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
