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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;



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