explain SELECT a, b, COUNT(*) FROM tbname

GROUP BY a, b

order by a DESC

limit 1

a 和 b 列已经设置联合索引， 为什么这种操作也会执行全表扫描呢？explain SELECT a, b FROM tbname

GROUP BY a, b

order by a DESC

去掉 COUNT 和 limit 则走索引没有扫描， 这是为什么呢？

这种场景有什么优化思路呢？

表结构CREATE TABLE `tbname` (

`id` INT(11) NOT NULL AUTO_INCREMENT,

`a` INT(11) NULL DEFAULT '0',

`b` INT(11) NULL DEFAULT '0',

PRIMARY KEY (`id`),

INDEX `testkey` (`a`, `b`)

)

COLLATE='utf8_general_ci'

ENGINE=InnoDB;

INSERT INTO `tbname` (`a`, `b`) VALUES ('1', '1');

INSERT INTO `tbname` (`a`, `b`) VALUES ('1', '1');

INSERT INTO `tbname` (`a`, `b`) VALUES ('1', '1');

INSERT INTO `tbname` (`a`, `b`) VALUES ('1', '1');

INSERT INTO `tbname` (`a`, `b`) VALUES ('1', '1');

查询1：explain SELECT a, b, COUNT(*) FROM tbname

GROUP BY a, b

order by a DESC

limit 1

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

| ---: | --- | --- | --- | --- | --- | --- | --- | --- | ---: | ---: | --- |

| 1 | SIMPLE | tbname | \N | index | testkey | testkey | 10 | \N | 5 | 100.00 | Using index; Using temporary; Using filesort |

查询2：explain SELECT a, b FROM tbname

GROUP BY a, b

order by a DESC

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

| ---: | --- | --- | --- | --- | --- | --- | --- | --- | ---: | ---: | --- |

| 1 | SIMPLE | tbname | \N | range | testkey | testkey | 10 | \N | 1 | 100.00 | Using index for group-by; Using temporary; Using filesort |