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  • 5.3 索引三表优化案例

    1、案例

    #删除之前建立的索引
    drop index X on class;
    
    CREATE TABLE IF NOT EXISTS `phone`(
    	`phoneid` int(10) UNSIGNED NOT NULL auto_increment,
    	`card` int(10) UNSIGNED NOT NULL,
    	PRIMARY KEY(`phoneid`)
    );
    
    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)));
    
    #explain 分析sql
    EXPLAIN SELECT * from class LEFT JOIN book on class.card = book.card LEFT JOIN phone on class.card = phone.card;
    

    结论:都是All ,需要优化。

    2、优化

    #建立索引
    ALTER TABLE `phone` ADD INDEX Z(`card`);
    ALTER TABLE `book` ADD INDEX Y(`card`);
    #再次分析sql
    EXPLAIN SELECT * from class LEFT JOIN book on class.card = book.card LEFT JOIN phone on class.card = phone.card;
    

    【结论】:

    join语句的优化
    尽可能减少join语句中的NestedLoop的循环总次数;“ 永远用小的结果集驱动大的结果集 ”。
    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;
    
    当无法保证被驱动的join条件字段被索引且内存资源充足的前提下,不要吝啬 JoinBuffer 的设置。

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  • 原文地址:https://www.cnblogs.com/huanchupkblog/p/7453184.html
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