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  • MySQL索引优化(索引两表优化案例)

    建表SQL

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

        

    # 下面开始explain分析
    EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card = book.card;

    #结论:type 有All

    # 添加索引优化
    ALTER TABLE `book` ADD INDEX Y ( `card`);

    # 第2次explain
    EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card = book.card;

    #可以看到第二行的 type 变为了 ref,rows 也变成了优化比较明显。
    #这是由左连接特性决定的。LEFT JOIN 条件用于确定如何从右表搜索行,左边一定都有,
    #所以右边是我们的关键点,一定需要建立索引。

    # 删除旧索引 + 新建 + 第3次explain
    DROP INDEX Y ON book;
    ALTER TABLE class ADD INDEX X (card);

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

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  • 原文地址:https://www.cnblogs.com/116970u/p/10986089.html
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