mysql> EXPLAIN SELECT DISTINCT(channel) FROM irclog;
+----+-------------+--------+-------+---------------+------------------------+---------+------+--------+------------------------------+
| id | select_type | table  | type  | possible_keys | key                    | key_len | ref  | rows   | Extra                        |
+----+-------------+--------+-------+---------------+------------------------+---------+------+--------+------------------------------+
|  1 | SIMPLE      | irclog | index | NULL          | irclog_day_channel_idx | 124     | NULL | 920939 | Using index; Using temporary | 
+----+-------------+--------+-------+---------------+------------------------+---------+------+--------+------------------------------+
1 row in set (0.00 sec)