在Mysql中使用索引

╰+攻爆jí腚メ 2021-10-18 21:16 245阅读 0赞

**MySQL查询的优化是个老生常谈的问题，方法更是多种多样，其中最直接的就是创建索引.**

这里通过一个简单的demo来实际用一下索引，看看索引在百万级别查询中速率的提升效果如何

所需数据可以从我前面的一篇博客中获取:[https://www.cnblogs.com/wangbaojun/p/11154515.html][https_www.cnblogs.com_wangbaojun_p_11154515.html]

有一张salaries,

查看表结构如下：

可以看到emp\_no，from\_date都是PRI（主键索引），这是在这个表中将这两个字段联合起来设置为主键，一张表中还是只能有一个主键

查看表的创建命令：

mysql> show create table salaries;
    +----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | Table    | Create Table                                                                                                                                                                                                                                                                                                                       |
    +----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | salaries | CREATE TABLE `salaries` (
      `emp_no` int(11) NOT NULL,
      `salary` int(11) NOT NULL,
      `from_date` date NOT NULL,
      `to_date` date NOT NULL,
      PRIMARY KEY (`emp_no`,`from_date`),
      CONSTRAINT `salaries_ibfk_1` FOREIGN KEY (`emp_no`) REFERENCES `employees` (`emp_no`) ON DELETE CASCADE
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
    +----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    1 row in set (0.00 sec)

注意我标红的地方，将emp\_no\`,\`from\_date设置为组合主键,组合索引遵从左前缀原则，查询emp\_no，或者查询(\`emp\_no\`,\`from\_date\`)会走索引，但是查from\_date不会走索引,可以看一下用explain命令查看:

+----+-------------+----------+-------+---------------+---------+---------+-------------+------+-------+ 
    1 row in set (0.00 sec)

to\_date字段没有设置索引，我们来测试一下加索引前后，该字段查询效率会不会有提升：

未加索引：

mysql> select * from salaries where to_date = '1986-06-26';                                                                                                                                          +--------+--------+------------+------------+
    | emp_no | salary | from_date  | to_date    |
    +--------+--------+------------+------------+
    |  25676 |  40000 | 1985-06-26 | 1986-06-26 |
    |  28757 |  40000 | 1985-06-26 | 1986-06-26 |
    |  30860 |  64620 | 1985-06-26 | 1986-06-26 |
    |  69209 |  40000 | 1985-06-26 | 1986-06-26 |
    |  80550 |  45292 | 1985-06-26 | 1986-06-26 |
    |  91204 |  47553 | 1985-06-26 | 1986-06-26 |
    |  96140 |  52908 | 1985-06-26 | 1986-06-26 |
    | 208352 |  42989 | 1985-06-26 | 1986-06-26 |
    | 213109 |  90133 | 1985-06-26 | 1986-06-26 |
    | 217498 |  80247 | 1985-06-26 | 1986-06-26 |
    | 219462 |  83880 | 1985-06-26 | 1986-06-26 |
    | 223150 |  40000 | 1985-06-26 | 1986-06-26 |
    | 227694 |  73897 | 1985-06-26 | 1986-06-26 |
    | 232856 |  73126 | 1985-06-26 | 1986-06-26 |
    | 237619 |  56982 | 1985-06-26 | 1986-06-26 |
    | 244087 |  40000 | 1985-06-26 | 1986-06-26 |
    | 253472 |  72004 | 1985-06-26 | 1986-06-26 |
    | 257395 |  40000 | 1985-06-26 | 1986-06-26 |
    | 261811 |  40000 | 1985-06-26 | 1986-06-26 |
    | 268968 |  40000 | 1985-06-26 | 1986-06-26 |
    | 269331 |  40000 | 1985-06-26 | 1986-06-26 |
    | 274805 |  40000 | 1985-06-26 | 1986-06-26 |
    | 279432 |  74530 | 1985-06-26 | 1986-06-26 |
    | 285685 |  83198 | 1985-06-26 | 1986-06-26 |
    | 286745 |  44082 | 1985-06-26 | 1986-06-26 |
    | 290901 |  49876 | 1985-06-26 | 1986-06-26 |
    | 400719 |  79168 | 1985-06-26 | 1986-06-26 |
    | 401448 |  49600 | 1985-06-26 | 1986-06-26 |
    | 427374 |  40000 | 1985-06-26 | 1986-06-26 |
    | 432024 |  40000 | 1985-06-26 | 1986-06-26 |
    | 432654 |  40000 | 1985-06-26 | 1986-06-26 |
    | 438461 |  44451 | 1985-06-26 | 1986-06-26 |
    | 446228 |  42733 | 1985-06-26 | 1986-06-26 |
    | 447391 |  62381 | 1985-06-26 | 1986-06-26 |
    | 448823 |  40000 | 1985-06-26 | 1986-06-26 |
    | 452355 |  40000 | 1985-06-26 | 1986-06-26 |
    | 453590 |  61615 | 1985-06-26 | 1986-06-26 |
    | 456521 |  40000 | 1985-06-26 | 1986-06-26 |
    | 464415 |  48955 | 1985-06-26 | 1986-06-26 |
    | 467901 |  52349 | 1985-06-26 | 1986-06-26 |
    | 472895 |  40000 | 1985-06-26 | 1986-06-26 |
    | 476501 |  40000 | 1985-06-26 | 1986-06-26 |
    | 477079 |  40000 | 1985-06-26 | 1986-06-26 |
    | 478934 |  55054 | 1985-06-26 | 1986-06-26 |
    | 480301 |  44177 | 1985-06-26 | 1986-06-26 |
    | 484507 |  40000 | 1985-06-26 | 1986-06-26 |
    | 486187 |  40000 | 1985-06-26 | 1986-06-26 |
    | 491159 |  46034 | 1985-06-26 | 1986-06-26 |
    | 493154 |  40000 | 1985-06-26 | 1986-06-26 |
    | 498140 |  81909 | 1985-06-26 | 1986-06-26 |
    | 498565 |  72853 | 1985-06-26 | 1986-06-26 |
    +--------+--------+------------+------------+
    51 rows in set (1.08 sec)

