在 MySQL 中将列类型 DATETIME 更改为 TIMESTAMP 时会发生什么？

Question

我想在超过一百万行的表中将列数据类型从 DATETIME 转换为 TIMESTAMP。

是否可以？慢吗？我会丢失旧信息还是 MySQL 可以自动转换？我可以做一个alter table还是我可以做一些不同的事情？

Answer 1

这是一个测试脚本：

use test
drop table if exists felipe_table;
create table felipe_table
(
    id int not null auto_increment,
    dt datetime not null,
    name varchar(25),
    primary key (id)
);
insert into felipe_table (dt,name) values
(NOW() + INTERVAL FLOOR(RAND()*10) DAY,'rolando'),
(NOW() + INTERVAL FLOOR(RAND()*100) DAY,'pamela'),
(NOW() + INTERVAL FLOOR(RAND()*1000) DAY,'dominique'),
(NOW() + INTERVAL FLOOR(RAND()*10000) DAY,'diamond');
select * from felipe_table;
SHOW CREATE TABLE felipe_table\G
ALTER TABLE felipe_table MODIFY dt TIMESTAMP;
select * from felipe_table;
SHOW CREATE TABLE felipe_table\G

这是它的执行（使用 MySQL 5.5.12-log for Windows）：

mysql> use test
Database changed
mysql> drop table if exists felipe_table;
Query OK, 0 rows affected (0.03 sec)

mysql> create table felipe_table
    -> (
    ->     id int not null auto_increment,
    ->     dt datetime not null,
    ->     name varchar(25),
    ->     primary key (id)
    -> );
Query OK, 0 rows affected (0.11 sec)

mysql> insert into felipe_table (dt,name) values
    -> (NOW() + INTERVAL FLOOR(RAND()*10) DAY,'rolando'),
    -> (NOW() + INTERVAL FLOOR(RAND()*100) DAY,'pamela'),
    -> (NOW() + INTERVAL FLOOR(RAND()*1000) DAY,'dominique'),
    -> (NOW() + INTERVAL FLOOR(RAND()*10000) DAY,'diamond');
Query OK, 4 rows affected (0.13 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from felipe_table;
+----+---------------------+-----------+
| id | dt                  | name      |
+----+---------------------+-----------+
|  1 | 2013-02-16 09:01:21 | rolando   |
|  2 | 2013-02-09 09:01:21 | pamela    |
|  3 | 2014-09-07 09:01:21 | dominique |
|  4 | 2036-03-07 09:01:21 | diamond   |
+----+---------------------+-----------+
4 rows in set (0.00 sec)

mysql> SHOW CREATE TABLE felipe_table\G
*************************** 1. row ***************************
       Table: felipe_table
Create Table: CREATE TABLE `felipe_table` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `dt` datetime NOT NULL,
  `name` varchar(25) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> ALTER TABLE felipe_table MODIFY dt TIMESTAMP;
Query OK, 4 rows affected (0.31 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from felipe_table;
+----+---------------------+-----------+
| id | dt                  | name      |
+----+---------------------+-----------+
|  1 | 2013-02-16 09:01:21 | rolando   |
|  2 | 2013-02-09 09:01:21 | pamela    |
|  3 | 2014-09-07 09:01:21 | dominique |
|  4 | 2036-03-07 09:01:21 | diamond   |
+----+---------------------+-----------+
4 rows in set (0.00 sec)

mysql> SHOW CREATE TABLE felipe_table\G
*************************** 1. row ***************************
       Table: felipe_table
Create Table: CREATE TABLE `felipe_table` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `dt` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `name` varchar(25) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql>

看起来它有效。如果您不信任这个小样本，请制作该表的副本并使用副本执行此操作。

试一试 ！！！