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;
SELECT          b, c FROM t1 WHERE b = 1 GROUP BY b, c;

EXPLAIN
SELECT          b, c FROM t1 WHERE b = 1;
EXPLAIN
SELECT DISTINCT b, c FROM t1 WHERE b = 1;

DROP TABLE t1;