mysql> EXPLAIN SELECT a.unique_identifier FROM Bilder a LEFT JOIN hotfolder_file                          list b ON (a.unique_identifier=b.barename) WHERE b.barename IS NULL;                                      +----+-------------+-------+-------+---------------+-------------------+--------                          -+------+--------+--------------------------+
| id | select_type | table | type  | possible_keys | key               | key_len                           | ref  | rows   | Extra                    |
+----+-------------+-------+-------+---------------+-------------------+--------                          -+------+--------+--------------------------+
|  1 | SIMPLE      | a     | index | NULL          | unique_identifier | 766                               | NULL | 158370 | Using index              |
|  1 | SIMPLE      | b     | index | NULL          | barename          | 51                                | NULL | 158377 | Using where; Using index |
+----+-------------+-------+-------+---------------+-------------------+--------                          -+------+--------+--------------------------+
2 rows in set (0.00 sec)