2020-11-08：在Mysql中，三个字段A、B、C的联合索引，查询条件是B、A、C，会用到索引吗？

vlambda
2020-11-09

2020-11-08：在Mysql中，三个字段A、B、C的联合索引，查询条件是B、A、C，会用到索引吗？

福哥答案2020-11-08：

会走索引，原因是mysql优化器会把BAC优化成ABC。

CREATE TABLE `t_testabc2` ( `id` int(11) NOT NULL AUTO_INCREMENT, `A` varchar(255) NOT NULL, `B` varchar(255) NOT NULL, `C` varchar(255) NOT NULL, `D` varchar(255) NOT NULL DEFAULT '''''', PRIMARY KEY (`id`), KEY `abc2` (`A`,`B`,`C`) USING BTREE) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1;
EXPLAIN SELECT * FROM t_testabc2 WHERE B='a2' AND A='a2' AND C='a2' --走索引
EXPLAIN SELECT * FROM t_testabc2 WHERE B >='a1' AND B<='a2' AND A >='a1' AND A<='a2' AND C >='a1' AND C<='a2' --走索引
EXPLAIN SELECT * FROM t_testabc2 WHERE B BETWEEN 'a1' AND 'a2' AND A BETWEEN 'a1' AND 'a2' AND C BETWEEN 'a1' AND 'a2' --走索引
EXPLAIN SELECT * FROM t_testabc2 WHERE B='a1' AND A LIKE 'a%' AND C='a1' --不走索引