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15.6.3.5 Temporary Tablespaces

InnoDB uses session temporary tablespaces and a global temporary tablespace.

Session Temporary Tablespaces

Session temporary tablespaces store user-created temporary tables and internal temporary tables created by the optimizer when InnoDB is configured as the storage engine for on-disk internal temporary tables. The internal_tmp_disk_storage_engine variable defines the storage engine for on-disk internal temporary tables and is set to InnoDB by default.

Session temporary tablespaces are allocated to a session from a pool of temporary tablespaces on the first request to create an on-disk temporary table. A maximum of two tablespaces is allocated to a session, one for user-created temporary tables and the other for internal temporary tables created by the optimizer. The temporary tablespaces allocated to a session are used for all on-disk temporary tables created by the session. When a session disconnects, its temporary tablespaces are truncated and released back to the pool. A pool of 10 temporary tablespaces is created when the server is started. The size of the pool never shrinks and tablespaces are added to the pool automatically as necessary. The pool of temporary tablespaces is removed on normal shutdown or on an aborted initialization. Session temporary tablespace files are five pages in size when created and have an .ibt file name extension.

A range of 400 thousand space IDs is reserved for session temporary tablespaces. Because the pool of session temporary tablespaces is recreated each time the server is started, space IDs for session temporary tablespaces are not persisted when the server is shut down and may be reused.

The innodb_temp_tablespaces_dir variable defines the location where session temporary tablespaces are created. The default location is the #innodb_temp directory in the data directory. Startup is refused if the pool of temporary tablespaces cannot be created.

shell> cd BASEDIR/data/#innodb_temp
shell> ls
temp_10.ibt  temp_2.ibt  temp_4.ibt  temp_6.ibt  temp_8.ibt
temp_1.ibt   temp_3.ibt  temp_5.ibt  temp_7.ibt  temp_9.ibt

In statement based replication (SBR) mode, temporary tables created on a slave reside in a single session temporary tablespace that is truncated only when the MySQL server is shut down.

The INNODB_SESSION_TEMP_TABLESPACES table provides metadata about session temporary tablespaces.

The INFORMATION_SCHEMA.INNODB_TEMP_TABLE_INFO table provides metadata about user-created temporary tables that are active in an InnoDB instance.

Global Temporary Tablespace

The global temporary tablespace (ibtmp1) stores rollback segments for changes made to user-created temporary tables.

The innodb_temp_data_file_path variable defines the relative path, name, size, and attributes for global temporary tablespace data files. If no value is specified for innodb_temp_data_file_path, the default behavior is to create a single auto-extending data file named ibtmp1 in the innodb_data_home_dir directory. The initial file size is slightly larger than 12MB.

The global temporary tablespace is removed on normal shutdown or on an aborted initialization, and recreated each time the server is started. The global temporary tablespace receives a dynamically generated space ID when it is created. Startup is refused if the global temporary tablespace cannot be created. The global temporary tablespace is not removed if the server halts unexpectedly. In this case, a database administrator can remove the global temporary tablespace manually or restart the MySQL server. Restarting the MySQL server removes and recreates the global temporary tablespace automatically.

The global temporary tablespace cannot reside on a raw device.

INFORMATION_SCHEMA.FILES provides metadata about the global temporary tablespace. Issue a query similar to this one to view global temporary tablespace metadata:

mysql> SELECT * FROM INFORMATION_SCHEMA.FILES WHERE TABLESPACE_NAME='innodb_temporary'\G

By default, the global temporary tablespace data file is autoextending and increases in size as necessary.

To determine if a global temporary tablespace data file is autoextending, check the innodb_temp_data_file_path setting:

mysql> SELECT @@innodb_temp_data_file_path;
+------------------------------+
| @@innodb_temp_data_file_path |
+------------------------------+
| ibtmp1:12M:autoextend        |
+------------------------------+

To check the size of global temporary tablespace data files, query the INFORMATION_SCHEMA.FILES table using a query similar to this one:

mysql> SELECT FILE_NAME, TABLESPACE_NAME, ENGINE, INITIAL_SIZE, TOTAL_EXTENTS*EXTENT_SIZE 
       AS TotalSizeBytes, DATA_FREE, MAXIMUM_SIZE FROM INFORMATION_SCHEMA.FILES 
       WHERE TABLESPACE_NAME = 'innodb_temporary'\G
*************************** 1. row ***************************
      FILE_NAME: ./ibtmp1
TABLESPACE_NAME: innodb_temporary
         ENGINE: InnoDB
   INITIAL_SIZE: 12582912
 TotalSizeBytes: 12582912
      DATA_FREE: 6291456
   MAXIMUM_SIZE: NULL

TotalSizeBytes shows the current size of the global temporary tablespace data file. For information about other field values, see Section 25.10, “The INFORMATION_SCHEMA FILES Table”.

Alternatively, check the global temporary tablespace data file size on your operating system. The global temporary tablespace data file is located in the directory defined by the innodb_temp_data_file_path variable.

To reclaim disk space occupied by a global temporary tablespace data file, restart the MySQL server. Restarting the server removes and recreates the global temporary tablespace data file according to the attributes defined by innodb_temp_data_file_path.

To limit the size of the global temporary tablespace data file, configure innodb_temp_data_file_path to specify a maximum file size. For example:

[mysqld]
innodb_temp_data_file_path=ibtmp1:12M:autoextend:max:500M

Configuring innodb_temp_data_file_path requires restarting the server.

Temporary Table Undo Logs

Temporary table undo logs reside in the global temporary tablespace (ibtmp1) and are used for user-created temporary tables and related objects. Temporary table undo logs are not redo-logged, as they are not required for crash recovery. They are only used for rollback while the server is running. This special type of undo log benefits performance by avoiding redo logging I/O.

The innodb_rollback_segments configuration option defines the number of rollback segments used by the global temporary tablespace.


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