用explain分析一下:

为to\_date字段加索引：

mysql> create index to_date on salaries(to_date);
    Query OK, 0 rows affected (5.31 sec)
    Records: 0  Duplicates: 0  Warnings: 0 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　创建索引会耗时，索然提升了查询速率，但是更新添加动作会效率降低

现在看一下表结构：

mysql> desc salaries;
    +-----------+---------+------+-----+---------+-------+
    | Field     | Type    | Null | Key | Default | Extra |
    +-----------+---------+------+-----+---------+-------+
    | emp_no    | int(11) | NO   | PRI | NULL    |       |
    | salary    | int(11) | NO   |     | NULL    |       |
    | from_date | date    | NO   | PRI | NULL    |       |
    | to_date   | date    | NO   | MUL | NULL    |       |            MUL表示非唯一索引
    +-----------+---------+------+-----+---------+-------+
    4 rows in set (0.00 sec)
    
    mysql> show create table salaries;
    +----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | Table    | Create Table                                                                                                                                                                                                                                                                                                                                                    |
    +----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    | salaries | CREATE TABLE `salaries` (
      `emp_no` int(11) NOT NULL,
      `salary` int(11) NOT NULL,
      `from_date` date NOT NULL,
      `to_date` date NOT NULL,
      PRIMARY KEY (`emp_no`,`from_date`),
      KEY `to_date` (`to_date`),                                                                                # 创建了索引key为to_date
      CONSTRAINT `salaries_ibfk_1` FOREIGN KEY (`emp_no`) REFERENCES `employees` (`emp_no`) ON DELETE CASCADE
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
    +----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    1 row in set (0.00 sec)

