MySQL索引最左匹配原则

原创转载请注明出处：https://www.cnblogs.com/agilestyle/p/11609103.html

创建一个测试表

 1 CREATE TABLE `test` (
 2   `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
 3   `a` varchar(32) NOT NULL,
 4   `b` varchar(32) NOT NULL,
 5   `c` varchar(64) NOT NULL,
 6   `d` varchar(128) NOT NULL,
 7   `e` varchar(256) NOT NULL,
 8   `create_time` timestamp(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3),  
 9   `update_time` timestamp(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3) ON UPDATE CURRENT_TIMESTAMP(3), 
10   PRIMARY KEY (`id`)
11 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

插入数据

 1 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
 2 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
 3 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
 4 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
 5 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
 6 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
 7 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
 8 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
 9 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
10 INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');

查看索引

1 show index from test;

查看执行计划

1 explain select * from test where id = "1";

1 explain select * from test where a = "1";

1 explain select * from test where b = "1";

1 explain select * from test where a = "1" and b = "1";

创建索引（a和b的联合索引）

1 create index idx_a_b on test(a, b);

查看索引

1 show index from test;

查看执行计划

1 explain select * from test where a = "1";

1 explain select * from test where b = "1";

1 explain select * from test where a = "1" and b = "1";

创建索引（b的索引）

1 create index idx_b on test(b);

查看索引

1 show index from test;

查看执行计划

1 explain select * from test where a = "1";

1 explain select * from test where b = "1";

1 explain select * from test where a = "1" and b = "1";

删除索引

1 drop index idx_a_b on test;
2 drop index idx_b on test;

删除索引后，创建a, b, c 3个字段的联合索引

1 create index idx_a_b_c on test(a, b, c);

查看如下语句的执行计划

 1 -- ref
 2 explain select * from test where a = "1";
 3 -- all 扫全表
 4 explain select * from test where b = "1";
 5 -- all 扫全表
 6 explain select * from test where c = "1";
 7 -- ref
 8 explain select * from test where a = "1" and b = "1" and c = "1";
 9 -- ref
10 explain select * from test where a = "1" and b = "1";
11 -- ref
12 explain select * from test where a = "1" and c = "1"
13 -- all 扫全表
14 explain select * from test where b = "1" and c = "1"