explain  select distinct count(*) from t2 where a between 10 and 20 group by a;

+------+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                                                               |

+------+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------+

|    1 | SIMPLE      | t2    | range | a             | a    | 5       | NULL |   10 | Using where; Using index; Distinct; Using temporary; Using filesort |

+------+-------------+-------+-------+---------------+------+---------+------+------+---------------------------------------------------------------------+