CREATE TABLE t1 ( a INT, b INT, c INT, KEY bc(b, c) )
PARTITION BY KEY (a, b) PARTITIONS 3
;
INSERT INTO t1 VALUES
(17, 1, -8),
(3,  1, -7),
(23, 1, -6),
(22, 1, -5),
(11, 1, -4),
(21, 1, -3),
(19, 1, -2),
(30, 1, -1),
(20, 1, 1),
(16, 1, 2),
(18, 1, 3),
(9,  1, 4),
(15, 1, 5),
(28, 1, 6),
(29, 1, 7),
(25, 1, 8),
(10, 1, 9),
(13, 1, 10),
(27, 1, 11),
(24, 1, 12),
(12, 1, 13),
(26, 1, 14),
(14, 1, 15)
;
SELECT          b, c FROM t1 WHERE b = 1;
b	c
1	-8
1	-7
1	-6
1	-5
1	-4
1	-3
1	-2
1	-1
1	1
1	2
1	3
1	4
1	5
1	6
1	7
1	8
1	9
1	10
1	11
1	12
1	13
1	14
1	15
SELECT          b, c FROM t1 WHERE b = 1 GROUP BY b, c;
b	c
1	-8
1	-7
1	-6
1	-5
1	-4
1	-3
1	-2
1	-1
1	1
1	2
1	3
1	4
1	5
1	6
1	7
1	8
1	9
1	10
1	11
1	12
1	13
1	14
1	15
EXPLAIN
SELECT          b, c FROM t1 WHERE b = 1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ref	bc	bc	5	const	61	Using where; Using index
EXPLAIN
SELECT DISTINCT b, c FROM t1 WHERE b = 1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	range	bc	bc	10	NULL	21	Using where; Using index for group-by
DROP TABLE t1;