My table schema is
CREATE TABLE test (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT ,
`class_id` int(11) unsigned NOT NULL DEFAULT '0' ,
`stu_id` int(11) unsigned NOT NULL DEFAULT '0' ,
`work_id` int(11) unsigned NOT NULL DEFAULT '0',
`img_id` int(11) unsigned NOT NULL DEFAULT '0' ,
`commit_id` int(11) unsigned NOT NULL DEFAULT '0' ,
`test_id` int(11) unsigned NOT NULL DEFAULT '0',
`audio_ids` varchar(500) NOT NULL DEFAULT '0' ,
`correct_time` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' ,
`is_wrong` tinyint(1) unsigned NOT NULL DEFAULT '0' ,
`status` tinyint(1) unsigned NOT NULL DEFAULT '0' ,
PRIMARY KEY (`id`),
KEY `commit_id` (`commit_id`),
KEY `correct_time` (`correct_time`),
KEY `commit_test` (`commit_id`,`test_id`)
) ENGINE=InnoDB AUTO_INCREMENT=242058182
my query this table for commit_id = 93317439 and commit_id = 93317438
explain select count(*) from test where commit_id=93317438
+----+-------------+------------------------+------------+------+---------------+-----------+---------+-------+------
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra
+----+-------------+------------------------+------------+------+---------------+-----------+---------+-------+------
| 1 | SIMPLE | test| NULL | ref | commit_id | commit_id | 4 | const | 1 | 100.00 | Using index |
+----+-------------+------------------------+------------+------+---------------+-----------+---------+-------+------
explain select count(*) from test where commit_id=93317439
+----+-------------+------------------------+------------+------+-----------------------+-----------+---------+-------
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------------+------------+------+-----------------------+-----------+---------+-------| 1 | SIMPLE | test| NULL | ref | commit_id,commit_test | commit_id | 4 | const | 44587972 | 100.00 | Using index |
+----+-------------+------------------------+------------+------+-----------------------+-----------+---------+-------
but commit_id=93317439 only 2 rows