
the value returned by the
mysql_insert_id() C API
function.
| Command-Line Format | --lc-messages=name | ||
| System Variable | Name | lc_messages | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
| Default | en_US | ||
The locale to use for error messages. The default is
en_US. The server converts the argument to
a language name and combines it with the value of
lc_messages_dir to produce
the location for the error message file. See
Section 10.2, “Setting the Error Message Language”.
| Command-Line Format | --lc-messages-dir=dir_name | ||
| System Variable | Name | lc_messages_dir | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | directory name | |
The directory where error messages are located. The server
uses the value together with the value of
lc_messages to produce the
location for the error message file. See
Section 10.2, “Setting the Error Message Language”.
| System Variable | Name | lc_time_names | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
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 10.7, “MySQL Server Locale Support”.
| System Variable | Name | license | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
| Default | GPL | ||
The type of license the server has.
| System Variable | Name | local_infile | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | true | ||
Whether LOCAL is supported for
LOAD DATA
INFILE statements. If this variable is disabled,
clients cannot use LOCAL in
LOAD DATA statements. While the
default for this variable is true, whether
LOAD DATA INFILE LOCAL is actually
permitted depends on how MySQL was compiled, as well as a
number of settings on both the server and the client; see
Section 6.1.6, “Security Issues with LOAD DATA LOCAL”, for details.
| Introduced | 5.5.3 | ||
| Command-Line Format | --lock_wait_timeout=# | ||
| System Variable | Name | lock_wait_timeout | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 31536000 | ||
| Min Value | 1 | ||
| Max Value | 31536000 | ||
This variable specifies the timeout in seconds for attempts to acquire metadata locks. The permissible values range from 1 to 31536000 (1 year). The default is 31536000.
This timeout applies to all statements that use metadata
locks. These include DML and DDL operations on tables, views,
stored procedures, and stored functions, as well as
LOCK TABLES,
FLUSH TABLES WITH READ
LOCK, and HANDLER
statements.
This timeout does not apply to implicit accesses to system
tables in the mysql database, such as grant
tables modified by GRANT or
REVOKE statements or table
logging statements. The timeout does apply to system tables
accessed directly, such as with
SELECT or
UPDATE.
The timeout value applies separately for each metadata lock
attempt. A given statement can require more than one lock, so
it is possible for the statement to block for longer than the
lock_wait_timeout value
before reporting a timeout error. When lock timeout occurs,
ER_LOCK_WAIT_TIMEOUT is
reported.
lock_wait_timeout does not
apply to delayed inserts, which always execute with a timeout
of 1 year. This is done to avoid unnecessary timeouts because
a session that issues a delayed insert receives no
notification of delayed insert timeouts.
This variable was added in MySQL 5.5.3.
| System Variable | Name | locked_in_memory | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Deprecated | 5.1.29, by general-log | ||
| Command-Line Format | --log[=file_name] | ||
| System Variable | Name | log | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | file name | |
Whether logging of all statements to the general query log is enabled. See Section 5.4.3, “The General Query Log”.
This variable is deprecated and is removed in MySQL 5.6. Use
general_log instead.
log_bin_trust_function_creators
| Command-Line Format | --log-bin-trust-function-creators | ||
| System Variable | Name | log_bin_trust_function_creators | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | FALSE | ||
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 permitted 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. This
variable also applies to trigger creation. See
Section 20.7, “Binary Logging of Stored Programs”.
log_bin_trust_routine_creators
This is the old name for
log_bin_trust_function_creators.
This variable is deprecated and was removed in MySQL 5.5.3.
| Command-Line Format | --log-error[=file_name] | ||
| System Variable | Name | log_error | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | file name | |
The location of the error log, or empty if the server is writing error message to the standard error output. See Section 5.4.2, “The Error Log”.
| Command-Line Format | --log-output=name | ||
| System Variable | Name | log_output | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | set | |
| Default | FILE | ||
| Valid Values | TABLE | ||
FILE | |||
NONE | |||
The destination for general query log and slow query log
output. The value can be a comma-separated list of one or more
of the words TABLE (log to tables),
FILE (log to files), or
NONE (do not log to tables or files). The
default value is FILE.
NONE, if present, takes precedence over any
other specifiers. If the value is NONE log
entries are not written even if the logs are enabled. If the
logs are not enabled, no logging occurs even if the value of
log_output is not
NONE. For more information, see
Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”.
| Command-Line Format | --log-queries-not-using-indexes | ||
| System Variable | Name | log_queries_not_using_indexes | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
Whether queries that do not use indexes are logged to the slow query log. See Section 5.4.5, “The Slow Query Log”.
| Deprecated | 5.1.29, by slow-query-log | ||
| Command-Line Format | --log-slow-queries[=name] | ||
| System Variable | Name | log_slow_queries | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
Whether slow queries should be logged. “Slow” is
determined by the value of the
long_query_time variable. See
Section 5.4.5, “The Slow Query Log”.
This variable is deprecated and is removed in MySQL 5.6. Use
slow_query_log instead.
| Command-Line Format | --log-warnings[=#] | ||
| System Variable | Name | log_warnings | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 1 | ||
| Min Value | 0 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 1 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 1 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709551615 | ||
Whether to produce additional warning messages to the error log. This variable is enabled (1) by default and can be disabled by setting it to 0. The server logs messages about statements that are unsafe for statement-based logging if the value is greater than 0. Aborted connections and access-denied errors for new connection attempts are logged if the value is greater than 1.
| Command-Line Format | --long_query_time=# | ||
| System Variable | Name | long_query_time | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | numeric | |
| Default | 10 | ||
| Min Value | 0 | ||
If a query takes longer than this many seconds, the server
increments the Slow_queries
status variable. If the slow query log is enabled, 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 and default
values of long_query_time are
0 and 10, respectively. The value can be specified to a
resolution of microseconds. For logging to a file, times are
written including the microseconds part. For logging to
tables, only integer times are written; the microseconds part
is ignored. See Section 5.4.5, “The Slow Query Log”.
| Command-Line Format | --low-priority-updates | ||
| System Variable | Name | low_priority_updates | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | FALSE | ||
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 (such as
MyISAM, MEMORY, and
MERGE). This variable previously was named
sql_low_priority_updates.
| System Variable | Name | lower_case_file_system | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | boolean | |
This variable describes the case sensitivity of file names on
the file system where the data directory is located.
OFF means file names are case sensitive,
ON means they are not case sensitive. This
variable is read only because it reflects a file system
attribute and setting it would have no effect on the file
system.
| Command-Line Format | --lower_case_table_names[=#] | ||
| System Variable | Name | lower_case_table_names | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 2 | ||
If set to 0, table names are stored as specified and comparisons are case sensitive. If set to 1, table names are stored in lowercase on disk and 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. For additional information, see Section 9.2.2, “Identifier Case Sensitivity”.
On Windows the default value is 1. On OS X, the default value is 2.
You should not set
lower_case_table_names to 0
if you are running MySQL on a system where the data directory
resides on a case-insensitive file system (such as on Windows
or OS X). It is an unsupported combination that could result
in a hang condition when running an INSERT INTO ...
SELECT ... FROM
operation with the wrong tbl_nametbl_name
letter case. With MyISAM, accessing table
names using different letter cases could cause index
corruption.
As of MySQL 5.5.46, an error message is printed and the server
exits if you attempt to start the server with
--lower_case_table_names=0 on
a case-insensitive file system.
If you are using InnoDB tables, you should
set this variable to 1 on all platforms to force names to be
converted to lowercase.
The setting of this variable has no effect on replication filtering options. This is a known issue which is fixed in MySQL 5.6. See Section 17.2.3, “How Servers Evaluate Replication Filtering Rules”, for more information.
You should not use different settings for
lower_case_table_names on replication
masters and slaves. In particular, you should not do this when
the slave uses a case-sensitive file system, as this can cause
replication to fail. This is a known issue which is fixed in
MySQL 5.6. For more information, see
Section 17.4.1.38, “Replication and Variables”.
| Command-Line Format | --max_allowed_packet=# | ||
| System Variable | Name | max_allowed_packet | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 1048576 | ||
| Min Value | 1024 | ||
| Max Value | 1073741824 | ||
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.
The value should be a multiple of 1024; nonmultiples are
rounded down to the nearest multiple.
When you change the message buffer size by changing the value
of the max_allowed_packet
variable, you should also change the buffer size on the client
side if your client program permits it. The default
max_allowed_packet value
built in to the client library is 1GB, but individual client
programs might override this. For example,
mysql and mysqldump have
defaults of 16MB and 24MB, respectively. They also enable you
to change the client-side value by setting
max_allowed_packet on the
command line or in an option file.
