mysql系列VIII-索引(6)-索引使用

发表于 更新于 分类于 ， Changyan： 本文字数： 7.7k 阅读时长 ≈ 28 分钟

索引是对数据库表中一列或多列的值进行排序的一种结构,使用索引可快速访问数据库表中的特定信息。

Mysql系列文章

  1. mysql系列I-概述

  2. mysql系列II-sql语法

    ①. mysql系列II-sql语法-(1)-概述

    ②. mysql系列II-sql语法-(2)-DDL

    ③. mysql系列II-sql语法-(3)-DML

    ④. mysql系列II-sql语法-(4)-DQL

    ⑤. mysql系列II-sql语法-(5)-DCL

  3. mysql系列III-函数

  4. mysql系列IV-约束

  5. mysql系列V-多表查询

  6. mysql系列VI-事务

  7. mysql系列VII-存储引擎

  8. mysql系列VIII-索引

    ①. mysql系列VIII-索引(1)-概述

    ②. mysql系列VIII-索引(2)-结构

    ③. mysql系列VIII-索引(3)-分类

    ④. mysql系列VIII-索引(4)-语法

    ⑤. mysql系列VIII-索引(5)-SQL性能分析

    ⑥. mysql系列VIII-索引(6)-索引使用

    ⑦. mysql系列VIII-索引(7)-设计原则

  9. mysql系列IX-SQL优化

  10. mysql系列X-视图

  11. mysql系列XI-存储过程

  12. mysql系列XII-存储函数

  13. mysql系列XIII-触发器

  14. mysql系列XIV-锁

验证索引效率

在讲解索引的使用原则之前，先通过一个简单的案例，来验证一下索引，看看是否能够通过索引来提升数据查询性能。在演示的时候，我们还是使用之前准备的一张表 tb_sku , 在这张表中准备了1000w的记录。

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mysql> select count(*) from tb_sku;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (17.93 sec)

这张表中id为主键，有主键索引，而其他字段是没有建立索引的。 我们先来查询其中的一条记录，看看里面的字段情况，执行如下SQL：

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mysql> select * from tb_sku where id = 1 \G;
*************************** 1. row ***************************
           id: 1
           sn: 100000003145001
         name: 华为Meta1
        price: 87901
          num: 9961
    alert_num: 100
        image: https://m.360buyimg.com/mobilecms/s720x720_jfs/t5590/64/5811657380/234462/5398e856/5965e173N34179777.jpg!q70.jpg.webp
       images: https://m.360buyimg.com/mobilecms/s720x720_jfs/t5590/64/5811657380/234462/5398e856/5965e173N34179777.jpg!q70.jpg.webp
       weight: 10
  create_time: 2019-05-01 00:00:00
  update_time: 2019-05-01 00:00:00
category_name: 真皮包
   brand_name: viney
         spec: 白色1
     sale_num: 39
  comment_num: 0
       status: 1
1 row in set (0.00 sec)

可以看到即使有1000w的数据,根据id进行数据查询,性能依然很快，因为主键id是有索引的。 那么接下来，我们再来根据 sn 字段进行查询，执行如下SQL：

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mysql> select *from tb_sku where sn = '100000003145001' \G;
*************************** 1. row ***************************
           id: 1
           sn: 100000003145001
         name: 华为Meta1
        price: 87901
          num: 9961
    alert_num: 100
        image: https://m.360buyimg.com/mobilecms/s720x720_jfs/t5590/64/5811657380/234462/5398e856/5965e173N34179777.jpg!q70.jpg.webp
       images: https://m.360buyimg.com/mobilecms/s720x720_jfs/t5590/64/5811657380/234462/5398e856/5965e173N34179777.jpg!q70.jpg.webp
       weight: 10
  create_time: 2019-05-01 00:00:00
  update_time: 2019-05-01 00:00:00
category_name: 真皮包
   brand_name: viney
         spec: 白色1
     sale_num: 39
  comment_num: 0
       status: 1
1 row in set (14.95 sec)

