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
- 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。
- 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:
- 默认参数为NULL
- 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。
时间: 2024-10-14 05:03:53