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:

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:

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.