mysql> explain select distinct channel from irclog;
+----+-------------+--------+-------+---------------+-------------+---------+------+------+--------------------------+
| id | select_type | table  | type  | possible_keys | key         | key_len | ref  | rows | Extra                    |
+----+-------------+--------+-------+---------------+-------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | irclog | range | NULL          | channel_idx | 93      | NULL |    5 | Using index for group-by | 
+----+-------------+--------+-------+---------------+-------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)