mysql explicit_defaults_for_timestamp 變量的做用

mysql 中有這樣的一個默認行爲，若是一行數據中某些列被更新了，若是這一行中有timestamp類型的列，那麼麼這個timestamp列的數據

也會被自動更新到 更新操做所發生的那個時間點；這個操做是由explicit_defaults_for_timestamp這個變動控制的

 

1、體驗一下mysql的默認更新行爲：

mysql> create table t(x int ,y timestamp);  -- 建立一個帶有timestamp列的表
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t(x) values(1);  -- 只插x列
Query OK, 1 row affected (0.00 sec)

mysql> select * from t; -- timestamp列會自動更新
+------+---------------------+
| x    | y                   |
+------+---------------------+
|    1 | 2017-06-07 13:48:56 |
+------+---------------------+
1 row in set (0.00 sec)

mysql> update t set x=2 where x=1; -- update 時timestamp列仍是會自動更新
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t;
+------+---------------------+
| x    | y                   |
+------+---------------------+
|    2 | 2017-06-07 13:49:21 |
+------+---------------------+
1 row in set (0.00 sec)

2、如何關閉這一默認行爲：

　　聰明的你一想到了，只要把explicit_defaults_for_timestamp這個變動設置爲on；對於timestamp列的值都要顯示指定，那麼這一默認行爲就

　　算是關閉了。來操做一把！

mysql> set @@global.explicit_defaults_for_timestamp=on; -- 把全局的設置爲on 那麼新的鏈接就會被設置成on
Query OK, 0 rows affected (0.00 sec)

mysql> set @@session.explicit_defaults_for_timestamp=on; -- 把當前鏈接explicit_defaults_for_timestamp設置爲on
Query OK, 0 rows affected (0.00 sec)

mysql> update t set x=3 ; -- 第一次更新 結果是變了
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t;
+------+---------------------+
| x    | y                   |
+------+---------------------+
|    3 | 2017-06-07 13:57:21 |
+------+---------------------+
row in set (0.00 sec)

mysql> update t set x=4 ; -- 第二次更新 結果仍是變了
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t;
+------+---------------------+
| x    | y                   |
+------+---------------------+
|    4 | 2017-06-07 13:57:52 |
+------+---------------------+
row in set (0.00 sec)

mysql> show variables like 'explicit_defaults_for_timestamp'; -- 全局發動生效
+---------------------------------+-------+
| Variable_name                   | Value |
+---------------------------------+-------+
| explicit_defaults_for_timestamp | ON    |
+---------------------------------+-------+
row in set (0.00 sec)

mysql> show global variables like 'explicit_defaults_for_timestamp'; -- session 級別改動也升效了
+---------------------------------+-------+
| Variable_name                   | Value |
+---------------------------------+-------+
| explicit_defaults_for_timestamp | ON    |
+---------------------------------+-------+
row in set (0.00 sec)

-- 發現了吧、就算把explicit_defaults_for_timestamp 設置成了on 也不會有效的

3、找問題出在了哪裏：

　　一、explicit_defaults_for_timestamp=off 時表結構

CREATE TABLE `t` (
  `x` int(11) DEFAULT NULL,
  `y` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- `y` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE

　　二、explicit_defaults_for_timestamp=on 時表結構

CREATE TABLE `t6` (
  `x` int(11) DEFAULT NULL,
  `y` timestamp NULL DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- `y` timestamp NULL DEFAULT NULL

 

4、結論：

　　explicit_defaults_for_timestamp 變量會直接影響表結構，也就是說explicit_defaults_for_timestamp的做用時間

　　是在表定義的時候；你的update | insert 想經過它去改變行爲已經太晚了！

 

5、如何解決這失控的場面：

　　一、改表結構

mysql> alter table t modify column y timestamp null default null; -- 解決辦法改表結構
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> update t set x=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t;
+------+---------------------+
| x    | y                   |
+------+---------------------+
|    1 | 2017-06-07 13:59:21 |
+------+---------------------+
row in set (0.00 sec)

mysql> update t set x=2; -- 再次對比，發現時間沒有變哦！
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t;
+------+---------------------+
| x    | y                   |
+------+---------------------+
|    2 | 2017-06-07 13:59:21 |
+------+---------------------+
row in set (0.00 sec)