mysql> explain SELECT id
    -> FROM file
    -> WHERE size IN
    ->     ( SELECT size FROM tmpTab );
+----+--------------------+--------+-------+---------------+----------+---------
+------+-------+--------------------------+
| id | select_type        | table  | type  | possible_keys | key      | key_len
| ref  | rows  | Extra                    |
+----+--------------------+--------+-------+---------------+----------+---------
+------+-------+--------------------------+
|  1 | PRIMARY            | file   | index | NULL          | filesize | 8
| NULL | 21168 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | tmpTab | ALL   | NULL          | NULL     | NULL
| NULL |  3197 | Using where              |
+----+--------------------+--------+-------+---------------+----------+---------
+------+-------+--------------------------+
2 rows in set (0.00 sec)