5.2 索引两表优化案例

1、案例

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)));

#两张表连接查询优化

2、下面使用explain 分析sql

explain SELECT * from class LEFT JOIN book ON class.card = book.card

结论：type 有All ，需要优化

3、优化

#添加索引优化
ALTER TABLE `book` ADD INDEX Y(`card`);

#第2次explain
explain SELECT * from class LEFT JOIN book ON class.card = book.card

结论：

#可以看到第二行的 type 变为了 ref , rows 也变成了 1 优化比较明显。
#这是由左连接特性决定的。LEFT JOIN 条件用于确定从右表搜索行，左边一定都有，
#所以右边是我们的关键点，一定需要建立索引。

3、再次分析

#左连接 改成 右连接
explain SELECT * from class RIGHT JOIN book ON class.card = book.card

#删除索引
drop index Y on book;
#重建索引
create index X on class(card);
#再次分析
explain SELECT * from class RIGHT JOIN book ON class.card = book.card

结论：优化比较明显。这是因为RIGHT JOIN 条件用于确定如何从左表搜索行，右边一定都有，所以左边是我们的关键点，一定需要建立索引。

综上所述 ：我们得到以下结论

左连接：索引建在右表上。
右连接：索引建在左表上。