The session value of this variable is read only. The client
can receive up to as many bytes as the session value. However,
the server will not send to the client more bytes than the
current global
max_allowed_packet value.
(The global value could be less than the session value if the
global value is changed after the client connects.)
| Command-Line Format | --max_connect_errors=# | ||
| System Variable | Name | max_connect_errors | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 10 | ||
| Min Value | 1 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 10 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 10 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709551615 | ||
If more than this many successive connection requests from a
host are interrupted without a successful connection, the
server blocks that host from further connections. You can
unblock blocked hosts by flushing the host cache. To do so,
issue a FLUSH
HOSTS statement or execute a mysqladmin
flush-hosts command. If a connection is established
successfully within fewer than
max_connect_errors attempts
after a previous connection was interrupted, the error count
for the host is cleared to zero. However, once a host is
blocked, flushing the host cache is the only way to unblock
it.
| Command-Line Format | --max_connections=# | ||
| System Variable | Name | max_connections | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 151 | ||
| Min Value | 1 | ||
| Max Value | 100000 | ||
The maximum permitted number of simultaneous client connections. By default, this is 151. See Section B.5.2.7, “Too many connections”, for more information.
Increasing this value increases the number of file descriptors that mysqld requires. See Section 8.4.3.1, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.
| Command-Line Format | --max_delayed_threads=# | ||
| System Variable | Name | max_delayed_threads | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 20 | ||
| Min Value | 0 | ||
| Max Value | 16384 | ||
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 was not specified. If you set this to 0, MySQL never
creates a thread to handle DELAYED rows; in
effect, this disables DELAYED entirely.
For the SESSION value of this variable, the
only valid values are 0 or the GLOBAL
value.
| Command-Line Format | --max_error_count=# | ||
| System Variable | Name | max_error_count | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 64 | ||
| Min Value | 0 | ||
| Max Value | 65535 | ||
The maximum number of error, warning, and note messages to be
stored for display by the SHOW
ERRORS and SHOW
WARNINGS statements.
| Command-Line Format | --max_heap_table_size=# | ||
| System Variable | Name | max_heap_table_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 16777216 | ||
| Min Value | 16384 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms) | Type | integer | |
| Default | 16777216 | ||
| Min Value | 16384 | ||
| Max Value | 1844674407370954752 | ||
This variable sets the maximum size to which user-created
MEMORY tables are permitted 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. A server
restart also sets the maximum size of existing
MEMORY tables to the global
max_heap_table_size value.
This variable is also used in conjunction with
tmp_table_size to limit the
size of internal in-memory tables. See
Section 8.4.4, “Internal Temporary Table Use in MySQL”.
max_heap_table_size is not replicated. See
Section 17.4.1.23, “Replication and MEMORY Tables”, and
Section 17.4.1.38, “Replication and Variables”, for more
information.
| System Variable | Name | max_insert_delayed_threads | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
This variable is a synonym for
max_delayed_threads.
| Command-Line Format | --max_join_size=# | ||
| System Variable | Name | max_join_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 18446744073709551615 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709551615 | ||
Do not permit 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 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.
| Command-Line Format | --max_length_for_sort_data=# | ||
| System Variable | Name | max_length_for_sort_data | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 1024 | ||
| Min Value | 4 | ||
| Max Value | 8388608 | ||
The cutoff on the size of index values that determines which
filesort algorithm to use. See
Section 8.2.1.11, “ORDER BY Optimization”.
| Introduced | 5.5.11 | ||
| Deprecated | 5.5.11 | ||
| Command-Line Format | --max_long_data_size=# | ||
| System Variable | Name | max_long_data_size | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 1048576 | ||
| Min Value | 1024 | ||
| Max Value | 4294967295 | ||
The maximum size of parameter values that can be sent with the
mysql_stmt_send_long_data() C
API function. If not set at server startup, the default is the
value of the
max_allowed_packet system
variable. This variable is deprecated. In MySQL 5.6, it is
removed and the maximum parameter size is controlled by
max_allowed_packet.
| Command-Line Format | --max_prepared_stmt_count=# | ||
| System Variable | Name | max_prepared_stmt_count | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 16382 | ||
| Min Value | 0 | ||
| Max Value | 1048576 | ||
This variable limits the total number of prepared statements in the server. (The sum of the number of prepared statements across all sessions.) 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. 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. The default value is 16,382. The permissible range of values is from 0 to 1 million. Setting the value to 0 disables prepared statements.
| Command-Line Format | --max_relay_log_size=# | ||
| System Variable | Name | max_relay_log_size | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 1073741824 | ||
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 17.2.1, “Replication Implementation Details”.
| Command-Line Format | --max_seeks_for_key=# | ||
| System Variable | Name | max_seeks_for_key | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 4294967295 | ||
| Min Value | 1 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 18446744073709547520 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 18446744073709551615 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709551615 | ||
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 13.7.5.23, “SHOW INDEX Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.
| Command-Line Format | --max_sort_length=# | ||
| System Variable | Name | max_sort_length | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 1024 | ||
| Min Value | 4 | ||
| Max Value | 8388608 | ||
The number of bytes to use when sorting data values. The
server uses only the first
max_sort_length bytes of each
value and ignores the rest. Consequently, values that differ
only after the first
max_sort_length bytes compare
as equal for GROUP BY, ORDER
BY, and DISTINCT operations.
Increasing the value of
max_sort_length may require
increasing the value of
sort_buffer_size as well. For
details, see Section 8.2.1.11, “ORDER BY Optimization”
| Command-Line Format | --max_sp_recursion_depth[=#] | ||
| System Variable | Name | max_sp_recursion_depth | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Max Value | 255 | ||
The number of times that any given stored procedure may be called recursively. The default value for this option is 0, which completely disables recursion in stored procedures. The maximum value is 255.
Stored procedure recursion increases the demand on thread
stack space. If you increase the value of
max_sp_recursion_depth, it
may be necessary to increase thread stack size by increasing
the value of thread_stack at
server startup.
This variable is unused.
| Command-Line Format | --max_user_connections=# | ||
| System Variable | Name | max_user_connections | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 4294967295 | ||
The maximum number of simultaneous connections permitted to any given MySQL user account. A value of 0 (the default) means “no limit.”
This variable has a global value that can be set at server startup or runtime. It also has a read-only session value that indicates the effective simultaneous-connection limit that applies to the account associated with the current session. The session value is initialized as follows:
If the user account has a nonzero
MAX_USER_CONNECTIONS resource limit,
the session
max_user_connections
value is set to that limit.
Otherwise, the session
max_user_connections
value is set to the global value.
Account resource limits are specified using the
GRANT statement. See
Section 6.3.4, “Setting Account Resource Limits”, and Section 13.7.1.3, “GRANT Syntax”.
| Command-Line Format | --max_write_lock_count=# | ||
| System Variable | Name | max_write_lock_count | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 4294967295 | ||
| Min Value | 1 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 18446744073709547520 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 18446744073709551615 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709551615 | ||
After this many write locks, permit some pending read lock requests to be processed in between.
| Introduced | 5.5.19 | ||
| System Variable | Name | metadata_locks_cache_size | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 1024 | ||
| Min Value | 1 | ||
| Max Value | 1048576 | ||
The size of the metadata locks cache. The server uses this cache to avoid creation and destruction of synchronization objects. This is particularly helpful on systems where such operations are expensive, such as Windows XP. This variable was added in MySQL 5.5.19.
| Command-Line Format | --min-examined-row-limit=# | ||
| System Variable | Name | min_examined_row_limit | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709551615 | ||
Queries that examine fewer than this number of rows are not logged to the slow query log.
| Command-Line Format | --multi_range_count=# | ||
| System Variable | Name | multi_range_count | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 256 | ||
| Min Value | 1 | ||
| Max Value | 4294967295 | ||
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 is
especially true for the
NDBCLUSTER table handler, which
needs to send the range requests to all nodes. Sending a batch
of those requests at once reduces communication costs
significantly.
This variable is deprecated in MySQL 5.1, and is no longer supported in MySQL 5.5, in which arbitrarily long lists of ranges can be processed.
| Command-Line Format | --myisam_data_pointer_size=# | ||
| System Variable | Name | myisam_data_pointer_size | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 6 | ||
| Min Value | 2 | ||
| Max Value | 7 | ||
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. See Section B.5.2.12, “The table is full”.
| Command-Line Format | --myisam_max_sort_file_size=# | ||
| System Variable | Name | myisam_max_sort_file_size | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 2147483648 | ||
| Permitted Values (64-bit platforms) | Type | integer | |
| Default | 9223372036854775807 | ||
The maximum size of the temporary file that MySQL is permitted
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.
