query_1index

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_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 | index  | test_index    | test_index | 10      | NULL                                        |  100 | Using where; Using index |
|  1 | SIMPLE      | users       | eq_ref | PRIMARY       | PRIMARY    | 4       | simplewants_development.memberships.user_id |    1 |                          |
+----+-------------+-------------+--------+---------------+------------+---------+---------------------------------------------+------+--------------------------+
2 rows in set (0.00 sec)