2020-11-08：在Mysql中，三個字段A、B、C的聯合索引，查詢條件是B、A、C，會用到索引嗎？

福哥答案2020-11-08：

會走索引，原因是mysql優化器會把BAC優化成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