If MyISAM index files exceed this size and
disk space is available, increasing the value may help
performance. The space must be available in the file system
containing the directory where the original index file is
located.
| Introduced | 5.5.1 | ||
| Command-Line Format | --myisam_mmap_size=# | ||
| System Variable | Name | myisam_mmap_size | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 4294967295 | ||
| Min Value | 7 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 18446744073709547520 | ||
| Min Value | 7 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 18446744073709551615 | ||
| Min Value | 7 | ||
| Max Value | 18446744073709551615 | ||
The maximum amount of memory to use for memory mapping
compressed MyISAM files. If many
compressed MyISAM tables are used, the
value can be decreased to reduce the likelihood of
memory-swapping problems. This variable was added in MySQL
5.5.1.
| System Variable | Name | myisam_recover_options | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
The value of the
--myisam-recover-options
option. See Section 5.1.4, “Server Command Options”.
| Command-Line Format | --myisam_repair_threads=# | ||
| System Variable | Name | myisam_repair_threads | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 1 | ||
| Min Value | 1 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 1 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 1 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709551615 | ||
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.
Multi-threaded repair is still beta-quality code.
| Command-Line Format | --myisam_sort_buffer_size=# | ||
| System Variable | Name | myisam_sort_buffer_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (Windows, 32-bit platforms) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 4096 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Windows, 64-bit platforms, <= 5.5.21) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 4096 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Windows, 64-bit platforms, >= 5.5.22) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 4096 | ||
| Max Value | 18446744073709551615 | ||
| Permitted Values (Other, 32-bit platforms) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 4096 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Other, 64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 4096 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (Other, 64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 8388608 | ||
| Min Value | 4096 | ||
| Max Value | 18446744073709551615 | ||
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.
The maximum permissible setting for
myisam_sort_buffer_size is
4GB−1. Larger values are permitted for 64-bit platforms
(except 64-bit Windows prior to MySQL 5.5.222, for which large
values are truncated to 4GB−1 with a warning).
| Command-Line Format | --myisam_stats_method=name | ||
| System Variable | Name | myisam_stats_method | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | enumeration | |
| Default | nulls_unequal | ||
| Valid Values | nulls_equal | ||
nulls_unequal | |||
nulls_ignored | |||
How the server treats NULL values when
collecting statistics about the distribution of index values
for MyISAM tables. This variable has three
possible values, nulls_equal,
nulls_unequal, and
nulls_ignored. 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. For nulls_ignored,
NULL values are ignored.
The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 8.3.7, “InnoDB and MyISAM Index Statistics Collection”.
| Command-Line Format | --myisam_use_mmap | ||
| System Variable | Name | myisam_use_mmap | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
Use memory mapping for reading and writing
MyISAM tables.
| System Variable | Name | named_pipe | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Platform Specific | Windows | ||
| Permitted Values (Windows) | Type | boolean | |
| Default | OFF | ||
(Windows only.) Indicates whether the server supports connections over named pipes.
| Command-Line Format | --net_buffer_length=# | ||
| System Variable | Name | net_buffer_length | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 16384 | ||
| Min Value | 1024 | ||
| Max Value | 1048576 | ||
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 session value of this variable is read only.
| Command-Line Format | --net_read_timeout=# | ||
| System Variable | Name | net_read_timeout | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 30 | ||
| Min Value | 1 | ||
The number of seconds to wait for more data from a connection
before aborting the read. 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.
| Command-Line Format | --net_retry_count=# | ||
| System Variable | Name | net_retry_count | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 10 | ||
| Min Value | 1 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 10 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 10 | ||
| Min Value | 1 | ||
| Max Value | 18446744073709551615 | ||
If a read or write 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.
| Command-Line Format | --net_write_timeout=# | ||
| System Variable | Name | net_write_timeout | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 60 | ||
| Min Value | 1 | ||
The number of seconds to wait for a block to be written to a
connection before aborting the write. See also
net_read_timeout.
| Command-Line Format | --new | ||
| System Variable | Name | new | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Disabled by | skip-new | ||
| Permitted Values | Type | boolean | |
| Default | FALSE | ||
This variable was used in MySQL 4.0 to turn on some 4.1
behaviors, and is retained for backward compatibility. Its
value is always OFF.
In MySQL Cluster, setting this variable to
ON makes it possible to employ partitioning
types other than KEY or LINEAR
KEY with NDB tables.
This feature is experimental only, and not supported
in production. For additional information, see
User-defined partitioning and the NDB storage engine (MySQL Cluster).
| Command-Line Format | --old | ||
| System Variable | Name | old | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
old is a compatibility
variable. It is disabled by default, but can be enabled at
startup to revert the server to behaviors present in older
versions.
When old is enabled, it
changes the default scope of index hints to that used prior to
MySQL 5.1.17. That is, index hints with no
FOR clause apply only to how indexes are
used for row retrieval and not to resolution of ORDER
BY or GROUP BY clauses. (See
Section 8.9.3, “Index Hints”.) Take care about enabling this
in a replication setup. With statement-based binary logging,
having different modes for the master and slaves might lead to
replication errors.
| Command-Line Format | --old-alter-table | ||
| System Variable | Name | old_alter_table | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
When this variable is enabled, the server does not use the
optimized method of processing an ALTER
TABLE operation. It reverts to using a temporary
table, copying over the data, and then renaming the temporary
table to the original, as used by MySQL 5.0 and earlier. For
more information on the operation of
ALTER TABLE, see
Section 13.1.7, “ALTER TABLE Syntax”.
| System Variable | Name | old_passwords | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 0 | ||
This variable controls the password hashing method used by the
PASSWORD() function. It also
influences password hashing performed by
CREATE USER and
GRANT statements that specify a
password using an IDENTIFIED BY clause.
The following table shows the permitted values of
old_passwords, the password
hashing method for each value, and which authentication
plugins use passwords hashed with each method.
| Value | Password Hashing Method | Associated Authentication Plugin |
|---|---|---|
0 or OFF | MySQL 4.1 native hashing | mysql_native_password |
1 or ON | Pre-4.1 (“old”) hashing | mysql_old_password |
If old_passwords=1,
PASSWORD(
returns the same value as
str)OLD_PASSWORD(.
The latter function is not affected by the value of
str)old_passwords.
For additional information about authentication plugins and hashing formats, see Section 6.3.6, “Pluggable Authentication”, and Section 6.1.2.4, “Password Hashing in MySQL”.
Passwords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.
This is not a variable, but it can be used when setting some variables. It is described in Section 13.7.4.1, “SET Syntax for Variable Assignment”.
| Command-Line Format | --open-files-limit=# | ||
| System Variable | Name | open_files_limit | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | platform dependent | ||
The number of files that the operating system permits
mysqld to open. This is the real value
permitted 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 cannot change the number
of open files.
| Command-Line Format | --optimizer_prune_level[=#] | ||
| System Variable | Name | optimizer_prune_level | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 1 | ||
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.
| Command-Line Format | --optimizer_search_depth[=#] | ||
| System Variable | Name | optimizer_search_depth | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 62 | ||
| Min Value | 0 | ||
| Max Value | 63 | ||
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 63, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for performing searches. The value of 63 is deprecated and will be treated as invalid in a future MySQL release.
| Command-Line Format | --optimizer_switch=value | ||
| System Variable | Name | optimizer_switch | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (<= 5.5.2) | Type | set | |
| Valid Values | index_merge={on|off} | ||
index_merge_intersection={on|off} | |||
index_merge_sort_union={on|off} | |||
index_merge_union={on|off} | |||
| Permitted Values (>= 5.5.3) | Type | set | |
| Valid Values | engine_condition_pushdown={on|off} | ||
index_merge={on|off} | |||
index_merge_intersection={on|off} | |||
index_merge_sort_union={on|off} | |||
index_merge_union={on|off} | |||
The optimizer_switch system
variable enables control over optimizer behavior. The value of
this variable is a set of flags, each of which has a value of
on or off to indicate
whether the corresponding optimizer behavior is enabled or
disabled. This variable has global and session values and can
be changed at runtime. The global default can be set at server
startup.
To see the current set of optimizer flags, select the variable value:
mysql> SELECT @@optimizer_switch\G
*************************** 1. row ***************************
@@optimizer_switch: index_merge=on,index_merge_union=on,
index_merge_sort_union=on,
index_merge_intersection=on,
engine_condition_pushdown=on
For more information about the syntax of this variable and the optimizer behaviors that it controls, see Section 8.9.2, “Controlling Switchable Optimizations”.
performance_schema_
xxx
Performance Schema system variables are listed in Section 22.10, “Performance Schema System Variables”. These variables may be used to configure Performance Schema operation.
| Command-Line Format | --pid-file=file_name | ||
| System Variable | Name | pid_file | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | file name | |
The path name of the process ID (PID) file. This variable can
be set with the --pid-file
option.
| Command-Line Format | --plugin_dir=dir_name | ||
| System Variable | Name | plugin_dir | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values (>= 5.5.5) | Type | directory name | |
| Default | BASEDIR/lib/plugin | ||
| Permitted Values (Windows, <= 5.5.4) | Type | directory name | |
| Default | BASEDIR/lib/plugin | ||
| Permitted Values (Other, <= 5.5.4) | Type | directory name | |
| Default | BASEDIR/lib/mysql/plugin | ||
The path name of the plugin directory.
