create table t1 (pk int primary key, a int, b int, key(a,b));

insert into t1 with recursive cte(a) as (select 1 as a union select a+1 from cte where a <100) select a, a*10, a*100 from cte;

explain select a, b, count(a) from t1 group by a,      b      order by a desc;

explain select a, b, count(a) from t1 group by a desc, b      order by a desc;

explain select a, b, count(a) from t1 group by a desc, b desc order by a desc;

MariaDB [test]> explain select a, b, count(a) from t1 group by a,      b      order by a desc;

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

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

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

|    1 | SIMPLE      | t1    | index | NULL          | a    | 10      | NULL | 100  | Using index; Using temporary; Using filesort |

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

1 row in set (0.000 sec)

MariaDB [test]> explain select a, b, count(a) from t1 group by a desc, b      order by a desc;

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

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

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

|    1 | SIMPLE      | t1    | index | NULL          | a    | 10      | NULL | 100  | Using index; Using temporary; Using filesort |

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

1 row in set (0.000 sec)

MariaDB [test]> explain select a, b, count(a) from t1 group by a desc, b desc order by a desc;

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

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

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

|    1 | SIMPLE      | t1    | index | NULL          | a    | 10      | NULL | 100  | Using index |

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