原创转载请注明出处：http://agilestyle.iteye.com/blog/2433805
创建一个测试表
CREATE TABLE `test` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `a` varchar(32) NOT NULL, `b` varchar(32) NOT NULL, `c` varchar(64) NOT NULL, `d` varchar(128) NOT NULL, `e` varchar(256) NOT NULL, `create_time` timestamp(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3), `update_time` timestamp(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3) ON UPDATE CURRENT_TIMESTAMP(3), PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
插入数据
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
INSERT INTO `test` (`a`, `b`, `c`, `d`, `e`) VALUES('a', 'b', 'c', 'd', 'e');
查看索引
show index from test;
查看执行计划
explain select * from test where id = "1";
explain select * from test where a = "1";
explain select * from test where b = "1";
explain select * from test where a = "1" and b = "1";
创建索引（a和b的联合索引）
create index idx_a_b on test(a, b);
查看索引
show index from test;
查看执行计划
explain select * from test where a = "1";
explain select * from test where b = "1";
explain select * from test where a = "1" and b = "1";
创建索引（b的索引）
create index idx_b on test(b);
查看索引
show index from test;
查看执行计划
explain select * from test where a = "1";
explain select * from test where b = "1";
explain select * from test where a = "1" and b = "1";
删除索引
drop index idx_a_b on test; 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
create index idx_a_b_c on test(a, b, c);
查看如下语句的执行计划
-- ref explain select * from test where a = "1"; -- all 扫全表 explain select * from test where b = "1"; -- all 扫全表 explain select * from test where c = "1"; -- ref explain select * from test where a = "1" and b = "1" and c = "1"; -- ref explain select * from test where a = "1" and b = "1"; -- ref explain select * from test where a = "1" and c = "1" -- all 扫全表 explain select * from test where b = "1" and c = "1"
