索引优化：关联查询

• 左外连接

# 创建分类表
CREATE TABLE IF NOT EXISTS `type` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY (`id`)
);
 
# 创建图书表
CREATE TABLE IF NOT EXISTS `book` (
`bookid` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY (`bookid`)
);
 
# 向分类表中添加20条记录
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO TYPE(card) VALUES(FLOOR(1 + (RAND() * 20)));
 
# 向图书表中添加20条记录
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)));
 
# 没有索引时，查询测试
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` LEFT JOIN book ON type.card = book.card;
# 为book表添加索引，为非驱动表添加索引查询效果更好
CREATE INDEX Y ON book(card);
# 测试
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` LEFT JOIN book ON type.card = book.card;
# 为type表添加索引，再次为驱动表添加索引
CREATE INDEX X ON `type`(card);
 
# 测试
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` LEFT JOIN book ON type.card = book.card;
# 删除book表的索引
DROP INDEX Y ON book;
# 测试
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` LEFT JOIN book ON type.card = book.card;
# 以上测试发现，left前面的字段作为驱动表，left后面的字段作为非驱动表
# 但在外连接中前面的表不一定作为驱动表，后面的表不一定作为被驱动表

• 内连接

# 删除索引
DROP INDEX X ON `type`;
 
# 没有索引时测试
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` INNER JOIN book ON type.card = book.card;
 
# 添加索引
CREATE INDEX Y ON book(card);
 
# 内连接测试
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` INNER JOIN book ON type.card = book.card;
 
# 再添加1个索引
CREATE INDEX X ON `type`(card);
 
# 结论：对于内连接来说，查询优化器可以决定谁作为驱动表，谁作为被驱动表出现的
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` INNER JOIN book ON type.card = book.card;
 
# 删除索引
DROP INDEX Y ON book;
# 对于内连接来讲，如果表的连接条件中只能有一个字段有索引，则有索引的字段所在的表会被作为被驱动表出现。
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` INNER JOIN book ON type.card = book.card;
 
# 再将Y索引添加上
CREATE INDEX Y ON book(card);
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` INNER JOIN book ON type.card = book.card;
 
# 向type表中添加数据（20条数据）
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO `type`(card) VALUES(FLOOR(1 + (RAND() * 20)));
 
# 对于内连接来说，在两个表的连接条件都存在索引的情况下，会选择小表作为驱动表。“小表驱动大表”
EXPLAIN SELECT SQL_NO_CACHE * FROM `type` INNER JOIN book ON type.card = book.card;