gistfile1.sql

mysql> explain SELECT `groups`.* FROM `groups` INNER JOIN `memberships` ON `groups`.id = `memberships`.group_id WHERE ((`memberships`.user_id = 653));
+----+-------------+-------------+--------+------------------------+------------+---------+----------------------------------------------+------+--------------------------+
| id | select_type | table       | type   | possible_keys          | key        | key_len | ref                                          | rows | Extra                    |
+----+-------------+-------------+--------+------------------------+------------+---------+----------------------------------------------+------+--------------------------+
|  1 | SIMPLE      | memberships | ref    | test_index,test_index1 | test_index | 5       | const                                        |    1 | Using where; Using index |
|  1 | SIMPLE      | groups      | eq_ref | PRIMARY                | PRIMARY    | 4       | simplewants_development.memberships.group_id |    1 |                          |
+----+-------------+-------------+--------+------------------------+------------+---------+----------------------------------------------+------+--------------------------+
2 rows in set (0.00 sec)

mysql> explain SELECT `users`.* FROM `users` INNER JOIN `memberships` ON `users`.id = `memberships`.user_id WHERE ((`memberships`.group_id = 449));
+----+-------------+-------------+--------+------------------------+-------------+---------+---------------------------------------------+------+--------------------------+
| id | select_type | table       | type   | possible_keys          | key         | key_len | ref                                         | rows | Extra                    |
+----+-------------+-------------+--------+------------------------+-------------+---------+---------------------------------------------+------+--------------------------+
|  1 | SIMPLE      | memberships | ref    | test_index,test_index1 | test_index1 | 5       | const                                       |    2 | Using where; Using index |
|  1 | SIMPLE      | users       | eq_ref | PRIMARY                | PRIMARY     | 4       | simplewants_development.memberships.user_id |    1 |                          |
+----+-------------+-------------+--------+------------------------+-------------+---------+---------------------------------------------+------+--------------------------+
2 rows in set (0.01 sec)