MySQL之优化案例

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单表简单案例

模拟数据

CREATE TABLE IF NOT EXISTS `article`(
`id` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
`author_id` INT (10) UNSIGNED NOT NULL,
`category_id` INT(10) UNSIGNED NOT NULL , 
`views` INT(10) UNSIGNED NOT NULL , 
`comments` INT(10) UNSIGNED NOT NULL,
`title` VARBINARY(255) NOT NULL,
`content` TEXT NOT NULL
);
INSERT INTO `article`(`author_id`,`category_id` ,`views` ,`comments` ,`title` ,`content` )VALUES
(1,1,1,1,'1','1'),
(2,2,2,2,'2','2'),
(3,3,3,3,'3','3');

模拟查询

• 查询category_id 为1且comments>1的情况下，观看数量最多的文章

explain select id,author_id from article where category_id = 1 and comments > 1 order by views desc limit 1 --分析sql

• type：all，全表扫描，情况不容乐观
• Using filesort：文件内排序，情况不容乐观*2

如何优化

show index from article --查看表索引

新建索引

ALTER TABLE article ADD INDEX idx_article_ccv (category_id, comments, views)  --第一种方式
CREATE INDEX idx_article_ccv ON article (category_id, comments, views)  --第二种方式

再次查看执行计划

• 全表扫描已解决，但是文件排序依然存在
• 索引不合适

删除并重建索引

DROP INDEX idx_article_ccv ON article -- 删除索引
CREATE INDEX idx_article_ccv ON article (category_id,views)  --重建索引

再次查看执行计划

完美收工

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双表简单案例

模拟数据

CREATE TABLE IF NOT EXISTS `class`(
`id` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
`card` INT (10) UNSIGNED NOT NULL
);
CREATE TABLE IF NOT EXISTS `book`(
`bookid` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
`card` INT (10) UNSIGNED NOT NULL
);
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)));

查看执行计划

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

由于是LEFT JOIN，所以左表是主表，因此第一次索引尝试加在主表上

• 只对左表class新增索引

CREATE INDEX idx_class_card ON class (card)
EXPLAIN SELECT * from class LEFT JOIN book ON class.card = book.card

结论：虽然type变为index，但是扫描行数依然是全表扫描。

• 只对右表book 新增索引

DROP INDEX idx_class_card on class --删除class表索引
CREATE INDEX idx_book_card ON book (card)
EXPLAIN SELECT * from class LEFT JOIN book ON class.card = book.card

结果：type变为ref，rows只扫描了一行。
结论：这是由于LEFT JOIN特性决定的，由于左表数据全都有，所以关键在于如何从右表进行搜索，所以右表一定要添加索引。

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三表简单案例

模拟数据

• 在双表的基础上新增一张phone表

CREATE TABLE IF NOT EXISTS `phone`(
`phoneid` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
`card` INT (10) UNSIGNED NOT NULL
)ENGINE = INNODB;

INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));
INSERT INTO phone(card)VALUES(FLOOR(1+(RAND()*20)));

• 三表均没有新建索引

EXPLAIN SELECT * from class LEFT JOIN book ON class.card = book.card LEFT JOIN phone ON book.card = phone.card

结论： 全表扫描，且使用了连接缓存

• 在phone和book表新增索引

CREATE INDEX idx_phone_card ON phone(card)
CREATE INDEX idx_book_card ON book (card)
EXPLAIN SELECT * from class LEFT JOIN book ON class.card = book.card LEFT JOIN phone ON book.card = phone.card

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总结

• 语句优化应尽可能减少join语句中NestedLoop的循环总次数，即“永远用小结果集驱动大结果集”。
• 优先优化NestedLoop的内层循环。
• 尽量保证join语句中被驱动表的条件字段添加了索引（即LEFT JOIN在右表上添加，反之亦然）。
• 当无法保证被驱动表的条件字段添加索引时，且内存资源充足的前提下，不妨调整join buffer以达到性能优化的目的。