我们可以看到根据sn字段进行查询，查询返回了一条数据，结果耗时 14.95 sec，就是因为sn没有索引，而造成查询效率很低。
那么我们可以针对于sn字段，建立一个索引，建立了索引之后，我们再次根据sn进行查询，再来看一下查询耗时情况。

创建索引：

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mysql> create index idx_sku_sn on tb_sku(sn);
Query OK, 0 rows affected (56.26 sec)
Records: 0  Duplicates: 0  Warnings: 0

创建索引成功，但是我们也看到了，创建这个索引我们花费了56.26 sec，由此可以看出索引的缺点，会占用一定空间，创建更新索引需要额外的时间。

然后再次执行相同的SQL语句，再次查看SQL的耗时

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mysql> select *from tb_sku where sn = '100000003145001' \G;
*************************** 1. row ***************************
           id: 1
           sn: 100000003145001
         name: 华为Meta1
        price: 87901
          num: 9961
    alert_num: 100
        image: https://m.360buyimg.com/mobilecms/s720x720_jfs/t5590/64/5811657380/234462/5398e856/5965e173N34179777.jpg!q70.jpg.webp
       images: https://m.360buyimg.com/mobilecms/s720x720_jfs/t5590/64/5811657380/234462/5398e856/5965e173N34179777.jpg!q70.jpg.webp
       weight: 10
  create_time: 2019-05-01 00:00:00
  update_time: 2019-05-01 00:00:00
category_name: 真皮包
   brand_name: viney
         spec: 白色1
     sale_num: 39
  comment_num: 0
       status: 1
1 row in set (0.00 sec)

我们明显会看到，sn字段建立了索引之后，查询性能大大提升。建立索引前后，查询耗时都不是一个数量级的。

最左前缀法则

如果索引了多列（联合索引），要遵守最左前缀法则。最左前缀法则指的是查询从索引的最左列开始，并且不跳过索引中的列。如果跳跃某一列，索引将会部分失效(后面的字段索引失效)。
以 tb_user 表为例，我们先来查看一下之前 tb_user 表所创建的索引。

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mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          0 | idx_user_phone       |            1 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_name        |            1 | name        | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email            |            1 | email       | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
7 rows in set (0.02 sec)

在 tb_user 表中，有一个联合索引，这个联合索引涉及到三个字段，顺序分别为：profession，age，status。
对于最左前缀法则指的是，查询时，最左边的列，也就是profession必须存在，否则索引全部失效。
而且中间不能跳过某一列，否则该列后面的字段索引将失效。 接下来，我们来演示几组案例，看一下具体的执行计划：

按顺序使用联合索引

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mysql> explain select * from tb_user where profession = '软件工程' and age = 31 and status
    -> = '0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref               | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

按顺序使用部分联合索引

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mysql> explain select * from tb_user where profession = '软件工程' and age = 31;
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref         | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 49      | const,const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

按顺序使用联合索引第一个

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mysql> explain select * from tb_user where profession = '软件工程';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | const |    4 |   100.00 | NULL  |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

以上的这三组测试中，我们发现只要联合索引最左边的字段 profession存在，索引就会生效，只不过索引的长度不同。 而且由以上三组测试，我们也可以推测出profession字段索引长度为47、age字段索引长度为2、status字段索引长度为5。

联合索引不使用第一个

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mysql> explain select * from tb_user where age = 31 and status = '0';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   24 |     4.17 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

联合索引不使用前两个

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mysql> explain select * from tb_user where status = '0';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   24 |    10.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

而通过上面的这两组测试，我们也可以看到索引并未生效，原因是因为不满足最左前缀法则，联合索引最左边的列profession不存在。

联合索引不使用中间部分索引

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mysql> explain select * from tb_user where profession = '软件工程' and status = '0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | const |    4 |    10.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

