enum类型占用几个字节？

[netxuning]

enum('男', '女');

[ACMAIN_CHM]

2 Bytes

[netxuning]

做了个实验，好像是7个字节:

create table(gender enum('男', '女'));
insert into tbl(gender) values('男', '男', '女', '女');
mysql> SHOW TABLE STATUS LIKE 'tbl'\G                               
*************************** 1. row ***************************
           Name: tbl
         Engine: MyISAM
        Version: 10
     Row_format: Fixed
           Rows: 4
 Avg_row_length: 7
    Data_length: 28
Max_data_length: 1970324836974591
   Index_length: 1024
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2009-12-16 17:09:48
    Update_time: 2009-12-16 17:13:29
     Check_time: NULL
      Collation: gb2312_chinese_ci
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)

Rows: 4
Avg_row_length: 7

[ACMAIN_CHM]

mysql> create table tbl(gender enum('男', '女')) engine=myisam;
Query OK, 0 rows affected (0.06 sec)

mysql> insert into tbl(gender) values('男'),( '男'), ('女'),( '女');
Query OK, 4 rows affected (0.05 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> show table status like 'tbl'\G
*************************** 1. row ***************************
           Name: tbl
         Engine: MyISAM
        Version: 10
     Row_format: Fixed
           Rows: 4
 Avg_row_length: 7
    Data_length: 28
Max_data_length: 1970324836974591
   Index_length: 1024
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2009-12-16 19:40:28
    Update_time: 2009-12-16 19:40:32
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options:
        Comment:
1 row in set (0.00 sec)

mysql> create table tbl1(gender SMALLINT) engine=myisam;
Query OK, 0 rows affected (0.06 sec)

mysql> insert into tbl1 values(1),(2),(3),(4);
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> show table status like 'tbl1'\G
*************************** 1. row ***************************
           Name: tbl1
         Engine: MyISAM
        Version: 10
     Row_format: Fixed
           Rows: 4
 Avg_row_length: 7
    Data_length: 28
Max_data_length: 1970324836974591
   Index_length: 1024
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2009-12-16 19:42:31
    Update_time: 2009-12-16 19:42:34
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options:
        Comment:
1 row in set (0.00 sec)

mysql>

[ACMAIN_CHM]

除了固定的字段会占用空间，另外还有一些其它记录用的信息需要存储。

The expected row length for dynamic-sized rows is calculated using the following expression:

3
+ (number of columns + 7) / 8
+ (number of char columns)
+ (packed size of numeric columns)
+ (length of strings)
+ (number of NULL columns + 7) / 8

[kazake]

说的不完全正确，在一张表中enum类型应该是占用1个或2个字节，取决于此表中enum类型的个数，enum类型字段占用的总字节数应该等于enum类型数量+1个字节。