If the plugin directory is writable by the server, it may be
possible for a user to write executable code to a file in the
directory using SELECT
... INTO DUMPFILE. This can be prevented by making
plugin_dir read only to the
server or by setting
--secure-file-priv to a
directory where SELECT writes
can be made safely.
| Command-Line Format | --port=# | ||
| System Variable | Name | port | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 3306 | ||
| Min Value | 0 | ||
| Max Value | 65535 | ||
The number of the port on which the server listens for TCP/IP
connections. This variable can be set with the
--port option.
| Command-Line Format | --preload_buffer_size=# | ||
| System Variable | Name | preload_buffer_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 32768 | ||
| Min Value | 1024 | ||
| Max Value | 1073741824 | ||
The size of the buffer that is allocated when preloading indexes.
If set to 0 or OFF (the default), statement
profiling is disabled. If set to 1 or ON,
statement profiling is enabled and the
SHOW PROFILE and
SHOW PROFILES statements
provide access to profiling information. See
Section 13.7.5.32, “SHOW PROFILES Syntax”.
The number of statements for which to maintain profiling
information if profiling is
enabled. The default value is 15. The maximum value is 100.
Setting the value to 0 effectively disables profiling. See
Section 13.7.5.32, “SHOW PROFILES Syntax”.
| System Variable | Name | protocol_version | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
The version of the client/server protocol used by the MySQL server.
| Introduced | 5.5.7 | ||
| System Variable | Name | proxy_user | |
| Variable Scope | Session | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
If the current client is a proxy for another user, this
variable is the proxy user account name. Otherwise, this
variable is NULL. See
Section 6.3.7, “Proxy Users”.
This variable was added in MySQL 5.5.7.
| Introduced | 5.5.30 | ||
| System Variable | Name | pseudo_slave_mode | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
This variable is for internal server use. It was added in MySQL 5.5.30.
| System Variable | Name | pseudo_thread_id | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
This variable is for internal server use.
| Command-Line Format | --query_alloc_block_size=# | ||
| System Variable | Name | query_alloc_block_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 8192 | ||
| Min Value | 1024 | ||
| Max Value | 4294967295 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 8192 | ||
| Min Value | 1024 | ||
| Max Value | 18446744073709547520 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 8192 | ||
| Min Value | 1024 | ||
| Max Value | 18446744073709551615 | ||
| Block Size | 1024 | ||
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 parameter.
| Command-Line Format | --query_cache_limit=# | ||
| System Variable | Name | query_cache_limit | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 1048576 | ||
| Min Value | 0 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 1048576 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 1048576 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709551615 | ||
Do not cache results that are larger than this number of bytes. The default value is 1MB.
| Command-Line Format | --query_cache_min_res_unit=# | ||
| System Variable | Name | query_cache_min_res_unit | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 4096 | ||
| Min Value | 512 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 4096 | ||
| Min Value | 512 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 4096 | ||
| Min Value | 512 | ||
| Max Value | 18446744073709551615 | ||
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 8.10.3.3, “Query Cache Configuration”.
| Command-Line Format | --query_cache_size=# | ||
| System Variable | Name | query_cache_size | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709551615 | ||
The amount of memory allocated for caching query results. The
default value is 0, which disables the query cache. To reduce
overhead significantly, you should also start the server with
query_cache_type=0 if you
will not be using the query cache. The permissible values are
multiples of 1024; other values are rounded down to the
nearest multiple.
query_cache_size bytes of
memory are allocated even if
query_cache_type is set to 0.
See Section 8.10.3.3, “Query Cache Configuration”, for more
information.
The query cache needs a minimum size of about 40KB to allocate
its structures. (The exact size depends on system
architecture.) If you set the value of
query_cache_size too small, a
warning will occur, as described in
Section 8.10.3.3, “Query Cache Configuration”.
| Command-Line Format | --query_cache_type=# | ||
| System Variable | Name | query_cache_type | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | enumeration | |
| Default | 1 | ||
| Valid Values | 0 | ||
1 | |||
2 | |||
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 | Do not 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 cacheable query results except for those that begin with
SELECT SQL_NO_CACHE. |
2 or DEMAND | Cache results only for cacheable queries that begin with SELECT
SQL_CACHE. |
This variable defaults to ON.
If the server is started with
query_cache_type set to 0, it does not
acquire the query cache mutex at all, which means that the
query cache cannot be enabled at runtime and there is reduced
overhead in query execution.
| Command-Line Format | --query_cache_wlock_invalidate | ||
| System Variable | Name | query_cache_wlock_invalidate | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | FALSE | ||
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.
| Command-Line Format | --query_prealloc_size=# | ||
| System Variable | Name | query_prealloc_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 8192 | ||
| Min Value | 8192 | ||
| Max Value | 4294967295 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 8192 | ||
| Min Value | 8192 | ||
| Max Value | 18446744073709547520 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 8192 | ||
| Min Value | 8192 | ||
| Max Value | 18446744073709551615 | ||
| Block Size | 1024 | ||
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 rand_seed1 and
rand_seed2 variables exist as
session variables only, and can be set but not read. The
variables—but not their values—are shown in the
output of SHOW VARIABLES.
The purpose of these variables is to support replication of
the RAND() function. For
statements that invoke RAND(),
the master passes two values to the slave, where they are used
to seed the random number generator. The slave uses these
values to set the session variables
rand_seed1 and
rand_seed2 so that
RAND() on the slave generates
the same value as on the master.
See the description for
rand_seed1.
| Command-Line Format | --range_alloc_block_size=# | ||
| System Variable | Name | range_alloc_block_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 4096 | ||
| Min Value | 4096 | ||
| Max Value | 4294967295 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 4096 | ||
| Min Value | 4096 | ||
| Max Value | 18446744073709551615 | ||
| Block Size | 1024 | ||
The size of blocks that are allocated when doing range optimization.
| Command-Line Format | --read_buffer_size=# | ||
| System Variable | Name | read_buffer_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 131072 | ||
| Min Value | 8200 | ||
| Max Value | 2147479552 | ||
Each thread that does a sequential scan for a
MyISAM table 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. The value of this variable should be a
multiple of 4KB. If it is set to a value that is not a
multiple of 4KB, its value will be rounded down to the nearest
multiple of 4KB.
This option is also used in the following context for all storage engines:
For caching the indexes in a temporary file (not a
temporary table), when sorting rows for ORDER
BY.
For bulk insert into partitions.
For caching results of nested queries.
and in one other storage engine-specific way: to determine the
memory block size for MEMORY
tables.
The maximum permissible setting for
read_buffer_size is 2GB.
For more information about memory use during different operations, see Section 8.12.4.1, “How MySQL Uses Memory”.
| Command-Line Format | --read_only | ||
| System Variable | Name | read_only | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
When the read_only system
variable is enabled, the server permits no client updates
except from users who have the
SUPER privilege. This variable
is disabled by default.
Even with read_only enabled,
the server permits these operations:
Updates performed by slave threads, if the server is a
replication slave. In replication setups, it can be useful
to enable read_only on
slave servers to ensure that slaves accept updates only
from the master server and not from clients.
Use of ANALYZE TABLE or
OPTIMIZE TABLE statements.
The purpose of read-only mode is to prevent changes to
table structure or contents. Analysis and optimization do
not qualify as such changes. This means, for example, that
consistency checks on read-only replication slaves can be
performed with mysqlcheck --all-databases
--analyze.
Operations on TEMPORARY tables.
Inserts into the log tables
(mysql.general_log and
mysql.slow_log; see
Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”).
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.
The following conditions apply to attempts to enable
read_only:
The attempt fails and an error occurs if you have any
explicit locks (acquired with LOCK
TABLES) or have a pending transaction.
The attempt blocks while other clients hold explicit table
locks or have pending transactions, until the locks are
released and the transactions end. While the attempt to
enable read_only is
pending, requests by other clients for table locks or to
begin transactions also block until
read_only has been set.
