mysql> EXPLAIN SELECT * FROM cupid.cupid c LEFT JOIN cupid.cupid_msgs cm ON (cm.receiver_id = c.member_id) WHERE cm.receiver_id IS NULL;+----+-------------+-------+------+---------------+---------+---------+-------------------+---------+--------------------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+---------+---------+-------------------+---------+--------------------------------------+| 1 | SIMPLE | c | ALL | NULL | NULL | NULL | NULL | 1840658 | || 1 | SIMPLE | cm | ref | PRIMARY | PRIMARY | 4 | cupid.c.member_id | 226603 | Using where; Using index; Not exists |+----+-------------+-------+------+---------------+---------+---------+-------------------+---------+--------------------------------------+2 rows in set (0.00 sec)mysql> EXPLAIN SELECT * FROM cupid.cupid c WHERE NOT EXISTS(SELECT * FROM cupid.cupid_msgs WHERE receiver_id = c.member_id); +----+--------------------+------------+------+---------------+---------+---------+-------------------+---------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+--------------------+------------+------+---------------+---------+---------+-------------------+---------+-------------+| 1 | PRIMARY | c | ALL | NULL | NULL | NULL | NULL | 1840658 | Using where || 2 | DEPENDENT SUBQUERY | cupid_msgs | ref | PRIMARY | PRIMARY | 4 | cupid.c.member_id | 226603 | Using index |+----+--------------------+------------+------+---------------+---------+---------+-------------------+---------+-------------+2 rows in set (0.00 sec)