【MySQL】探究之TIMESTAMP

背景

  之前有業務反饋表中start_time,end_time時間字段隨着時間的推移被自動更新，這可不是業務意願，說的嚴重點是要出故障的。

MySQL中有DATE,DATETIME,TIMESTAMP時間類型

看看官方文檔怎么說

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.

這里我們重點介紹下DATATIME和TIMESTMAP

業務反饋start_time,end_time缺失就使TIMESTAMP類型，結合TIMESTAMP的特性，我們來分析原因。

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字段默認為NOT NULL,如果你在定義“t_null_1 TIMESTAMP DEFAULT NULL” 會提升“ERROR 1067 (42000): Invalid default value for 't_null_1'”。 可以指定為空 null ，“t_null_1 TIMESTAMP NULL" ,這時可以再添加語句改變默認值。
2. 如果不做特殊說明，同一個表中會對第一個TIMESTAMP字段設置DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP屬性，第二個字段設置DEFAULT '0000-00-00 00:00:00'，如果對兩個字段都顯示指定“DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP”是會被告知“ERROR 1293 (HY000): Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause”，測試發現在MySQL5.6版本中並沒有該限制。

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

那如何解決業務遇到的問題呢，其實很簡單，既然“DEFAULT CURRENT_TIMESTAMP”有限制，那不用好了，換成NULL DEFAULT CURRENT_TIMESTAMP去掉自動更新屬性，或者顯式的允許這兩個字段為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

經過學習，發現在5.6版中中多了一個叫“explicit_defaults_for_timestamp”的系統變量，但默認是OFF的，但是也可以支持一個表中多個字段的同時更新。

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

那將這個參數打開會有什么不同？

mysql> set @@global.explicit_defaults_for_timestamp=ON;   
ERROR 1238 (HY000): Variable 'explicit_defaults_for_timestamp' is a read only variable
該參數並不支持動態修改！！！

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
;

發現如下改變：

1. 默認參數為NULL
2. 並不會自動添加DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP等屬性

結論

那考慮的版本的兼容性，測試發現，在5，5版本中按默認屬性創建的表，在5.6版本中遷移是沒有任何問題的，是完全兼容的。這說明，使用Mysql5.6以后的版本，應立即將explicit_defaults_for_timestamp參數設置為True，並及時反饋開發TIMESTAMP的行為。