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。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | 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 |
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