Unnecessary filesort on derived table materialization

create table t1 (

  groups_20 int NOT NULL,

  groups_20_2 int NOT NULL,

  b int,

  PRIMARY KEY (groups_20, groups_20_2)

);

insert into t1 select seq/1000, seq+1, seq from seq_1_to_20000;

create table t2 (a int, b int, index(a));

insert into t2 select seq, seq from seq_0_to_1000;

analyze table t1, t2;

MariaDB [test]> analyze select a, sum(b) from    (     select groups_20 from t1       group by groups_20       having count(*)  != 1000   ) DT    join t2 on a = groups_20 group by a;

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

| id   | select_type | table      | type | possible_keys | key  | key_len | ref       | rows  | r_rows   | filtered | r_filtered | Extra                                        |

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

|    1 | PRIMARY     | t2         | ALL  | a             | NULL | NULL    | NULL      | 1001  | 1001.00  |   100.00 |     100.00 | Using where; Using temporary; Using filesort |

|    1 | PRIMARY     | <derived2> | ref  | key0          | key0 | 4       | test.t2.a | 1     | 0.00     |   100.00 |     100.00 |                                              |

|    2 | DERIVED     | t1         | ALL  | PRIMARY       | NULL | NULL    | NULL      | 19735 | 20000.00 |   100.00 |     100.00 | Using temporary; Using filesort              |

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

3 rows in set (0.917 sec)

select groups_20 from t1       group by groups_20       having count(*)  != 1000