上述的SQL查询时，存在profession字段，最左边的列是存在的，索引满足最左前缀法则的基本条件。但是查询时，跳过了age这个列，所以后面的列索引是不会使用的，也就是索引部分生效，所以索引的长度就是47。

思考：

当执行SQL语句: explain select * from tb_user where age = 31 and status = '0' and profession = '软件工程'; 时，是否满足最左前缀法则，走不走上述的联合索引，索引长度？

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mysql> explain select * from tb_user where age = 31 and status = '0' and profession = '软件工程';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref               | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

可以看到，是完全满足最左前缀法则的，索引长度54，联合索引是生效的。
注意 ： 最左前缀法则中指的最左边的列，是指在查询时，联合索引的最左边的字段(即是第一个字段)必须存在，与我们编写SQL时，条件编写的先后顺序无关。

范围查询

联合索引中，出现范围查询(>,<)，范围查询右侧的列索引失效。

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mysql> explain select * from tb_user where profession = '软件工程' and age > 30 and status
    -> = '0';
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_pro_age_sta | idx_user_pro_age_sta | 49      | NULL |    2 |    10.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

当范围查询使用> 或 < 时，走联合索引了，但是索引的长度为49，就说明范围查询右边的status字段是没有走索引的。

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mysql> explain select * from tb_user where profession = '软件工程' and age >= 30 and
    -> status = '0';
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_pro_age_sta | idx_user_pro_age_sta | 54      | NULL |    2 |    10.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

当范围查询使用>= 或 <= 时，走联合索引了，但是索引的长度为54，就说明所有的字段都是走索引的。

所以，**在业务允许的情况下，尽可能的使用类似于 >= 或 <= 这类的范围查询，而避免使用 > 或 <**。

索引失效情况

索引列运算

不要在索引列上进行运算操作， 索引将失效。
在tb_user表中，除了前面介绍的联合索引之外，还有一个索引，是phone字段的单列索引。

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mysql> show index from tb_user where key_name = "idx_user_phone";
+---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name       | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | idx_user_phone |            1 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
1 row in set (0.01 sec)

当根据phone字段进行等值匹配查询时, 索引生效。

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mysql> explain select * from tb_user where phone = '17799990015';
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type  | possible_keys  | key            | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | const | idx_user_phone | idx_user_phone | 46      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

当根据phone字段进行函数运算操作之后，索引失效。

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mysql> explain select * from tb_user where substring(phone,10,2) = '15';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   24 |   100.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

字符串不加引号

字符串类型字段使用时，不加引号，索引将失效。
接下来，我们通过两组示例，来看看对于字符串类型的字段，加单引号与不加单引号的区别：

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mysql> explain select * from tb_user where profession = '软件工程' and age = 31 and status
    -> = '0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref               | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where profession = '软件工程' and age = 31 and status
    -> = 0;
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref         | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 49      | const,const |    1 |    10.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------+------+----------+-----------------------+
1 row in set, 2 warnings (0.00 sec)

手机号

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mysql> explain select * from tb_user where phone = '17799990015';
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type  | possible_keys  | key            | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | const | idx_user_phone | idx_user_phone | 46      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where phone = 17799990015;
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys  | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | idx_user_phone | NULL | NULL    | NULL |   24 |    10.00 | Using where |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)

经过上面两组示例，我们会明显的发现，如果字符串不加单引号，对于查询结果，没什么影响，但是数据库存在隐式类型转换，索引将失效。

模糊查询

如果仅仅是尾部模糊匹配，索引不会失效。如果是头部模糊匹配，索引失效。
接下来，我们来看一下这三条SQL语句的执行效果，查看一下其执行计划：
由于下面查询语句中，都是根据profession字段查询，符合最左前缀法则，联合索引是可以生效的，我们主要看一下，模糊查询时，%加在关键字之前，和加在关键字之后的影响。