As of MySQL 5.5.3, the attempt blocks if there are active transactions that hold metadata locks, until those transactions end.
read_only can be enabled
while you hold a global read lock (acquired with
FLUSH TABLES WITH
READ LOCK) because that does not involve table
locks.
| Command-Line Format | --read_rnd_buffer_size=# | ||
| System Variable | Name | read_rnd_buffer_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (<= 5.5.2) | Type | integer | |
| Default | 262144 | ||
| Min Value | 8200 | ||
| Max Value | 2147483647 | ||
| Permitted Values (>= 5.5.3) | Type | integer | |
| Default | 262144 | ||
| Min Value | 1 | ||
| Max Value | 2147483647 | ||
When reading rows from a MyISAM table in
sorted order following a key-sorting operation, the rows are
read through this buffer to avoid disk seeks. See
Section 8.2.1.11, “ORDER BY Optimization”. 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.
The maximum permissible setting for
read_rnd_buffer_size is 2GB.
For more information about memory use during different operations, see Section 8.12.4.1, “How MySQL Uses Memory”.
| Command-Line Format | --relay_log_purge | ||
| System Variable | Name | relay_log_purge | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | TRUE | ||
Disables or enables automatic purging of relay log files as
soon as they are not needed any more. The default value is 1
(ON).
| Command-Line Format | --relay_log_space_limit=# | ||
| System Variable | Name | relay_log_space_limit | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 4294967295 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709547520 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 18446744073709551615 | ||
The maximum amount of space to use for all relay logs.
| Command-Line Format | --report-host=host_name | ||
| System Variable | Name | report_host | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The value of the --report-host
option.
| Command-Line Format | --report-password=name | ||
| System Variable | Name | report_password | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The value of the
--report-password option. Not
the same as the password used for the MySQL replication user
account.
| Command-Line Format | --report-port=# | ||
| System Variable | Name | report_port | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values (<= 5.5.22) | Type | integer | |
| Default | 3306 | ||
| Min Value | 0 | ||
| Max Value | 65535 | ||
| Permitted Values (>= 5.5.23) | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 65535 | ||
The value of the --report-port
option.
| Command-Line Format | --report-user=name | ||
| System Variable | Name | report_user | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The value of the --report-user
option. Not the same as the name for the MySQL replication
user account.
| System Variable | Name | rpl_semi_sync_master_enabled | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
Controls whether semisynchronous replication is enabled on the
master. To enable or disable the plugin, set this variable to
ON or OFF (or 1 or 0),
respectively. The default is OFF.
This variable is available only if the master-side semisynchronous replication plugin is installed.
| System Variable | Name | rpl_semi_sync_master_timeout | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 10000 | ||
A value in milliseconds that controls how long the master waits on a commit for acknowledgment from a slave before timing out and reverting to asynchronous replication. The default value is 10000 (10 seconds).
This variable is available only if the master-side semisynchronous replication plugin is installed.
rpl_semi_sync_master_trace_level
| System Variable | Name | rpl_semi_sync_master_trace_level | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 32 | ||
The semisynchronous replication debug trace level on the master. Four levels are defined:
1 = general level (for example, time function failures)
16 = detail level (more verbose information)
32 = net wait level (more information about network waits)
64 = function level (information about function entry and exit)
This variable is available only if the master-side semisynchronous replication plugin is installed.
rpl_semi_sync_master_wait_no_slave
| System Variable | Name | rpl_semi_sync_master_wait_no_slave | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | ON | ||
With semisynchronous replication, for each transaction, the master waits until timeout for acknowledgment of receipt from some semisynchronous slave. If no response occurs during this period, the master reverts to normal replication. This variable controls whether the master waits for the timeout to expire before reverting to normal replication even if the slave count drops to zero during the timeout period.
If the value is ON (the default), it is
permissible for the slave count to drop to zero during the
timeout period (for example, if slaves disconnect). The master
still waits for the timeout, so as long as some slave
reconnects and acknowledges the transaction within the timeout
interval, semisynchronous replication continues.
If the value is OFF, the master reverts to
normal replication if the slave count drops to zero during the
timeout period.
This variable is available only if the master-side semisynchronous replication plugin is installed.
| System Variable | Name | rpl_semi_sync_slave_enabled | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
Controls whether semisynchronous replication is enabled on the
slave. To enable or disable the plugin, set this variable to
ON or OFF (or 1 or 0),
respectively. The default is OFF.
This variable is available only if the slave-side semisynchronous replication plugin is installed.
rpl_semi_sync_slave_trace_level
| System Variable | Name | rpl_semi_sync_slave_trace_level | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 32 | ||
The semisynchronous replication debug trace level on the
slave. See
rpl_semi_sync_master_trace_level
for the permissible values.
This variable is available only if the slave-side semisynchronous replication plugin is installed.
| Command-Line Format | --secure-auth | ||
| System Variable | Name | secure_auth | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
If this variable is enabled, the server blocks connections by clients that attempt to use accounts that have passwords stored in the old (pre-4.1) format.
Enable this variable 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 variable is enabled and the privilege tables are in pre-4.1 format. See Section B.5.2.4, “Client does not support authentication protocol”.
Passwords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.
| Command-Line Format | --secure-file-priv=dir_name | ||
| System Variable | Name | secure_file_priv | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values (<= 5.5.52) | Type | string | |
| Default | empty | ||
| Valid Values | empty | ||
dirname | |||
| Permitted Values (>= 5.5.53) | Type | string | |
| Default | platform specific | ||
| Valid Values | empty | ||
dirname | |||
NULL | |||
This variable is used to limit the effect of data import and
export operations, such as those performed by the
LOAD DATA and
SELECT ... INTO
OUTFILE statements and the
LOAD_FILE() function. These
operations are permitted only to users who have the
FILE privilege.
secure_file_priv may be set
as follows:
If empty, the variable has no effect. This is not a secure setting.
If set to the name of a directory, the server limits import and export operations to work only with files in that directory. The directory must exist; the server will not create it.
If set to NULL, the server disables
import and export operations. This value is permitted as
of MySQL 5.5.53.
Before MySQL 5.5.53, this variable is empty by default. As of
5.5.53, the default value is platform specific and depends on
the value of the INSTALL_LAYOUT
CMake option, as shown in the following
table. To specify the default
secure_file_priv value
explicitly if you are building from source, use the
INSTALL_SECURE_FILE_PRIVDIR
CMake option.
INSTALL_LAYOUT Value | Default secure_file_priv Value |
|---|---|
STANDALONE, WIN | NULL |
DEB, RPM, SLES,
SVR4 | /var/lib/mysql-files |
| Otherwise | mysql-files under the
CMAKE_INSTALL_PREFIX
value |
To set the default
secure_file_priv value for
the libmysqld embedded server, use the
INSTALL_SECURE_FILE_PRIV_EMBEDDEDDIR
CMake option. The default value for this
option is NULL.
As of MySQL 5.5.53, the server checks the value of
secure_file_priv at startup
and writes a warning to the error log if the value is
insecure. A non-NULL value is considered
insecure if it is empty, or the value is the data directory or
a subdirectory of it, or a directory that is accessible by all
users. If secure_file_priv is
set to a nonexistent path, the server writes an error message
to the error log and exits.
| Command-Line Format | --server-id=# | ||
| System Variable | Name | server_id | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 4294967295 | ||
The server ID, used in replication to give each master and
slave a unique identity. This variable is set by the
--server-id option. For each
server participating in replication, you should pick a
positive integer in the range from 1 to
232 − 1 to act as that
server's ID.
| Command-Line Format | --shared_memory[={0,1}] | ||
| System Variable | Name | shared_memory | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Platform Specific | Windows | ||
| Permitted Values | Type | boolean | |
| Default | FALSE | ||
(Windows only.) Whether the server permits shared-memory connections.
| Command-Line Format | --shared_memory_base_name=name | ||
| System Variable | Name | shared_memory_base_name | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Platform Specific | Windows | ||
| Permitted Values | Type | string | |
| Default | MYSQL | ||
(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.
| Command-Line Format | --skip-external-locking | ||
| System Variable | Name | skip_external_locking | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | boolean | |
| Default | ON | ||
This is OFF if mysqld
uses external locking (system locking), ON
if external locking is disabled. This affects only
MyISAM table access.
This variable is set by the
--external-locking or
--skip-external-locking
option. External locking is disabled by default.
External locking affects only
MyISAM table access. For more
information, including conditions under which it can and
cannot be used, see Section 8.11.5, “External Locking”.
| Command-Line Format | --skip-name-resolve | ||
| System Variable | Name | skip_name_resolve | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
This variable is set from the value of the
--skip-name-resolve option. If
it is OFF, mysqld
resolves host names when checking client connections. If it is
ON, mysqld uses only IP
numbers; in this case, all Host column
values in the grant tables must be IP addresses or
localhost. See
Section 8.12.5.2, “DNS Lookup Optimization and the Host Cache”.
