MySQL--索引两表优化案例分析

素颜马尾好姑娘i 2021-08-14 01:29 518阅读 0赞

### 索引两表优化案例分析 ###

**建表及准备数据：**

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`)
    );
    
    #card为外键
    
    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 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)));
    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)));

**案例：**

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

![在这里插入图片描述][20191124174524349.png]  
结论：type存在ALL全表扫描

先在book的card字段创建索引：

CREATE INDEX idx_book ON book(card);

![在这里插入图片描述][20191124175051755.png]**大致的结论：A表left joinB表，索引加在B表的字段上**

删除索引，在class的card字段创建索引：

DROP INDEX idx_book ON book;
    CREATE INDEX idx_class ON class(card);

![在这里插入图片描述][20191124175350178.png]  
虽然用上了索引，但是效果没那么好。  
所以，结论如下  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70]

[20191124174524349.png]: /images/20210813/4982509da547419fa86a8a2a5f66d1a4.png
[20191124175051755.png]: /images/20210813/65cead7a296642b4ac819fb00b3824b0.png
[20191124175350178.png]: /images/20210813/5daf1cd92fba441a840f0de42829323d.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NvbGRfX19wbGF5_size_16_color_FFFFFF_t_70]: /images/20210813/de0d57890277450698e8a426bc72ba35.png