mysql> EXPLAIN -- .... (truncated) .... index: (`userID`).
   -> ;
+----+-------------+-------+--------+---------------------+-----------+---------+-----------------------+------+-------------+
| id | select_type | table | type   | possible_keys       | key       | key_len | ref                   | rows | Extra       |
+----+-------------+-------+--------+---------------------+-----------+---------+-----------------------+------+-------------+
|  1 | SIMPLE      | cm    | ref    | IX_byClub,IX_byUser | IX_byUser | 4       | const                 |  672 | Using where |
|  1 | SIMPLE      | c     | eq_ref | PRIMARY             | PRIMARY   | 4       | invisionapp.cm.clubID |    1 | NULL        |
+----+-------------+-------+--------+---------------------+-----------+---------+-----------------------+------+-------------+
2 rows in set (0.00 sec)