运行时参数tmp_table_size和max_heap_table_size

 

可用到的语句：

SET GLOBAL max_heap_table_size=522715200;512M

SHOW status like 'created%';

SHOW variables LIKE '%table_size';

 

以下内容整理自mysql手册（tmp_table_size/max_heap_table_size）： 

tmp_table_size

Command-Line Format	--tmp_table_size=#
Option-File Format	tmp_table_size
System Variable Name	tmp_table_size
Variable Scope	Global, Session
Dynamic Variable	Yes
	Permitted Values
Type	numeric
Default	system dependent
Range	1024 .. 4294967295

The maximum size of internal in-memory temporary tables. (The actual limit is determined as the minimum oftmp_table_size and max_heap_table_size.) If an in-memory temporary table exceeds the limit, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size (andmax_heap_table_size if necessary) if you do many advanced GROUP BY queries and you have lots of memory. This variable does not apply to user-created MEMORY tables.

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_tablesvariables.

 

 

 max_heap_table_size

Command-Line Format	--max_heap_table_size=#
Option-File Format	max_heap_table_size
System Variable Name	max_heap_table_size
Variable Scope	Global, Session
Dynamic Variable	Yes
	Permitted Values
Platform Bit Size	32
Type	numeric
Default	16777216
Range	16384 .. 4294967295
	Permitted Values
Platform Bit Size	64
Type	numeric
Default	16777216
Range	16384 .. 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 existingMEMORY table, unless the table is re-created with a statement such as CREATE TABLE or altered with ALTER TABLEor TRUNCATE TABLE. A server restart also sets the maximum size of existing MEMORY tables to the globalmax_heap_table_size value.

This variable is also used in conjunction with tmp_table_size to limit the size of internal in-memory tables. SeeSection 8.8.5, “How MySQL Uses Internal Temporary Tables”.

 

 

8.8.5. How MySQL Uses Internal Temporary Tables

In some cases, the server creates internal temporary tables while processing queries. Such a table can be held in memory and processed by the MEMORY storage engine, or stored on disk and processed by the MyISAM storage engine. The server may create a temporary table initially as an in-memory table, then convert it to an on-disk table if it becomes too large. Users have no direct control over when the server creates an internal temporary table or which storage engine the server uses to manage it.

Temporary tables can be created under conditions such as these:

• If there is an ORDER BY clause and a different GROUP BY clause, or if the ORDER BY or GROUP BY contains columns from tables other than the first table in the join queue, a temporary table is created.

• DISTINCT combined with ORDER BY may require a temporary table.

• If you use the SQL_SMALL_RESULT option, MySQL uses an in-memory temporary table, unless the query also contains elements (described later) that require on-disk storage.

To determine whether a query requires a temporary table, use EXPLAIN and check the Extra column to see whether it says Using temporary. See Section 8.2.1, “Optimizing Queries with EXPLAIN”.

Some conditions prevent the use of an in-memory temporary table, in which case the server uses an on-disk table instead:

• Presence of a BLOB or TEXT column in the table

• Presence of any string column in a GROUP BY or DISTINCT clause larger than 512 bytes

• Presence of any string column with a maximum length larger than 512 (bytes for binary strings, characters for nonbinary strings) in the SELECT list, if UNION or UNION ALL is used

The SHOW COLUMNS and The DESCRIBE statements use BLOB as the type for some columns, thus the temporary table used for the results is an on-disk table.

If an internal temporary table is created initially as an in-memory table but becomes too large, MySQL automatically converts it to an on-disk table. The maximum size for in-memory temporary tables is the minimum of the tmp_table_size and max_heap_table_size values. This differs from MEMORY tables explicitly created withCREATE TABLE: For such tables, the max_heap_table_size system variable determines how large the table is permitted to grow and there is no conversion to on-disk format.

When the server creates an internal temporary table (either in memory or on disk), it increments theCreated_tmp_tables status variable. If the server creates the table on disk (either initially or by converting an in-memory table) it increments the Created_tmp_disk_tables status variable.