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mysql> explain select * from tb_user where profession like '软件%';
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | NULL |    4 |   100.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where profession like '%工程';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   24 |    11.11 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where profession like '%工%';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   24 |    11.11 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

经过上述的测试，我们发现，在like模糊查询中，在关键字后面加%，索引可以生效。而如果在关键字前面加了%，索引将会失效。

or连接条件

用or分割开的条件， 如果or前的条件中的列有索引，而后面的列中没有索引，那么涉及的索引都不会被用到。

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mysql> explain select * from tb_user where id = 10 or age = 23;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | PRIMARY       | NULL | NULL    | NULL |   24 |    13.75 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where phone = '17799990017' or age = 23;
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys  | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | idx_user_phone | NULL | NULL    | NULL |   24 |    13.75 | Using where |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

由于age没有索引，所以即使id、phone有索引，索引也会失效。所以需要针对于age也要建立索引。

然后，我们可以对age字段建立索引。

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mysql> create index idx_user_age on tb_user(age);
Query OK, 0 rows affected (0.03 sec)

mysql> show index from tb_user where Key_name="idx_user_age";
+---------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name     | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          1 | idx_user_age |            1 | age         | A         |          19 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
1 row in set (0.00 sec)

建立了索引之后，我们再次执行上述的SQL语句，看看前后执行计划的变化。

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mysql> explain select * from tb_user where id = 10 or age = 23;
+----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table   | partitions | type        | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                                          |
+----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+
|  1 | SIMPLE      | tb_user | NULL       | index_merge | PRIMARY,idx_user_age | PRIMARY,idx_user_age | 4,2     | NULL |    3 |   100.00 | Using union(PRIMARY,idx_user_age); Using where |
+----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where phone = '17799990017' or age = 23;
+----+-------------+---------+------------+-------------+-----------------------------+-----------------------------+---------+------+------+----------+-------------------------------------------------------+
| id | select_type | table   | partitions | type        | possible_keys               | key                         | key_len | ref  | rows | filtered | Extra                                                 |
+----+-------------+---------+------------+-------------+-----------------------------+-----------------------------+---------+------+------+----------+-------------------------------------------------------+
|  1 | SIMPLE      | tb_user | NULL       | index_merge | idx_user_phone,idx_user_age | idx_user_phone,idx_user_age | 46,2    | NULL |    3 |   100.00 | Using union(idx_user_phone,idx_user_age); Using where |
+----+-------------+---------+------------+-------------+-----------------------------+-----------------------------+---------+------+------+----------+-------------------------------------------------------+

最终，我们发现，当or连接的条件，左右两侧字段都有索引时，索引才会生效。

数据分布影响

如果MySQL评估使用索引比全表更慢，则不使用索引。

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mysql> explain select * from tb_user where phone >= '17799990005';
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys  | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | idx_user_phone | NULL | NULL    | NULL |   24 |    79.17 | Using where |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where phone >= '17799990015';
+----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys  | key            | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_phone | idx_user_phone | 46      | NULL |    9 |   100.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

经过测试我们发现，相同的SQL语句，只是传入的字段值不同，最终的执行计划也完全不一样，这是为什么呢？

就是因为MySQL在查询时，会评估使用索引的效率与走全表扫描的效率，如果走全表扫描更快，则放弃索引，走全表扫描。 因为索引是用来索引少量数据的，如果通过索引查询返回大批量的数据，则还不如走全表扫描来的快，此时索引就会失效。
接下来，我们再来看看 is null 与 is not null 操作是否走索引。
执行如下两条语句 ：

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mysql> explain select * from tb_user where profession is null;
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where profession is not null;
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys        | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | idx_user_pro_age_sta | NULL | NULL    | NULL |   24 |   100.00 | Using where |
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

可以看到专业为空的比较少走了索引，不为空的数据太多，直接走了全表扫描

接下来，我们做一个操作将profession字段值全部更新为null。

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mysql> update tb_user set profession = null;
Query OK, 24 rows affected (0.00 sec)
Rows matched: 24  Changed: 24  Warnings: 0

