一篇搞定MySQL索引长度（key_len）计算规则

2023-06-03 13:11 由 e鸣惊人发表于 #数据库

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

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;

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       |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+

插入测试数据

insert into test3(id, name, sex, email, age) values(1, 'tom', 1, '[email protected]', 16),(2, 'lucy', 2, '[email protected]', 18);

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 = '[email protected]'; 
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+
| 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

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;

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       |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+

插入测试数据

1 insert into test4(id, name, sex, email, age) values(1, 'tom', 1, '[email protected]', 16),(2, 'lucy', 2, '[email protected]', 18);

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 = '[email protected]'; 
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
| 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  |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+