Converting HEAP to MyISAM

The state "converting HEAP to MyISAM" happens when a query that needs a temporary table is converting from an in-memory temporary table to a disk-based temporary table.

MySQL uses memory-based temporary tables up to the size limit set by the tmp_table_size system variable. If a query needs a temporary table larger than this it will be converted to a disk-based temporary table using the MyISAM storage engine.

GROUP BY queries and ORDER BY queries that can't use an index for the ordering are the most common causes of temporary table creation.

Solution

You could consider raising the per-session value of tmp_table_size if you have sufficient memory. Use the SHOW GLOBAL STATUSstatement to see the value of the Created_tmp_tables variable. It will show the total number of temporary tables that have been created:

SHOW GLOBAL STATUS LIKE 'Created_tmp_tables';

+--------------------+-------+

| Variable_name      | Value |

+--------------------+-------+

| Created_tmp_tables | 13    | 

+--------------------+-------+