然后，再次执行上述的两条SQL，查看SQL语句的执行计划。

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mysql> explain select * from tb_user where profession is null;
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys        | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | idx_user_pro_age_sta | NULL | NULL    | NULL |   24 |   100.00 | Using where |
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where profession is not null;
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | NULL |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

最终我们看到，一模一样的SQL语句，先后执行了两次，结果查询计划是不一样的，为什么会出现这种现象，这是和数据库的数据分布有关系。查询时MySQL会评估，走索引快，还是全表扫描快，如果全表扫描更快，则放弃索引走全表扫描。 因此，is null 、is not null是否走索引，得具体情况具体分析，并不是固定的。

SQL提示

目前tb_user表的数据情况如下:

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mysql> select * from tb_user;
+----+--------+-------------+-----------------------+--------------------+------+--------+--------+---------------------+
| id | name   | phone       | email                 | profession         | age  | gender | status | createtime          |
+----+--------+-------------+-----------------------+--------------------+------+--------+--------+---------------------+
|  1 | 吕布   | 17799990000 | lvbu666@163.com       | 软件工程           |   23 | 1      | 6      | 2001-02-02 00:00:00 |
|  2 | 曹操   | 17799990001 | caocao666@qq.com      | 通讯工程           |   33 | 1      | 0      | 2001-03-05 00:00:00 |
|  3 | 赵云   | 17799990002 | 17799990@139.com      | 英语               |   34 | 1      | 2      | 2002-03-02 00:00:00 |
|  4 | 孙悟空 | 17799990003 | 17799990@sina.com     | 工程造价           |   54 | 1      | 0      | 2001-07-02 00:00:00 |
|  5 | 花木兰 | 17799990004 | 19980729@sina.com     | 软件工程           |   23 | 2      | 1      | 2001-04-22 00:00:00 |
|  6 | 大乔   | 17799990005 | daqiao666@sina.com    | 舞蹈               |   22 | 2      | 0      | 2001-02-07 00:00:00 |
|  7 | 露娜   | 17799990006 | luna_love@sina.com    | 应用数学           |   24 | 2      | 0      | 2001-02-08 00:00:00 |
|  8 | 程咬金 | 17799990007 | chengyaojin@163.com   | 化工               |   38 | 1      | 5      | 2001-05-23 00:00:00 |
|  9 | 项羽   | 17799990008 | xiaoyu666@qq.com      | 金属材料           |   43 | 1      | 0      | 2001-09-18 00:00:00 |
| 10 | 白起   | 17799990009 | baiqi666@sina.com     | 机械工程及其自动化 |   27 | 1      | 2      | 2001-08-16 00:00:00 |
| 11 | 韩信   | 17799990010 | hanxin520@163.com     | 无机非金属材料工程 |   27 | 1      | 0      | 2001-06-12 00:00:00 |
| 12 | 荆轲   | 17799990011 | jingke123@163.com     | 会计               |   29 | 1      | 0      | 2001-05-11 00:00:00 |
| 13 | 兰陵王 | 17799990012 | lanlinwang666@126.com | 工程造价           |   44 | 1      | 1      | 2001-04-09 00:00:00 |
| 14 | 狂铁   | 17799990013 | kuangtie@sina.com     | 应用数学           |   43 | 1      | 2      | 2001-04-10 00:00:00 |
| 15 | 貂蝉   | 17799990014 | 84958948374@qq.com    | 软件工程           |   40 | 2      | 3      | 2001-02-12 00:00:00 |
| 16 | 妲己   | 17799990015 | 2783238293@qq.com     | 软件工程           |   31 | 2      | 0      | 2001-01-30 00:00:00 |
| 17 | 芈月   | 17799990016 | xiaomin2001@sina.com  | 工业经济           |   35 | 2      | 0      | 2000-05-03 00:00:00 |
| 18 | 嬴政   | 17799990017 | 8839434342@qq.com     | 化工               |   38 | 1      | 1      | 2001-08-08 00:00:00 |
| 19 | 狄仁杰 | 17799990018 | jujiamlm8166@163.com  | 国际贸易           |   30 | 1      | 0      | 2007-03-12 00:00:00 |
| 20 | 安琪拉 | 17799990019 | jdodm1h@126.com       | 城市规划           |   51 | 2      | 0      | 2001-08-15 00:00:00 |
| 21 | 典韦   | 17799990020 | ycaunanjian@163.com   | 城市规划           |   52 | 1      | 2      | 2000-04-12 00:00:00 |
| 22 | 廉颇   | 17799990021 | lianpo321@126.com     | 土木工程           |   19 | 1      | 3      | 2002-07-18 00:00:00 |
| 23 | 后羿   | 17799990022 | altycj2000@139.com    | 城市园林           |   20 | 1      | 0      | 2002-03-10 00:00:00 |
| 24 | 姜子牙 | 17799990023 | 37483844@qq.com       | 工程造价           |   29 | 1      | 4      | 2003-05-26 00:00:00 |
+----+--------+-------------+-----------------------+--------------------+------+--------+--------+---------------------+
24 rows in set (0.00 sec)

