单表优化案例

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
);

先删掉索引

两表优化案例

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)));

EXPLAIN SELECT * FROM class LEFT JOIN book ON book.card = class.card;

CREATE INDEX Y ON book(card);

DROP INDEX Y ON book;

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

DROP INDEX Y ON class;

上图有trpe：ALL

添加索引优化(左连接将索引创建在右表上更合适，右连接将索引创建在左表上更合适。)
CREATE INDEX Y ON book(card); #左外添加右表

删除索引
DROP INDEX Y ON book;

同样的查询，换为左表索引
ALTER TABLE class ADD INDEX Y(card);

type变为了index了。

三表优化案例

先删除上两张表索引
没有建立索引的情况

建立索引后

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

/* 在book表创建索引 */
CREATE INDEX Y ON book(card);

/* 在phone表上创建索引 */
CREATE INDEX X ON phone(card);

DROP INDEX Y ON book;
DROP INDEX X ON phone;

结论

Join优化：

• 尽量可能减少JOIN语句的NestLoop的循环次数，“永远小表驱动大表”
• 优先优化NestedLoop的内层循环
• 保证JOIN语句中被驱动表上JOIN条件字段已经被索引。
• 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Join Buffer 的设置。