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
[*]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, 'tom@163.com', 16),(2, 'lucy', 2, 'lucy@163.com', 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 = 2mysql> 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 = 63mysql> 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
[*]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, 'tom@163.com', 16),(2, 'lucy', 2, 'lucy@163.com', 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 = 82mysql> 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 = 83mysql> 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|
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
来源:https://www.cnblogs.com/it-ws/p/17453801.html
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