In some cases, the server creates internal temporary tables while processing queries. A temporary table can be held in memory and processed by the MEMORY storage engine, or stored on disk and processed by the MyISAM storage engine. Temporary tables can be created under conditions such as these:

You can tell whether a query requires a temporary table by using EXPLAIN and checking the Extra column to see whether it says Using temporary. See Section 12.3.2, “EXPLAIN Syntax”.

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Some conditions prevent the use of a MEMORY temporary table, in which case the server uses a MyISAM table instead:

A temporary table that is created initially as a MEMORY table might be converted to a MyISAM table and stored on disk if it becomes too large. The max_heap_table_size system variable determines how large MEMORY tables are allowed to grow. It applies to all MEMORY tables, including those created with CREATE TABLE. However, for internal MEMORY tables, the actual maximum size is determined by max_heap_table_size in combination with tmp_table_size: Whichever value is smaller is the one that applies. If the size of an internal MEMORY table exceeds the limit, MySQL automatically converts it to an on-disk MyISAM table.