【MySQL】探究之TIMESTAMP

背景

  之前有业务反馈表中start_time,end_time时间字段随着时间的推移被自动更新，这可不是业务意愿，说的严重点是要出故障的。

MySQL中有DATE,DATETIME,TIMESTAMP时间类型

看看官方文档怎么说

The DATE type is used for values with a date part but no time part. MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The supported range is '1000-01-01' to '9999-12-31'.
The DATETIME type is used for values that contain both date and time parts. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'.
The TIMESTAMP data type is used for values that contain both date and time parts. TIMESTAMP has a range of '1970-01-01 00:00:01' UTC to '2038-01-19 03:14:07' UTC.

这里我们重点介绍下DATATIME和TIMESTMAP

业务反馈start_time,end_time缺失就使TIMESTAMP类型，结合TIMESTAMP的特性，我们来分析原因。

Automatic Initialization and Updating for TIMESTAMP

One TIMESTAMP column in a table can have the current timestamp as the default value for initializing the column, as the auto-update value, or both. 
If the column is auto-initialized, it is set to the current timestamp for inserted rows that specify no value for the column.
If the column is auto-updated, it is automatically updated to the current timestamp when the value of any other column in the row is changed from its current value. 

什么意思呢？

create table gbtest_with_force_default_val
(
    id bigint AUTO_INCREMENT,
    t_null_1 timestamp,
    t_null_2 timestamp,
    t_with_not_null timestamp not null,
    t_with_not_null_default timestamp not null default '2016-12-21',
    primary key(id)
);

mysql> show create table gbtest_with_force_default_val;

CREATE TABLE `gbtest_with_force_default_val` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `t_null_1` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `t_null_2` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `t_with_not_null` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `t_with_not_null_default` timestamp NOT NULL DEFAULT '2016-12-21 00:00:00',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 

在MySQL5.5版本中，TIMESTAMP特性

1. TIMESTAMP字段默认为NOT NULL,如果你在定义“t_null_1 TIMESTAMP DEFAULT NULL” 会提升“ERROR 1067 (42000): Invalid default value for 't_null_1'”。 可以指定为空 null ，“t_null_1 TIMESTAMP NULL" ,这时可以再添加语句改变默认值。
2. 如果不做特殊说明，同一个表中会对第一个TIMESTAMP字段设置DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP属性，第二个字段设置DEFAULT '0000-00-00 00:00:00'，如果对两个字段都显示指定“DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP”是会被告知“ERROR 1293 (HY000): Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause”，测试发现在MySQL5.6版本中并没有该限制。

mysql> select version();
+------------------+
| version()        |
+------------------+
| 5.6.16.7-rc0-log |
+------------------+
1 row in set (0.00 sec)

CREATE TABLE `gbtest_with_force_default_val_null` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `t_null_1` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `t_null_2` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `t_with_not_null` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `t_with_not_null_default` timestamp NOT NULL DEFAULT '2016-12-21 00:00:00',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

那如何解决业务遇到的问题呢，其实很简单，既然“DEFAULT CURRENT_TIMESTAMP”有限制，那不用好了，换成NULL DEFAULT CURRENT_TIMESTAMP去掉自动更新属性，或者显式的允许这两个字段为NULL。

create table gbtest_with_force_default_val_null ( 
    id bigint AUTO_INCREMENT, 
    t_null_1 timestamp NULL CURRENT_TIMESTAMP, 
    t_null_2 timestamp NULL DEFAULT 0, 
    t_with_not_null timestamp not null, 
    t_with_not_null_default timestamp not null default '2016-12-21', primary key(id) );

mysql> show create table gbtest_with_force_default_val_null;

CREATE TABLE `gbtest_with_force_default_val_null` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `t_null_1` timestamp NULL CURRENT_TIMESTAMP,
  `t_null_2` timestamp NULL DEFAULT '0000-00-00 00:00:00',
  `t_with_not_null` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `t_with_not_null_default` timestamp NOT NULL DEFAULT '2016-12-21 00:00:00',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

MySQL5.6版本中TIMESTAMP特性

经过学习，发现在5.6版中中多了一个叫“explicit_defaults_for_timestamp”的系统变量，但默认是OFF的，但是也可以支持一个表中多个字段的同时更新。

mysql> show variables like '%explicit_defaults_for_timestamp%';
+---------------------------------+-------+
| Variable_name                   | Value |
+---------------------------------+-------+
| explicit_defaults_for_timestamp | OFF   |
+---------------------------------+-------+
1 row in set (0.00 sec)

那将这个参数打开会有什么不同？

mysql> set @@global.explicit_defaults_for_timestamp=ON;   
ERROR 1238 (HY000): Variable 'explicit_defaults_for_timestamp' is a read only variable
该参数并不支持动态修改！！！

create table gbtest_with_noforce(
    id bigint AUTO_INCREMENT,
    t_null_1 timestamp,
    t_null_2 timestamp,
    t_with_not_null timestamp not null,
    t_with_not_null_default timestamp not null default '2016-12-21',
    primary key(id)
);

CREATE TABLE `gbtest_with_noforce` (
    `id` BIGINT(20) NOT NULL AUTO_INCREMENT,
    `t_null_1` TIMESTAMP NULL DEFAULT NULL,
    `t_null_2` TIMESTAMP NULL DEFAULT NULL,
    `t_with_not_null` TIMESTAMP NOT NULL,
    `t_with_not_null_default` TIMESTAMP NOT NULL DEFAULT '2016-12-21 00:00:00',
    PRIMARY KEY (`id`)
)ENGINE=InnoDB DEFAULT CHARSET=utf8
;

发现如下改变：

1. 默认参数为NULL
2. 并不会自动添加DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP等属性

结论

那考虑的版本的兼容性，测试发现，在5，5版本中按默认属性创建的表，在5.6版本中迁移是没有任何问题的，是完全兼容的。这说明，使用Mysql5.6以后的版本，应立即将explicit_defaults_for_timestamp参数设置为True，并及时反馈开发TIMESTAMP的行为。