mysql:wikiadmin@db1051 [enwiki]> EXPLAIN SELECT ct_tag,count(*) AS `hitcount` FROM `change_tag` GROUP BY ct_tag ORDER BY hitcount DESC;
+------+-------------+------------+-------+---------------+--------+---------+------+----------+----------------------------------------------+
| id   | select_type | table      | type  | possible_keys | key    | key_len | ref  | rows     | Extra                                        |
+------+-------------+------------+-------+---------------+--------+---------+------+----------+----------------------------------------------+
|    1 | SIMPLE      | change_tag | index | NULL          | ct_tag | 272     | NULL | 24735709 | Using index; Using temporary; Using filesort |
+------+-------------+------------+-------+---------------+--------+---------+------+----------+----------------------------------------------+
1 row in set (0.00 sec)