一篇搞定MySQL索引长度（key_len）计算规则

MySQL索引长度（key_len）计算

 计算规则

• 索引字段：没有设置 NOT NULL，则需要加 1 个字节。
• 定长字段：tinyint 占 1 个字节、int 占 4个字节、bitint 占 8 个字节、date 占 3个字节、datetime 占 5 个字节、char(n) 占 n 个字节。
• 变长字段：varchar (n) 占 n 个字符 + 2 个 字节。

注意（字符和字节在不同编码之间的转换） 

不同的字符集，一个字符占用的字节数不同

• latin1 编码： 每个字符占用一个字节。
• gbk编码：每个字符占用两个字节。
• utf8编码：每个字符占用三个字节。
• utf8mb4编码：每个字符占用四个字节。

案例分析

（一）、编码为utf8

1. 创建测试表

   CREATE TABLE `test3` (
     `id` int NOT NULL,
     `name` varchar(20) NOT NULL COMMENT '姓名',
     `sex` tinyint NOT NULL COMMENT '性别,1：男，2：女',
     `email` varchar(20) DEFAULT NULL,
     `age` tinyint default 0,
     PRIMARY KEY (`id`)
   ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

2. 创建索引

   mysql> show index from test3;
   +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
   | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
   +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
   | test3 |          0 | PRIMARY  |            1 | id          | A         |           2 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
   | test3 |          1 | idx_age  |            1 | age         | A         |           2 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
   | test3 |          1 | idx_name |            1 | name        | A         |           2 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
   | test3 |          1 | idx_eml  |            1 | email       | A         |           2 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
   +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+

3. 插入测试数据

   insert into test3(id, name, sex, email, age) values(1, 'tom', 1, 'tom@163.com', 16),(2, 'lucy', 2, 'lucy@163.com', 18); 

4. 分析查询计划

• 根据主键 id 分析，由于 id 为 int 类型，设置的NOT NULL ，key_len为 4 

  mysql> explain select * from test3 where id =1; 
  +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
  | id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
  +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
  |  1 | SIMPLE      | test3 | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
  +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

 

• 根据 age 分析，由于 age 为 tinyint 类型，可以为NULL ，key_len为 1 + 1  = 2

  mysql> explain select * from test3 where age =16;
  +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+
  | id | select_type | table | partitions | type | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
  +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+
  |  1 | SIMPLE      | test3 | NULL       | ref  | idx_age       | idx_age | 2       | const |    1 |   100.00 | NULL  |
  +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+

 

• 根据 name 分析，由于 name 为 varchar 类型，设置NOT NULL ，key_len为 20 *3 + 2  =  62 

  mysql> explain select * from test3 where name = 'tom'; 
  +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
  | id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra |
  +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
  |  1 | SIMPLE      | test3 | NULL       | ref  | idx_name      | idx_name | 62      | const |    1 |   100.00 | NULL  |
  +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+

 

• 根据 email 分析，由于 email  为 varchar 类型，没有设置NOT NULL  ，key_len为 20 *3 + 2 + 1  =  63

  mysql> explain select * from test3 where email = 'tom@163.com'; 
  +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+
  | id | select_type | table | partitions | type | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
  +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+
  |  1 | SIMPLE      | test3 | NULL       | ref  | idx_eml       | idx_eml | 63      | const |    1 |   100.00 | NULL  |
  +----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+

 

（二）、编码为utf8mb4

1. 创建测试表

    CREATE TABLE `test4` (
     `id` int NOT NULL,
     `name` varchar(20) NOT NULL COMMENT '姓名',
     `sex` tinyint NOT NULL COMMENT '性别,1：男，2：女',
     `email` varchar(20) DEFAULT NULL,
     `age` tinyint default 0,
     PRIMARY KEY (`id`)
   ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

2. 创建索引

   mysql> show index from test3;
   +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
   | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
   +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
   | test3 |          0 | PRIMARY  |            1 | id          | A         |           2 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
   | test3 |          1 | idx_age  |            1 | age         | A         |           2 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
   | test3 |          1 | idx_name |            1 | name        | A         |           2 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
   | test3 |          1 | idx_eml  |            1 | email       | A         |           2 |     NULL |   NULL | YES  | BTREE      |         |               | YES     | NULL       |
   +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+

3. 插入测试数据

   insert into test4(id, name, sex, email, age) values(1, 'tom', 1, 'tom@163.com', 16),(2, 'lucy', 2, 'lucy@163.com', 18); 

4. 分析查询计划

• 根据 name 分析，由于 name 为 varchar 类型，设置NOT NULL ，key_len为 20 *4 + 2  =  82

  mysql> explain select * from test4 where name = 'tom'; 
  +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
  | id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra |
  +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
  |  1 | SIMPLE      | test4 | NULL       | ref  | idx_name      | idx_name | 82      | const |    1 |   100.00 | NULL  |
  +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+

 

• 根据 email 分析，由于 email 为 varchar 类型，没有设置NOT NULL ，key_len为 20 *4 + 2 +1  =  83

  mysql> explain select * from test4 where email = 'tom@163.com'; 
  +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
  | id | select_type | table | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra |
  +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
  |  1 | SIMPLE      | test4 | NULL       | ref  | idx_email     | idx_email | 83      | const |    1 |   100.00 | NULL  |
  +----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+