Sometimes it really counts to restructure the queries made to your MySQL database, especially so that they do make use of indexes which are present on the table.
You can obtain information on which keys are being used with a query by using the EXPLAIN statement before your SELECT statement. Here are some examples of it’s output.
mysql> EXPLAIN SELECT * FROM USERS WHERE area_id = 2;
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | USERS | ALL | NULL | NULL | NULL | NULL | 6 | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.03 sec)
mysql> EXPLAIN SELECT * FROM USERS WHERE group_id = 16515319;
+----+-------------+---------+------+---------------------------+--------------------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------------------+--------------------------+---------+-------+------+-------+
| 1 | SIMPLE | USERS | ref | index_users_on_group_id | index_users_on_group_id | 4 | const | 1 | |
+----+-------------+---------+------+---------------------------+--------------------------+---------+-------+------+-------+
1 row in set (0.00 sec)