Unneeded filesort when executing a GROUP BY query

[Igor Babaev]

If to create tables t1 and t2 in MariaDB 5.5 as with the following commands

CREATE TABLE t1 (
  col_int_key INT,
  pk INT,
  PRIMARY KEY (pk),
  KEY (col_int_key)
) ENGINE=INNODB;
INSERT INTO t1 VALUES (2,3),(3,2),(3,5),(4,6);
CREATE TABLE t2 (
  col_int_key INT,
  pk INT,
  PRIMARY KEY (pk),
  KEY (col_int_key)
) ENGINE=INNODB;
INSERT INTO t2 VALUES (0,9),(3,10),(4,6),(6,1),(100,3),(200,5);

and

set join_cache_level=0;

then the execution plan for the query

SELECT t2.col_int_key AS field1
FROM t2 USE INDEX(col_int_key) STRAIGHT_JOIN t1 ON t2.col_int_key
WHERE t2.pk < 7 AND t2.col_int_key <> 7
GROUP BY field1;

will use unneeded filesort:

MariaDB [test]> explain SELECT t2.col_int_key AS field1 FROM t2 USE INDEX(col_int_key) STRAIGHT_JOIN t1 ON t2.t_key WHERE t2.pk < 7 AND t2.col_int_key <> 7 GROUP BY field1;
+------+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------------------------------------+
| 1 | SIMPLE | t2 | range | col_int_key | col_int_key | 5 | NULL | 5 | Using where; Usinx; Using temporary; Using filesort |
| 1 | SIMPLE | t1 | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
+------+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------------------------------------+