mysql вопросы tmp

Temporary tables are created on disk when:

TEXT or BLOB fields are present (because MEMORY doesn't support these types)
the size of the resulting implicit temporary table exceeds the lesser of tmp_table_size or max_heap_table_size
If a column w/ more than 512 bytes is used with either a GROUP BY or UNION

mysql> SHOW GLOBAL STATUS LIKE 'Created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 787   |
| Created_tmp_files       | 6     |
| Created_tmp_tables      | 4866  |
+-------------------------+-------+
3 rows in set (0.00 sec)

tmpfs                 512M     0  512M   0% /tmp/mysql_tmpfs