Session System Variables
Several system variables exist only as session variables. These cannot be set
at server startup but can be assigned values at runtime using the SET
statement (except for those that are read only).
Most of them are not displayed by SHOW
, but you can obtain their values using
VARIABLESSELECT
. This section describes the session system
variables. For information about setting or displaying their values, see Section 5.1.5, “Using System
Variables”. For example:
mysql>SELECT @@autocommit;
+--------------+
| @@autocommit |
+--------------+
| 1 |
+--------------+
The lettercase of these variables does not matter.
The following table lists the system variables that have only session scope:
Table 5.3. mysqld Session System Variable
Summary
Name | Cmd-Line | Option file | System Var | Dynamic |
---|---|---|---|---|
autocommit | Yes | Yes | ||
big-tables | Yes | Yes | ||
- Variable: big_tables | Yes | Yes | ||
error_count | Yes | No | ||
foreign_key_checks | Yes | Yes | ||
identity | Yes | Yes | ||
insert_id | Yes | Yes | ||
last_insert_id | Yes | Yes | ||
ndb_table_no_logging | Yes | Yes | ||
ndb_table_temporary | Yes | Yes | ||
profiling | Yes | Yes | ||
rand_seed1 | Yes | Yes | ||
rand_seed2 | Yes | Yes | ||
sql_auto_is_null | Yes | Yes | ||
sql_big_selects | Yes | Yes | ||
sql_big_tables | Yes | Yes | ||
sql_buffer_result | Yes | Yes | ||
sql_log_bin | Yes | Yes | ||
sql_log_off | Yes | Yes | ||
sql_log_update | Yes | Yes | ||
sql_notes | Yes | Yes | ||
sql_quote_show_create | Yes | Yes | ||
sql_safe_updates | Yes | Yes | ||
sql_warnings | Yes | Yes | ||
timestamp | Yes | Yes | ||
transaction_allow_batching | Yes | Yes | ||
unique_checks | Yes | Yes | ||
warning_count | Yes | No |
-
The autocommit mode. If set to 1, all changes to a table take effect
immediately. If set to 0, you must useCOMMIT
to accept a
transaction orROLLBACK
to cancel it.
By default, client connections begin withautocommit
set
to 1. If you changeautocommit
mode from 0 to 1, MySQL performs an
automaticCOMMIT
of any open
transaction. Another way to begin a transaction is to use aSTART TRANSACTION
or
BEGIN
statement. See
Section 12.4.1, “START
,
TRANSACTIONCOMMIT
, andROLLBACK
Syntax”. -
If set to 1, all temporary tables are stored on disk
rather than in memory. This is a little slower, but the errorThe table
does not occur fortbl_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.Note
This variable was formerly named
sql_big_tables
. -
The number of errors that resulted from the last statement that generated
messages. This variable is read only. See Section 12.5.5.18, “SHOW
Syntax”.
ERRORS -
If set to 1 (the default), foreign key constraints for
InnoDB
tables are checked. If set to 0, they are ignored.
Disabling foreign key checking can be useful for reloadingInnoDB
tables in an order different from that required by
their parent/child relationships. See Section 13.6.4.4, “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, andDROP TABLE
drops
tables that have foreign keys that are referred to by other tables.Note
Setting
foreign_key_checks
to 1 does not trigger a scan of the
existing table data. Therefore, rows added to the table whileforeign_key_checks = 0
will not be verified for
consistency. -
This variable is a synonym for the
last_insert_id
variable. It exists for compatibility
with other database systems. You can read its value withSELECT @@identity
, and set it usingSET
.
identity -
The value to be used by the following
INSERT
orALTER TABLE
statement when inserting anAUTO_INCREMENT
value. This is mainly used with the binary
log. -
The value to be returned from
LAST_INSERT_ID()
. This is stored in the binary log when
you useLAST_INSERT_ID()
in a statement that updates a table.
Setting this variable does not update the value returned by themysql_insert_id()
C API function. -
If set to 0 (the default), statement profiling is disabled. If set to 1,
statement profiling is enabled and theSHOW PROFILES
andSHOW PROFILE
statements provide access to profiling
information. See Section 12.5.5.33, “SHOW
Syntax”. This variable was added in MySQL 5.1.24.
PROFILES -
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 12.5.5.33, “SHOW
Syntax”. This variable was added in MySQL 5.1.24.
PROFILES -
The
rand_seed1
andrand_seed2
variables exist as session variables only, and can be set but not read.
Beginning with MySQL 5.1.18, the variables — but not their values — are shown in
the output ofSHOW
.
VARIABLESThe purpose of these variables is to support replication of the
RAND()
function. For statements that invokeRAND()
, 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 variablesrand_seed1
andrand_seed2
so
thatRAND()
on the slave generates the same value as on the master. -
See the description for
rand_seed1
. -
If set to 1 (the default), you can find the last inserted row for a table
that contains anAUTO_INCREMENT
column by using the
following construct:WHERE
auto_increment_column
IS NULLThis behavior is used by some ODBC programs, such as Access.
-
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 ofmax_join_size
). This is useful when an inadvisable
WHERE
statement has been issued. The default value
for a new connection is 1, which allows allSELECT
statements.If you set the
max_join_size
system variable to a value other than
DEFAULT
,sql_big_selects
is set to 0. -
If set to 1,
sql_buffer_result
forces results fromSELECT
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. -
If set to 0, no logging is done to the binary log for the client. The client
must have theSUPER
privilege to set this option. The default value
is 1. -
If set to 1, no logging is done to the general query log for this client. The
client must have theSUPER
privilege to set this option. The default value
is 0. -
This variable is deprecated, and is mapped to
sql_log_bin
. -
If set to 1 (the default), warnings of
Note
level
are recorded. If set to 0,Note
warnings are
suppressed. 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
and
TABLESHOW CREATE
statements. If set to 0, quoting is disabled. This option is
DATABASE
enabled by default so that replication works for identifiers that require
quoting. See Section 12.5.5.12, “SHOW CREATE TABLE
Syntax”, and Section 12.5.5.8, “SHOW CREATE DATABASE
Syntax”. -
If set to 1, MySQL aborts
UPDATE
orDELETE
statements that do not use a key in theWHERE
clause or aLIMIT
clause.
This makes it possible to catchUPDATE
orDELETE
statements where keys are not used properly and
that would probably change or delete a large number of rows. The default value
is 0. -
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. -
timestamp = {
timestamp_value
| DEFAULT}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, not a MySQL timestamp.SET timestamp
affects the value returned byNOW()
but not by
SYSDATE()
. This means that timestamp settings in the
binary log have no effect on invocations ofSYSDATE()
.
The server can be started with the--sysdate-is-now
option to causeSYSDATE()
to
be an alias forNOW()
, in which caseSET
affects both functions.
timestamp -
If set to 1 (the default), uniqueness checks for secondary indexes in
InnoDB
tables are performed. If set to 0, storage engines
are allowed 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 toInnoDB
.Note that setting this variable to 0 does not require storage engines to ignore duplicate keys.
An engine is still allowed to check for them and issue duplicate-key errors if
it detects them. -
The number of errors, warnings, and notes that resulted from the last
statement that generated messages. This variable is read only. See Section 12.5.5.42, “SHOW
Syntax”.
WARNINGS