mysql> explain SELECT * FROM user_test AS u1 LEFT OUTER JOIN user_test AS u2	ON u1.date_of_birth = u2.date_of_birth WHERE u1.first_name = 'cee'	;
+----+-------------+-------+------+---------------------+---------------------+---------+---------------------------+------+-----------------------+
| id | select_type | table | type | possible_keys       | key                 | key_len | ref                       | rows | Extra                 |
+----+-------------+-------+------+---------------------+---------------------+---------+---------------------------+------+-----------------------+
|  1 | SIMPLE      | u1    | ref  | first_name_index    | first_name_index    | 152     | const                     |   44 | Using index condition |
|  1 | SIMPLE      | u2    | ref  | date_of_birth_index | date_of_birth_index | 4       | rap_test.u1.date_of_birth |    9 | NULL                  |
+----+-------------+-------+------+---------------------+---------------------+---------+---------------------------+------+-----------------------+
2 rows in set (0.00 sec)