索引情况如下:

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mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          0 | idx_user_phone       |            1 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_name        |            1 | name        | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
6 rows in set (0.00 sec)

执行SQL : explain select * from tb_user where profession = '软件工程';

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mysql> explain select * from tb_user where profession = '软件工程';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | const |    4 |   100.00 | NULL  |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

查询走了联合索引。

执行SQL，创建profession的单列索引：create index idx_user_pro on tb_user(profession);

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mysql> create index idx_user_pro on tb_user(profession);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          0 | idx_user_phone       |            1 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_name        |            1 | name        | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro         |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
7 rows in set (0.00 sec)

创建单列索引后，再次执行A中的SQL语句，查看执行计划，看看到底走哪个索引。

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mysql> explain select * from tb_user where profession = '软件工程';
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys                     | key                  | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta,idx_user_pro | idx_user_pro_age_sta | 47      | const |    4 |   100.00 | NULL  |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

测试结果，我们可以看到，possible_keys中 idx_user_pro_age_sta,idx_user_pro 这两个索引都可能用到，最终MySQL选择了idx_user_pro_age_sta索引。这是MySQL自动选择的结果。

那么，我们能不能在查询的时候，自己来指定使用哪个索引呢？ 答案是肯定的，此时就可以借助于MySQL的SQL提示来完成。 接下来，介绍一下SQL提示。

SQL提示，是优化数据库的一个重要手段，简单来说，就是在SQL语句中加入一些人为的提示来达到优化操作的目的。

use index

建议MySQL使用哪一个索引完成此次查询（仅仅是建议，mysql内部还会再次进行评估）。

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mysql> explain select * from tb_user use index(idx_user_pro) where profession = '软件工程';
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key          | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro  | idx_user_pro | 47      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

ignore index

忽略指定的索引。

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mysql> explain select * from tb_user ignore index(idx_user_pro) where profession = '软件工程';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

force index

强制使用索引。

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mysql> explain select * from tb_user force index(idx_user_pro) where profession = '软件工程';
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key          | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro  | idx_user_pro | 47      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