再次查询：

mysql> select * from salaries where to_date = '1986-06-26';
    +--------+--------+------------+------------+
    | emp_no | salary | from_date  | to_date    |
    +--------+--------+------------+------------+
    |  25676 |  40000 | 1985-06-26 | 1986-06-26 |
    |  28757 |  40000 | 1985-06-26 | 1986-06-26 |
    |  30860 |  64620 | 1985-06-26 | 1986-06-26 |
    |  69209 |  40000 | 1985-06-26 | 1986-06-26 |
    |  80550 |  45292 | 1985-06-26 | 1986-06-26 |
    |  91204 |  47553 | 1985-06-26 | 1986-06-26 |
    |  96140 |  52908 | 1985-06-26 | 1986-06-26 |
    | 208352 |  42989 | 1985-06-26 | 1986-06-26 |
    | 213109 |  90133 | 1985-06-26 | 1986-06-26 |
    | 217498 |  80247 | 1985-06-26 | 1986-06-26 |
    | 219462 |  83880 | 1985-06-26 | 1986-06-26 |
    | 223150 |  40000 | 1985-06-26 | 1986-06-26 |
    | 227694 |  73897 | 1985-06-26 | 1986-06-26 |
    | 232856 |  73126 | 1985-06-26 | 1986-06-26 |
    | 237619 |  56982 | 1985-06-26 | 1986-06-26 |
    | 244087 |  40000 | 1985-06-26 | 1986-06-26 |
    | 253472 |  72004 | 1985-06-26 | 1986-06-26 |
    | 257395 |  40000 | 1985-06-26 | 1986-06-26 |
    | 261811 |  40000 | 1985-06-26 | 1986-06-26 |
    | 268968 |  40000 | 1985-06-26 | 1986-06-26 |
    | 269331 |  40000 | 1985-06-26 | 1986-06-26 |
    | 274805 |  40000 | 1985-06-26 | 1986-06-26 |
    | 279432 |  74530 | 1985-06-26 | 1986-06-26 |
    | 285685 |  83198 | 1985-06-26 | 1986-06-26 |
    | 286745 |  44082 | 1985-06-26 | 1986-06-26 |
    | 290901 |  49876 | 1985-06-26 | 1986-06-26 |
    | 400719 |  79168 | 1985-06-26 | 1986-06-26 |
    | 401448 |  49600 | 1985-06-26 | 1986-06-26 |
    | 427374 |  40000 | 1985-06-26 | 1986-06-26 |
    | 432024 |  40000 | 1985-06-26 | 1986-06-26 |
    | 432654 |  40000 | 1985-06-26 | 1986-06-26 |
    | 438461 |  44451 | 1985-06-26 | 1986-06-26 |
    | 446228 |  42733 | 1985-06-26 | 1986-06-26 |
    | 447391 |  62381 | 1985-06-26 | 1986-06-26 |
    | 448823 |  40000 | 1985-06-26 | 1986-06-26 |
    | 452355 |  40000 | 1985-06-26 | 1986-06-26 |
    | 453590 |  61615 | 1985-06-26 | 1986-06-26 |
    | 456521 |  40000 | 1985-06-26 | 1986-06-26 |
    | 464415 |  48955 | 1985-06-26 | 1986-06-26 |
    | 467901 |  52349 | 1985-06-26 | 1986-06-26 |
    | 472895 |  40000 | 1985-06-26 | 1986-06-26 |
    | 476501 |  40000 | 1985-06-26 | 1986-06-26 |
    | 477079 |  40000 | 1985-06-26 | 1986-06-26 |
    | 478934 |  55054 | 1985-06-26 | 1986-06-26 |
    | 480301 |  44177 | 1985-06-26 | 1986-06-26 |
    | 484507 |  40000 | 1985-06-26 | 1986-06-26 |
    | 486187 |  40000 | 1985-06-26 | 1986-06-26 |
    | 491159 |  46034 | 1985-06-26 | 1986-06-26 |
    | 493154 |  40000 | 1985-06-26 | 1986-06-26 |
    | 498140 |  81909 | 1985-06-26 | 1986-06-26 |
    | 498565 |  72853 | 1985-06-26 | 1986-06-26 |
    +--------+--------+------------+------------+
    51 rows in set (0.00 sec)                                        # 创建索引后同样的查询条件从1.08s变为了0.00s，惊讶吧

explain分析：

这个demo从数据上直观的体现了索引带来的查询效率提升有多可观,但是索引也是有利必有害，更多索引的底层知识可以参考这位大牛的博客：[https://www.cnblogs.com/Aiapple/p/5693239.html][https_www.cnblogs.com_Aiapple_p_5693239.html]

转载于:https://www.cnblogs.com/wangbaojun/p/11157199.html

[https_www.cnblogs.com_wangbaojun_p_11154515.html]: https://www.cnblogs.com/wangbaojun/p/11154515.html
[https_www.cnblogs.com_Aiapple_p_5693239.html]: https://www.cnblogs.com/Aiapple/p/5693239.html