This variable was added in MySQL 5.5.5.
| Command-Line Format | --skip-networking | ||
| System Variable | Name | skip_networking | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
This is ON if the server permits 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. This variable can be set to
ON with the
--skip-networking option.
| Command-Line Format | --skip-show-database | ||
| System Variable | Name | skip_show_database | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
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
permitted only to users who have the SHOW
DATABASES privilege, and the statement displays all
database names. If the value is OFF,
SHOW DATABASES is permitted to
all users, but displays the names of only those databases for
which the user has the SHOW
DATABASES or other privilege. (Note that
any global privilege is considered a
privilege for the database.)
| Command-Line Format | --slow_launch_time=# | ||
| System Variable | Name | slow_launch_time | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 2 | ||
If creating a thread takes longer than this many seconds, the
server increments the
Slow_launch_threads status
variable.
| Command-Line Format | --slow-query-log | ||
| System Variable | Name | slow_query_log | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
Whether the slow query log is enabled. The value can be 0 (or
OFF) to disable the log or 1 (or
ON) to enable the log. The default value
depends on whether the
--slow_query_log option is
given. The destination for log output is controlled by the
log_output system variable;
if that value is NONE, no log entries are
written even if the log is enabled.
“Slow” is determined by the value of the
long_query_time variable. See
Section 5.4.5, “The Slow Query Log”.
| Command-Line Format | --slow-query-log-file=file_name | ||
| System Variable | Name | slow_query_log_file | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | file name | |
| Default | host_name-slow.log | ||
The name of the slow query log file. The default value is
host_name-slow.log--slow_query_log_file option.
| Command-Line Format | --socket={file_name|pipe_name} | ||
| System Variable | Name | socket | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
| Default | /tmp/mysql.sock | ||
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).
| Command-Line Format | --sort_buffer_size=# | ||
| System Variable | Name | sort_buffer_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (Windows) | Type | integer | |
| Default | 2097144 | ||
| Min Value | 32768 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Other, 32-bit platforms) | Type | integer | |
| Default | 2097144 | ||
| Min Value | 32768 | ||
| Max Value | 4294967295 | ||
| Permitted Values (Other, 64-bit platforms) | Type | integer | |
| Default | 2097144 | ||
| Min Value | 32768 | ||
| Max Value | 18446744073709551615 | ||
Each session that must perform a sort allocates a buffer of
this size. sort_buffer_size
is not specific to any storage engine and applies in a general
manner for optimization. At minimum the
sort_buffer_size value must
be large enough to accommodate fifteen tuples in the sort
buffer. Also, increasing the value of
max_sort_length may require
increasing the value of
sort_buffer_size. For more
information, see Section 8.2.1.11, “ORDER BY Optimization”
If you see many
Sort_merge_passes per second
in SHOW GLOBAL
STATUS output, you can consider increasing the
sort_buffer_size value to
speed up ORDER BY or GROUP
BY operations that cannot be improved with query
optimization or improved indexing. The entire buffer is
allocated even if it is not all needed, so setting it larger
than required globally will slow down most queries that sort.
It is best to increase it as a session setting, and only for
the sessions that need a larger size. On Linux, there are
thresholds of 256KB and 2MB where larger values may
significantly slow down memory allocation, so you should
consider staying below one of those values. Experiment to find
the best value for your workload. See
Section B.5.3.5, “Where MySQL Stores Temporary Files”.
The maximum permissible setting for
sort_buffer_size is
4GB−1. Larger values are permitted for 64-bit platforms
(except 64-bit Windows, for which large values are truncated
to 4GB−1 with a warning).
| System Variable (<= 5.5.2) | Name | sql_auto_is_null | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | sql_auto_is_null | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (<= 5.5.2) | Type | boolean | |
| Default | 1 | ||
| Permitted Values (>= 5.5.3) | Type | boolean | |
| Default | 0 | ||
If this variable is set to 1, then after a statement that
successfully inserts an automatically generated
AUTO_INCREMENT value, you can find that
value by issuing a statement of the following form:
SELECT * FROMtbl_nameWHEREauto_colIS NULL
If the statement returns a row, the value returned is the same
as if you invoked the
LAST_INSERT_ID() function. For
details, including the return value after a multiple-row
insert, see Section 12.14, “Information Functions”. If no
AUTO_INCREMENT value was successfully
inserted, the SELECT statement
returns no row.
The behavior of retrieving an
AUTO_INCREMENT value by using an
IS NULL comparison is used by
some ODBC programs, such as Access. See
Obtaining Auto-Increment Values.
This behavior can be disabled by setting
sql_auto_is_null to 0.
The default value of
sql_auto_is_null is 0 as of
MySQL 5.5.3, and 1 for earlier versions.
| System Variable (<= 5.5.2) | Name | sql_big_selects | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | sql_big_selects | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 1 | ||
If set to 0, MySQL aborts
SELECT statements that are
likely to take a very long time to execute (that is,
statements for which the optimizer estimates that the number
of examined rows exceeds the value of
max_join_size). This is
useful when an inadvisable WHERE statement
has been issued. The default value for a new connection is 1,
which permits all SELECT
statements.
If you set the max_join_size
system variable to a value other than
DEFAULT,
sql_big_selects is set to 0.
| System Variable (<= 5.5.2) | Name | sql_buffer_result | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | sql_buffer_result | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 0 | ||
If set to 1,
sql_buffer_result forces
results from SELECT statements
to be put into temporary tables. This helps MySQL free the
table locks early and can be beneficial in cases where it
takes a long time to send results to the client. The default
value is 0.
| System Variable (<= 5.5.2) | Name | sql_log_bin | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3, <= 5.5.40) | Name | sql_log_bin | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.41) | Name | sql_log_bin | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
This variable controls whether logging to the binary log is
done. The default value is 1 (do logging). To change logging
for the current session, change the session value of this
variable. The session user must have the
SUPER privilege to set this
variable.
Beginning with MySQL 5.5.5, it is no longer possible to set
@@session.sql_log_bin within a transaction
or subquery. (Bug #53437)
| System Variable (<= 5.5.2) | Name | sql_log_off | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | sql_log_off | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 0 | ||
This variable controls whether logging to the general query
log is done. The default value is 0 (do logging). To change
logging for the current session, change the session value of
this variable. The session user must have the
SUPER privilege to set this
option. The default value is 0.
| Deprecated | 5.0.0, by sql_log_bin | ||
| Removed | 5.5.3 | ||
| System Variable | Name | sql_log_update | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
This variable is deprecated, and is mapped to
sql_log_bin. It was removed
in MySQL 5.5.3.
| Command-Line Format | --sql-mode=name | ||
| System Variable | Name | sql_mode | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | set | |
| Default | '' | ||
| Valid Values | ALLOW_INVALID_DATES | ||
ANSI_QUOTES | |||
ERROR_FOR_DIVISION_BY_ZERO | |||
HIGH_NOT_PRECEDENCE | |||
IGNORE_SPACE | |||
NO_AUTO_CREATE_USER | |||
NO_AUTO_VALUE_ON_ZERO | |||
NO_BACKSLASH_ESCAPES | |||
NO_DIR_IN_CREATE | |||
NO_ENGINE_SUBSTITUTION | |||
NO_FIELD_OPTIONS | |||
NO_KEY_OPTIONS | |||
NO_TABLE_OPTIONS | |||
NO_UNSIGNED_SUBTRACTION | |||
NO_ZERO_DATE | |||
NO_ZERO_IN_DATE | |||
ONLY_FULL_GROUP_BY | |||
PAD_CHAR_TO_FULL_LENGTH | |||
PIPES_AS_CONCAT | |||
REAL_AS_FLOAT | |||
STRICT_ALL_TABLES | |||
STRICT_TRANS_TABLES | |||
The current server SQL mode, which can be set dynamically. For details, see Section 5.1.8, “Server SQL Modes”.
MySQL installation programs may configure the SQL mode during the installation process. If the SQL mode differs from the default or from what you expect, check for a setting in an option file that the server reads at startup.
If set to 1 (the default), warnings of Note
level increment warning_count and the
server records them. If set to 0, Note
warnings do not increment
warning_count and the server
does not record them. mysqldump includes
output to set this variable to 0 so that reloading the dump
file does not produce warnings for events that do not affect
the integrity of the reload operation.
If set to 1 (the default), the server quotes identifiers for
SHOW CREATE TABLE and
SHOW CREATE DATABASE
statements. If set to 0, quoting is disabled. This option is
enabled by default so that replication works for identifiers
that require quoting. See Section 13.7.5.12, “SHOW CREATE TABLE Syntax”,
and Section 13.7.5.8, “SHOW CREATE DATABASE Syntax”.
If set to 1, MySQL aborts
UPDATE or
DELETE statements that do not
use a key in the WHERE clause or a
LIMIT clause. (Specifically,
UPDATE statements must have a
WHERE clause that uses a key or a
LIMIT clause, or both.
