在数据库设计的时候会遇到很多只需要0、1、2这种固定几个值的状态字段，基本上都建议设置为只占一字节的tinyint类型，有些觉得char(1)是一样，毕竟char(1)存储数字和字母时一个字符也只是占一个字节

mysql是用c++写的，而在c++中字符类型是存放对应ascii码的二进制到存储空间，而整型数字是直接存数字的二进制，虽然最终都是二进制存储，但是环节上有少许不同，同样在msyql查找时也会有所不同，下图摘自小白版c++教程《c++ primer plus》:

今天对tinyint和char(1)做了个简单测试，分表建两个表t1、t2，结构如下：

mysql> show create table t1\G

*************************** 1. row ***************************

Table: t1

Create Table: CREATE TABLE `t1` (

`_id` int(11) NOT NULL AUTO_INCREMENT,

`id` tinyint(4) DEFAULT NULL,

`title` text,

PRIMARY KEY (`_id`),

KEY `id` (`id`)

) ENGINE=InnoDB AUTO_INCREMENT=2400096 DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

mysql> show create table t2\G

*************************** 1. row ***************************

Table: t2

Create Table: CREATE TABLE `t2` (

`_id` int(11) NOT NULL AUTO_INCREMENT,

`id` char(1) DEFAULT NULL,

`title` text,

PRIMARY KEY (`_id`),

KEY `id` (`id`)

) ENGINE=InnoDB AUTO_INCREMENT=2400096 DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

两个表唯一不同为id字段类型，总数据量都为2400096，id只有0、1、2三个，为了两个表的数据一样且磁盘上分布也一样，降低IO对测试的影响，分别加载的数据如下：

mysql> select id,count(*) from t1 group by id;

+------+----------+

| id   | count(*) |

+------+----------+

|    0 |  1199998 |

|    1 |  1199998 |

|    2 |       99 |

+------+----------+

3 rows in set (0.55 sec)

mysql> select id,count(*) from t2 group by id;

+------+----------+

| id   | count(*) |

+------+----------+

| 0    |  1199998 |

| 1    |  1199998 |

| 2    |       99 |

+------+----------+

3 rows in set (0.77 sec)

查看执行计划：

mysql> explain select _id from test.t2 where id='1';

+----+-------------+-------+------+---------------+------+---------+-------+---------+--------------------------+

| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows    | Extra                    |

+----+-------------+-------+------+---------------+------+---------+-------+---------+--------------------------+

|  1 | SIMPLE      | t2    | ref  | id            | id   | 4       | const | 1170900 | Using where; Using index |

+----+-------------+-------+------+---------------+------+---------+-------+---------+--------------------------+

1 row in set (0.00 sec)

mysql> explain select _id from test.t1 where id=1;

+----+-------------+-------+------+---------------+------+---------+-------+---------+-------------+

| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows    | Extra       |

+----+-------------+-------+------+---------------+------+---------+-------+---------+-------------+

|  1 | SIMPLE      | t1    | ref  | id            | id   | 2       | const | 1170601 | Using index |

+----+-------------+-------+------+---------------+------+---------+-------+---------+-------------+

1 row in set (0.00 sec)

两个表都使用了id索引，再看看information_schema.tables的信息是否和之前理解的存储字节大小是否有出入：

mysql> select DATA_LENGTH/1024/1024,INDEX_LENGTH/1024/1024,data_free from tables where table_name in ('t1','t2');

+-----------------------+------------------------+-----------+

| DATA_LENGTH/1024/1024 | INDEX_LENGTH/1024/1024 | data_free |

+-----------------------+------------------------+-----------+

|          310.81250000 |            27.56250000 |         0 |

|          313.81250000 |            29.56250000 |         0 |

+-----------------------+------------------------+-----------+

2 rows in set (0.00 sec)

两个表大小相差不多，确认char(1)和tinyint占字节数相同，现在直接看执行时间：

mysql> show profiles;

+----------+------------+---------------------------------------------------------------+

| Query_ID | Duration   | Query                                                         |

+----------+------------+---------------------------------------------------------------+

|        1 | 0.60804275 | select count(*) from (select _id from test.t1 where id=1) a   |

|        2 | 0.59277575 | select count(*) from (select _id from test.t1 where id=1) a   |

|        3 | 0.60398000 | select count(*) from (select _id from test.t1 where id=1) a   |

|        4 | 0.69068025 | select count(*) from (select _id from test.t2 where id='1') a |

|        5 | 0.69654200 | select count(*) from (select _id from test.t2 where id='1') a |

|        6 | 0.67788800 | select count(*) from (select _id from test.t2 where id='1') a |

+----------+------------+---------------------------------------------------------------+

这样就很明显可以看出为char(1)字段的t2表查询时消耗时间偏多，如果几条几百条的情况根本看不出char(1)和tinyint的差别，毕竟现在CPU的效率是非常高的，这里测试的利用了id=1的数据，有1199998条，这样就可以看出点差别了！！虽然效率差别不是很大，为了生产环境统一以及提升QPS还是使用短小的整型更好