数据准备

CREATE TABLE IF NOT EXISTS `class` (

`id` int(10) unsigned NOT NULL AUTO_INCREMENT,

`card` int(10) unsigned NOT NULL,

PRIMARY KEY (`id`)

);

CREATE TABLE IF NOT EXISTS `book` (

`bookid` int(10) unsigned NOT NULL AUTO_INCREMENT,

`card` int(10) unsigned NOT NULL,

PRIMARY KEY (`bookid`)

);

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into class(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

insert into book(card) values(floor(1+(RAND()*20)));

两表关联

左连接优化

1、一个左连接查询

explain select * from class left join book on class.card = book.card;

image.png

两个select的type都是ALL

2、在右表book表的card字段上建立索引

ALTER TABLE `book` ADD INDEX y ( `card`);

3、查看索引情况

SHOW INDEX FROM book

image.png

4、再次查看执行计划

explain select * from class left join book on class.card = book.card;

image.png

往左连接的右表上加索引效果非常理想！

5、删除旧索引，在class表上加上新索引。

DROP INDEX y ON book;

ALTER TABLE `class` ADD INDEX x ( `card`);

查询索引情况

SHOW INDEX FROM book

image.png

6、再次查看执行计划

explain select * from class left join book on class.card = book.card;

image.png

在left join的左表class添加索引，效果并不理想！

右连接优化

1、接着上面的操作，class表里添加了索引。

SHOW INDEX FROM class

image.png

查看执行计划

explain select * from class right join book on class.card = book.card;

image.png

2、将class表和table表调换位置，再次查看执行计划

explain select * from book right join class on class.card = book.card;

image.png

三表关联

1、在上面的表结构和基础上，继续创建一个phone表

CREATE TABLE IF NOT EXISTS `phone` (

`phoneid` int(10) unsigned NOT NULL AUTO_INCREMENT,

`card` int(10) unsigned NOT NULL,

PRIMARY KEY (`phoneid`)

) engine = innodb;

2、book、phone 两表添加索引

ALTER TABLE `book` ADD INDEX y ( `card`);

ALTER TABLE `phone` ADD INDEX z ( `card`);

3、查看执行计划

explain select * from class left join book on class.card=book.card left j

oin phone on book.card = phone.card;

image.png

后 2 行的 type 都是 ref 且总 rows 优化很好，效果不错。因此索引最好设置在需要经常查询的字段中

join的优化：

1、小表驱动大表；left join的小表放到左边；因此这里使用class驱动book再是phone ；

2、尽可能减少join语句中的NestedLoop的循环总次数；

3、优先优化NestedLoop的内层循环；

4、保证join语句中“被驱动表”上join条件字段已经被索引(key字段非NULL)；

5、当无法保证“被驱动表”的join条件字段被索引且内存资源充足的前提下，不要太吝惜JoinBuffer的设置；修改my.conf文件

结论

多表join添加索引原则：左连接加右表，右连接加左表

不要添加多余的索引

image.png