DELETE statements must have
both.) This makes it possible to catch
UPDATE or
DELETE statements where keys
are not used properly and that would probably change or delete
a large number of rows. The default value is 0.
| System Variable | Name | sql_select_limit | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
The maximum number of rows to return from
SELECT statements. The default
value for a new connection is the maximum number of rows that
the server permits per table. Typical default values are
(232)−1 or
(264)−1. If you have changed
the limit, the default value can be restored by assigning a
value of DEFAULT.
If a SELECT has a
LIMIT clause, the LIMIT
takes precedence over the value of
sql_select_limit.
This variable controls whether single-row
INSERT statements produce an
information string if warnings occur. The default is 0. Set
the value to 1 to produce an information string.
| Command-Line Format | --ssl-ca=file_name | ||
| System Variable | Name | ssl_ca | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | file name | |
The path to a file with a list of trusted SSL CAs.
| Command-Line Format | --ssl-capath=dir_name | ||
| System Variable | Name | ssl_capath | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | directory name | |
The path to a directory that contains trusted SSL CA certificates in PEM format.
| Command-Line Format | --ssl-cert=file_name | ||
| System Variable | Name | ssl_cert | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | file name | |
The name of the SSL certificate file to use for establishing a secure connection.
| Command-Line Format | --ssl-cipher=name | ||
| System Variable | Name | ssl_cipher | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
A list of permissible ciphers to use for SSL encryption.
| Command-Line Format | --ssl-key=file_name | ||
| System Variable | Name | ssl_key | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | file name | |
The name of the SSL key file to use for establishing a secure connection.
| System Variable | Name | storage_engine | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (<= 5.5.4) | Type | enumeration | |
| Default | MyISAM | ||
| Permitted Values (>= 5.5.5) | Type | enumeration | |
| Default | InnoDB | ||
The default storage engine (table type). To set the storage
engine at server startup, use the
--default-storage-engine
option. See Section 5.1.4, “Server Command Options”.
This variable is deprecated as of MySQL 5.5.3. Use
default_storage_engine
instead.
| Introduced | 5.5.21 | ||
| Command-Line Format | --stored-program-cache=# | ||
| System Variable | Name | stored_program_cache | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 256 | ||
| Min Value | 256 | ||
| Max Value | 524288 | ||
Sets a soft upper limit for the number of cached stored routines per connection. The value of this variable is specified in terms of the number of stored routines held in each of the two caches maintained by the MySQL Server for, respectively, stored procedures and stored functions.
Whenever a stored routine is executed this cache size is checked before the first or top-level statement in the routine is parsed; if the number of routines of the same type (stored procedures or stored functions according to which is being executed) exceeds the limit specified by this variable, the corresponding cache is flushed and memory previously allocated for cached objects is freed. This allows the cache to be flushed safely, even when there are dependencies between stored routines.
| Command-Line Format | --sync-frm | ||
| System Variable | Name | sync_frm | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | TRUE | ||
If this variable is set to 1, when any nontemporary 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.
| System Variable | Name | system_time_zone | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
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 10.6, “MySQL Server Time Zone Support”.
| System Variable | Name | table_definition_cache | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 400 | ||
| Min Value | 400 | ||
| Max Value | 524288 | ||
The number of table definitions (from
.frm files) that can be stored in the
definition cache. If you use a large number of tables, you can
create a large table definition cache to speed up opening of
tables. The table definition cache takes less space and does
not use file descriptors, unlike the normal table cache. The
minimum and default values are both 400.
| Removed | 5.5.3 | ||
| Command-Line Format | --table_lock_wait_timeout=# | ||
| System Variable | Name | table_lock_wait_timeout | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 50 | ||
| Min Value | 1 | ||
| Max Value | 1073741824 | ||
This variable is unused. It was removed in 5.5.3.
| System Variable | Name | table_open_cache | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 400 | ||
| Min Value | 1 | ||
| Max Value | 524288 | ||
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 5.1.7, “Server Status Variables”. If
the value of Opened_tables
is large and you do not use
FLUSH TABLES
often (which just forces all tables to be closed and
reopened), then you should increase the value of the
table_open_cache variable.
For more information about the table cache, see
Section 8.4.3.1, “How MySQL Opens and Closes Tables”.
This variable was removed in MySQL 5.5.3. Use
storage_engine instead.
| Command-Line Format | --thread_cache_size=# | ||
| System Variable | Name | thread_cache_size | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 16384 | ||
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 does not provide a notable performance
improvement if you have a good thread implementation. However,
if your server sees hundreds of connections per second you
should normally set
thread_cache_size high enough
so that most new connections use cached threads. 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 5.1.7, “Server Status Variables”.
| Command-Line Format | --thread_concurrency=# | ||
| System Variable | Name | thread_concurrency | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 10 | ||
| Min Value | 1 | ||
| Max Value | 512 | ||
This variable is specific to Solaris 8 and earlier systems,
for which mysqld invokes the
thr_setconcurrency() function with the
variable 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. Current Solaris versions
document this as having no effect.
| Command-Line Format | --thread_handling=name | ||
| System Variable | Name | thread_handling | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values (<= 5.5.13) | Type | enumeration | |
| Default | one-thread-per-connection | ||
| Valid Values | no-threads | ||
one-thread-per-connection | |||
| Permitted Values (>= 5.5.16) | Type | enumeration | |
| Default | one-thread-per-connection | ||
| Valid Values | no-threads | ||
one-thread-per-connection | |||
dynamically-loaded | |||
The thread-handling model used by the server for connection
threads. The permissible user-settable values are
no-threads (the server uses a single thread
to handle one connection) and
one-thread-per-connection (the server uses
one thread to handle each client connection; this is the
default). no-threads is useful for
debugging under Linux; see Section 24.5, “Debugging and Porting MySQL”.
If the thread pool plugin is enabled, the server sets the
thread_handling value to
dynamically-loaded. See
Section 5.5.4.2, “Thread Pool Installation”.
| Introduced | 5.5.16 | ||
| Command-Line Format | --thread_pool_algorithm=# | ||
| System Variable | Name | thread_pool_algorithm | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 1 | ||
This variable controls which algorithm the thread pool plugin uses:
A value of 0 (the default) uses a conservative low-concurrency algorithm which is most well tested and is known to produce very good results.
A value of 1 increases the concurrency and uses a more aggressive algorithm which at times has been known to perform 5–10% better on optimal thread counts, but has degrading performance as the number of connections increases. Its use should be considered as experimental and not supported.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”
thread_pool_high_priority_connection
| Introduced | 5.5.16 | ||
| Command-Line Format | --thread_pool_high_priority_connection=# | ||
| System Variable | Name | thread_pool_high_priority_connection | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 1 | ||
This variable affects queuing of new statements prior to execution. If the value is 0 (false, the default), statement queuing uses both the low-priority and high-priority queues. If the value is 1 (true), queued statements always go to the high-priority queue.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”
thread_pool_max_unused_threads
| Introduced | 5.5.16 | ||
| Command-Line Format | --thread_pool_max_unused_threads=# | ||
| System Variable | Name | thread_pool_max_unused_threads | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 0 | ||
| Min Value | 0 | ||
| Max Value | 4096 | ||
The maximum permitted number of unused threads in the thread pool. This variable makes it possible to limit the amount of memory used by sleeping threads.
A value of 0 (the default) means no limit on the number of
sleeping threads. A value of N
where N is greater than 0 means 1
consumer thread and N−1
reserve threads. In this case, if a thread is ready to sleep
but the number of sleeping threads is already at the maximum,
the thread exits rather than going to sleep.
