GROUP_MIN_MAX is not applied for ENGINE=HEAP when it could

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (id INT NOT NULL, a INT NOT NULL) ENGINE=HEAP;

INSERT INTO t1 VALUES (1, 1);

INSERT INTO t1 VALUES (1, 2);

INSERT INTO t1 VALUES (1, 3);

INSERT INTO t1 VALUES (1, 4);

INSERT INTO t1 VALUES (2, 1);

INSERT INTO t1 VALUES (2, 2);

INSERT INTO t1 VALUES (2, 3);

INSERT INTO t1 VALUES (2, 4);

INSERT INTO t1 VALUES (3, 1);

INSERT INTO t1 VALUES (3, 2);

INSERT INTO t1 VALUES (3, 3);

INSERT INTO t1 VALUES (3, 4);

INSERT INTO t1 VALUES (4, 1);

INSERT INTO t1 VALUES (4, 2);

INSERT INTO t1 VALUES (4, 3);

INSERT INTO t1 VALUES (4, 4);

ALTER TABLE t1 ADD KEY (id,a) USING BTREE;

EXPLAIN SELECT id,MIN(a) FROM t1 WHERE a=4 GROUP BY id;

EXPLAIN SELECT id,MIN(a) FROM t1 WHERE a>=4 GROUP BY id;

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

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

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

|    1 | SIMPLE      | t1    | index | NULL          | id   | 8       | NULL |   16 | Using where |

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