覆盖索引

尽量使用覆盖索引，减少select *。 那么什么是覆盖索引呢？ 覆盖索引是指 查询使用了索引，并且需要返回的列，在该索引中已经全部能够找到 。

接下来，我们来看一组SQL的执行计划，看看执行计划的差别，然后再来具体做一个解析。

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mysql> explain select id, profession from tb_user where profession = '软件工程' and age =
    -> 31 and status = '0' ;
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+--------------------------+
| id | select_type | table   | partitions | type | possible_keys                     | key                  | key_len | ref               | rows | filtered | Extra                    |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+--------------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta,idx_user_pro | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using where; Using index |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select id,profession,age, status from tb_user where profession = '软件工程'
    -> and age = 31 and status = '0' ;
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+--------------------------+
| id | select_type | table   | partitions | type | possible_keys                     | key                  | key_len | ref               | rows | filtered | Extra                    |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+--------------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta,idx_user_pro | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using where; Using index |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select id,profession,age, status, name from tb_user where profession = '软件工程' and age = 31 and status = '0' ;
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys                     | key                  | key_len | ref               | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta,idx_user_pro | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from tb_user where profession = '软件工程' and age = 31 and status = '0';
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys                     | key                  | key_len | ref               | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta,idx_user_pro | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

从上述的执行计划我们可以看到，这四条SQL语句的执行计划前面所有的指标都是一样的，看不出来差异。但是此时，我们主要关注的是后面的Extra，前面两天SQL的结果为 Using where; Using Index ; 而后面两条SQL的结果为: Using index condition 。

Extra含义
Using where; Using Index查找使用了索引，但是需要的数据都在索引列中能找到，所以不需要回表查询数据
Using index condition查找使用了索引，但是需要回表查询数据

因为，在tb_user表中有一个联合索引 idx_user_pro_age_sta，该索引关联了三个字段profession、age、status，而这个索引也是一个二级索引，所以叶子节点下面挂的是这一行的主键id。 所以当我们查询返回的数据在 id、profession、age、status 之中，则直接走二级索引直接返回数据了。 如果超出这个范围，就需要拿到主键id，再去扫描聚集索引，再获取额外的数据了，这个过程就是回表。 而我们如果一直使用select * 查询返回所有字段值，很容易就会造成回表查询（除非是根据主键查询，此时只会扫描聚集索引）。

为了大家更清楚的理解，什么是覆盖索引，什么是回表查询，我们一起再来看下面的这组SQL的执行过
程。
A. 表结构及索引示意图:

id是主键，是一个聚集索引。 name字段建立了普通索引，是一个二级索引（辅助索引）。
B. 执行SQL : select * from tb_user where id = 2;

根据id查询，直接走聚集索引查询，一次索引扫描，直接返回数据，性能高。

C. 执行SQL：selet id,name from tb_user where name = 'Arm';

虽然是根据name字段查询，查询二级索引，但是由于查询返回在字段为 id，name，在name的二级索引中，这两个值都是可以直接获取到的，因为覆盖索引，所以不需要回表查询，性能高。

D. 执行SQL：selet id,name,gender from tb_user where name = 'Arm';

image-20220802134051219

由于在name的二级索引中，不包含gender，所以，需要两次索引扫描，也就是需要回表查询，性能相对较差一点。

思考：

一张表, 有四个字段(id, username, password, status), 由于数据量大, 需要对以下SQL语句进行优化, 该如何进行才是最优方案:

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select id,username,password from tb_user where username ='itcast';

答案: 针对于 username, password建立联合索引, sql为: create index idx_user_name_pass on tb_user(username,password);
这样可以避免上述的SQL语句，在查询的过程中，出现回表查询。

前缀索引

当字段类型为字符串（varchar，text，longtext等）时，有时候需要索引很长的字符串，这会让索引变得很大，查询时，浪费大量的磁盘IO， 影响查询效率。此时可以只将字符串的一部分前缀，建立索引，这样可以大大节约索引空间，从而提高索引效率。
1). 语法

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create index idx_xxxx on table_name(column(n)) ;

示例:
为tb_user表的email字段，建立长度为5的前缀索引。

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mysql> create index idx_email_5 on tb_user(email(5));
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          0 | idx_user_phone       |            1 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_name        |            1 | name        | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro         |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email_5          |            1 | email       | A         |          23 |        5 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
8 rows in set (0.00 sec)

