1CREATE TABLE t2970 (a INT, b INT, c INT, d INT, KEY(a), KEY(a, b)) ENGINE=InnoDB; 2INSERT INTO t2970 VALUES (1,1,1,1), (1,2,3,4); 3EXPLAIN SELECT a, COUNT(b), MAX(b) FROM t2970 WHERE a > 0 GROUP BY a ORDER BY a;; 4id 1 5select_type SIMPLE 6table t2970 7type index 8possible_keys a,a_2 9key a_2 10key_len 10 11ref NULL 12rows 2 13Extra Using where; Using index 14DROP TABLE t2970; 15