A sleeping thread is either sleeping as a consumer thread or a reserve thread. The thread pool permits one thread to be the consumer thread when sleeping. If a thread goes to sleep and there is no existing consumer thread, it will sleep as a consumer thread. When a thread must be woken up, a consumer thread is selected if there is one. A reserve thread is selected only when there is no consumer thread to wake up.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”
| Introduced | 5.5.16 | ||
| Command-Line Format | --thread_pool_prio_kickup_timer=# | ||
| System Variable | Name | thread_pool_prio_kickup_timer | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 1000 | ||
| Min Value | 0 | ||
| Max Value | 4294967294 | ||
This variable affects statements waiting for execution in the low-priority queue. The value is the number of milliseconds before a waiting statement is moved to the high-priority queue. The default is 1000 (1 second). The range of values is 0 to 232 − 2.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”
| Introduced | 5.5.16 | ||
| Command-Line Format | --thread_pool_size=# | ||
| System Variable | Name | thread_pool_size | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | integer | |
| Default | 16 | ||
| Min Value | 1 | ||
| Max Value | 64 | ||
The number of thread groups in the thread pool. This is the most important parameter controlling thread pool performance. It affects how many statements can execute simultaneously. The default value is 16, with a range from 1 to 64 of permissible values. If a value outside this range is specified, the thread pool plugin does not load and the server writes a message to the error log.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”
| Introduced | 5.5.16 | ||
| Command-Line Format | --thread_pool_stall_limit=# | ||
| System Variable | Name | thread_pool_stall_limit | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 6 | ||
| Min Value | 4 | ||
| Max Value | 600 | ||
This variable affects executing statements. The value is the amount of time a statement has to finish after starting to execute before it becomes defined as stalled, at which point the thread pool permits the thread group to begin executing another statement. The value is measured in 10 millisecond units, so a value of 6 (the default) means 60ms. The range of values is 4 to 600 (40ms to 6s). Short wait values permit threads to start more quickly. Short values are also better for avoiding deadlock situations. Long wait values are useful for workloads that include long-running statements, to avoid starting too many new statements while the current ones execute.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”
| Command-Line Format | --thread_stack=# | ||
| System Variable | Name | thread_stack | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values (32-bit platforms) | Type | integer | |
| Default | 196608 | ||
| Min Value | 131072 | ||
| Max Value | 4294967295 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 262144 | ||
| Min Value | 131072 | ||
| Max Value | 18446744073709547520 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, >= 5.5.3) | Type | integer | |
| Default | 262144 | ||
| Min Value | 131072 | ||
| Max Value | 18446744073709551615 | ||
| Block Size | 1024 | ||
The stack size for each thread. Many of the limits detected by
the crash-me test are dependent on this
value. See Section 8.13.2, “The MySQL Benchmark Suite”. The default of
192KB (256KB for 64-bit systems) is large enough for normal
operation. If the thread stack size is too small, it limits
the complexity of the SQL statements that the server can
handle, the recursion depth of stored procedures, and other
memory-consuming actions.
This variable is unused.
| System Variable | Name | time_zone | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | string | |
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 10.6, “MySQL Server Time Zone Support”.
| Deprecated | 5.5.39 | ||
| Command-Line Format | --timed_mutexes | ||
| System Variable | Name | timed_mutexes | |
| Variable Scope | Global | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | OFF | ||
This variable is deprecated; it has no use. It will be removed in a future MySQL release.
| System Variable | Name | timestamp | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | numeric | |
Set the time for this client. This is used to get the original
timestamp if you use the binary log to restore rows.
timestamp_value should be a Unix
epoch timestamp (a value like that returned by
UNIX_TIMESTAMP(), not a value
in 'YYYY-MM-DD hh:mm:ss' format) or
DEFAULT.
Setting timestamp to a
constant value causes it to retain that value until it is
changed again. Setting
timestamp to
DEFAULT causes its value to be the current
date and time as of the time it is accessed.
SET timestamp affects the value returned by
NOW() but not by
SYSDATE(). This means that
timestamp settings in the binary log have no effect on
invocations of SYSDATE(). The
server can be started with the
--sysdate-is-now option to
cause SYSDATE() to be an alias
for NOW(), in which case
SET timestamp affects both functions.
| Command-Line Format | --tmp_table_size=# | ||
| System Variable | Name | tmp_table_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | integer | |
| Default | 16777216 | ||
| Min Value | 1024 | ||
| Max Value | 18446744073709551615 | ||
The maximum size of internal in-memory temporary tables. This
variable does not apply to user-created
MEMORY tables.
The actual limit is determined from whichever of the values of
tmp_table_size and
max_heap_table_size is
smaller. 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.
You can compare the number of internal on-disk temporary
tables created to the total number of internal temporary
tables created by comparing the values of the
Created_tmp_disk_tables and
Created_tmp_tables
variables.
See also Section 8.4.4, “Internal Temporary Table Use in MySQL”.
| Command-Line Format | --tmpdir=dir_name | ||
| System Variable | Name | tmpdir | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | directory name | |
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.
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 file system 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. You can set the slave's temporary directory using the
slave_load_tmpdir variable.
In that case, the slave will not use the general
tmpdir value and you can set
tmpdir to a nonpermanent
location.
| Command-Line Format | --transaction_alloc_block_size=# | ||
| System Variable | Name | transaction_alloc_block_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (>= 5.5.43) | Type | integer | |
| Default | 8192 | ||
| Min Value | 1024 | ||
| Max Value | 131072 | ||
| Block Size | 1024 | ||
| Permitted Values (32-bit platforms, <= 5.5.42) | Type | integer | |
| Default | 8192 | ||
| Min Value | 1024 | ||
| Max Value | 4294967295 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 8192 | ||
| Min Value | 1024 | ||
| Max Value | 18446744073709547520 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, >= 5.5.3, <= 5.5.42) | Type | integer | |
| Default | 8192 | ||
| Min Value | 1024 | ||
| Max Value | 18446744073709551615 | ||
| Block Size | 1024 | ||
The amount in bytes by which to increase a per-transaction
memory pool which needs memory. See the description of
transaction_prealloc_size.
| Command-Line Format | --transaction_prealloc_size=# | ||
| System Variable | Name | transaction_prealloc_size | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (>= 5.5.43) | Type | integer | |
| Default | 4096 | ||
| Min Value | 1024 | ||
| Max Value | 131072 | ||
| Block Size | 1024 | ||
| Permitted Values (32-bit platforms, <= 5.5.42) | Type | integer | |
| Default | 4096 | ||
| Min Value | 1024 | ||
| Max Value | 4294967295 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, <= 5.5.2) | Type | integer | |
| Default | 4096 | ||
| Min Value | 1024 | ||
| Max Value | 18446744073709547520 | ||
| Block Size | 1024 | ||
| Permitted Values (64-bit platforms, >= 5.5.3, <= 5.5.42) | Type | integer | |
| Default | 4096 | ||
| Min Value | 1024 | ||
| Max Value | 18446744073709551615 | ||
| Block Size | 1024 | ||
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.
| System Variable | Name | tx_isolation | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | enumeration | |
| Default | REPEATABLE-READ | ||
| Valid Values | READ-UNCOMMITTED | ||
READ-COMMITTED | |||
REPEATABLE-READ | |||
SERIALIZABLE | |||
The default transaction isolation level. Defaults to
REPEATABLE-READ.
This variable can be set directly, or indirectly using the
SET TRANSACTION statement. See
Section 13.3.6, “SET TRANSACTION Syntax”. If you set
tx_isolation directly to an
isolation level name that contains a space, the name should be
enclosed within quotation marks, with the space replaced by a
dash. For example:
SET tx_isolation = 'READ-COMMITTED';
Any unique prefix of a valid value may be used to set the value of this variable.
The default transaction isolation level can also be set at
startup using the
--transaction-isolation server
option.
| System Variable (<= 5.5.2) | Name | unique_checks | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| System Variable (>= 5.5.3) | Name | unique_checks | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 1 | ||
If set to 1 (the default), uniqueness checks for secondary
indexes in InnoDB tables are performed. If
set to 0, storage engines are permitted to assume that
duplicate keys are not present in input data. If you know for
certain that your data does not contain uniqueness violations,
you can set this to 0 to speed up large table imports to
InnoDB.
Setting this variable to 0 does not require storage engines to ignore duplicate keys. An engine is still permitted to check for them and issue duplicate-key errors if it detects them.
| Command-Line Format | --updatable_views_with_limit=# | ||
| System Variable | Name | updatable_views_with_limit | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 1 | ||
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.
The version number for the server. The value might also
include a suffix indicating server build or configuration
information. -log indicates that one or
more of the general log, slow query log, or binary log are
enabled. -debug indicates that the server was
built with debugging support enabled.
| System Variable | Name | version_comment | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The CMake configuration program has a
COMPILATION_COMMENT option that
permits a comment to be specified when building MySQL. This
variable contains the value of that comment. See
Section 2.9.4, “MySQL Source-Configuration Options”.
| System Variable | Name | version_compile_machine | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The type of machine or architecture on which MySQL was built.
| System Variable | Name | version_compile_os | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The type of operating system on which MySQL was built.
| Command-Line Format | --wait_timeout=# | ||
| System Variable | Name | wait_timeout | |
| Variable Scope | Global, Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values (Windows) | Type | integer | |
| Default | 28800 | ||
| Min Value | 1 | ||
| Max Value | 2147483 | ||
| Permitted Values (Other) | Type | integer | |
| Default | 28800 | ||
| Min Value | 1 | ||
| Max Value | 31536000 | ||
The number of seconds the server waits for activity on a noninteractive connection before closing it.
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.
The number of errors, warnings, and notes that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.41, “SHOW WARNINGS Syntax”.