2). 前缀长度
可以根据索引的选择性来决定，而选择性是指不重复的索引值（基数）和数据表的记录总数的比值，索引选择性越高则查询效率越高， 唯一索引的选择性是1，这是最好的索引选择性，性能也是最好的。

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select count(distinct email) / count(*) from tb_user ;
select count(distinct substring(email,1,5)) / count(*) from tb_user ;

3). 前缀索引的查询流程

单列索引与联合索引

单列索引：即一个索引只包含单个列。
联合索引：即一个索引包含了多个列。
我们先来看看 tb_user 表中目前的索引情况:

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mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table   | Non_unique | Key_name             | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user |          0 | PRIMARY              |            1 | id          | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          0 | idx_user_phone       |            1 | phone       | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_name        |            1 | name        | A         |          24 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            2 | age         | A         |          22 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro_age_sta |            3 | status      | A         |          24 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_user_pro         |            1 | profession  | A         |          16 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
| tb_user |          1 | idx_email_5          |            1 | email       | A         |          23 |        5 |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
8 rows in set (0.00 sec)

在查询出来的索引中，既有单列索引，又有联合索引。
接下来，我们来执行一条SQL语句，看看其执行计划：

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mysql> explain select id, phone, name from tb_user where phone = '17799990010' and name ='韩信';
+----+-------------+---------+------------+-------+------------------------------+----------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type  | possible_keys                | key            | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+-------+------------------------------+----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | const | idx_user_phone,idx_user_name | idx_user_phone | 46      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+-------+------------------------------+----------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

通过上述执行计划我们可以看出来，在and连接的两个字段 phone、name上都是有单列索引的，但是最终mysql只会选择一个索引，也就是说，只能走一个字段的索引，此时是会回表查询的。

紧接着，我们再来创建一个phone和name字段的联合索引来查询一下执行计划。

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mysql> create unique index idx_user_phone_name on tb_user(phone,name);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select id, phone, name from tb_user use index(idx_user_phone_name) where phone='17799990010' and name='韩信';
+----+-------------+---------+------------+-------+---------------------+---------------------+---------+-------------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys       | key                 | key_len | ref         | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------------+---------------------+---------+-------------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | const | idx_user_phone_name | idx_user_phone_name | 248     | const,const |    1 |   100.00 | Using index |
+----+-------------+---------+------------+-------+---------------------+---------------------+---------+-------------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

此时，查询时，就走了联合索引，而在联合索引中包含 phone、name的信息，在叶子节点下挂的是对应的主键id，所以查询是无需回表查询的。

在业务场景中，如果存在多个查询条件，考虑针对于查询字段建立索引时，建议建立联合索引，而非单列索引。

如果查询使用的是联合索引，具体的结构示意图如下：

Mysql系列文章

  1. mysql系列I-概述

  2. mysql系列II-sql语法

    ①. mysql系列II-sql语法-(1)-概述

    ②. mysql系列II-sql语法-(2)-DDL

    ③. mysql系列II-sql语法-(3)-DML

    ④. mysql系列II-sql语法-(4)-DQL

    ⑤. mysql系列II-sql语法-(5)-DCL

  3. mysql系列III-函数

  4. mysql系列IV-约束

  5. mysql系列V-多表查询

  6. mysql系列VI-事务

  7. mysql系列VII-存储引擎

  8. mysql系列VIII-索引

    ①. mysql系列VIII-索引(1)-概述

    ②. mysql系列VIII-索引(2)-结构

    ③. mysql系列VIII-索引(3)-分类

    ④. mysql系列VIII-索引(4)-语法

    ⑤. mysql系列VIII-索引(5)-SQL性能分析

    ⑥. mysql系列VIII-索引(6)-索引使用

    ⑦. mysql系列VIII-索引(7)-设计原则

  9. mysql系列IX-SQL优化

  10. mysql系列X-视图

  11. mysql系列XI-存储过程

  12. mysql系列XII-存储函数

  13. mysql系列XIII-触发器

  14. mysql系列XIV-锁