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MySQL 3.23, 4.0, 4.1 Reference Manual :: 7 Optimization :: 7.5 Optimizing the MySQL Server :: 7.5.8 How MySQL Uses Internal Temporary Tables
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7.5.8. 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. A temporary table created initially as an in-memory table may be converted to an on-disk table if it becomes too large.

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 7.2.1, “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 column in a GROUP BY or DISTINCT clause larger than 512 bytes

  • Presence of any column larger than 512 bytes in the SELECT list, if UNION or UNION ALL is used

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 with CREATE TABLE: The max_heap_table_size system variable determines how large the table is allowed 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 the Created_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.


User Comments

Posted by Baron Schwartz on August 14 2007 3:44pm[Delete] [Edit]

MySQL also uses temporary tables when processing subqueries in the FROM clause (derived tables), some UNION queries, and some VIEW queries.

Posted by Kevin Pettit on December 23 2009 3:24pm[Delete] [Edit]

"Presence of a BLOB or TEXT column in the table" alone does not prevent the use of an in-memory temporary table unless the BLOB or TEXT column is in the select list and the query uses a group by or distinct clause.

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