mysql添加索引

1.添加PRIMARY KEY（主键索引）

mysql>ALTER TABLE `table_name` ADD PRIMARY KEY ( `column` )

 2.添加UNIQUE(唯一索引) 
mysql>ALTER TABLE `table_name` ADD UNIQUE ( `column` )

3.添加INDEX(普通索引) 

mysql>ALTER TABLE `table_name` ADD INDEX index_name ( `column` )

4.添加FULLTEXT(全文索引) 

mysql>ALTER TABLE `table_name` ADD FULLTEXT ( `column`)

5.添加多列索引 
mysql>ALTER TABLE `table_name` ADD INDEX index_name ( `column1`, `column2`, `column3` )

https://www.cnblogs.com/henrylinux/p/9746872.html

 

where 语句里面如果带有or条件, myisam引擎表能用到索引，5.7.x版本可以，但是我是了5.6.x没有用到索引

CREATE TABLE IF NOT EXISTS `t_myisam` (
     `id` int(1) NOT NULL AUTO_INCREMENT,
     `uid` int(11) NOT NULL,
     `aNum` char(20) DEFAULT NULL,
     PRIMARY KEY (`id`),
     KEY `uid` (`uid`)
 ) ENGINE=MyISAM  DEFAULT CHARSET=utf8mb4  AUTO_INCREMENT=1 ;

insert into  t_myisam (uid,aNum) values(2,22);
insert into  t_myisam (uid,aNum) values(3,33);
insert into  t_myisam (uid,aNum) values(4,44);

mysql> explain select * from t_myisam  where id=1 or uid =2;

或者拆分成union语句

explain select * from t_myisam  where id=1 UNION SELECT * from t_myisam where uid =2;

 

 摘自https://blog.csdn.net/hguisu/article/details/7106159