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HomeDatabaseMysql TutorialOracle中表添加带有默认值列的讨论(包括11G新特性)

这个问题为什么要讨论呢,是有历史根源的,我们从Oracle 10G开始说起。在11G以前,一个大表,如果要添加一个新列,且新列有默认值

这个问题为什么要讨论呢,是有历史根源的,我们从Oracle 10G开始说起。在11G以前,一个大表,如果要添加一个新列,且新列有默认值,如果使用alter table xx add xx_1 number default 1000; 这类语句设置,那将非常杯具!

它的动作其实就是,添加一个列,然后对这一列进行更新,如果这个表非常大,那么将会产生一个非常大的事务。

而且因为它是ddl操作,它甚至会导致关于该表的select语句都被堵塞!后果非常可怕。

我们在10G中,遇到这种需求怎么办呢?

简单来说,就是先添加一列(不设置默认值)

之后通过使用PL/SQL 对这一列进行批量提交更新(比如1000行 一提交)

好处是,不会锁太多的行,而且每个事务都不大,出现故障之类的,不会产生死事务

或者可以使用在线重定义的方式。 ,参考我的这篇博客

到了11G中,我们对于表添加一列 带有默认值,有了新特性

我们来测试一下,创建表t,100多万数据。

SQL> select count(*) from t;

  COUNT(*)
----------
  1207317

对表加了一列,并设置not null 加默认值

SQL> alter table t add MrDai number default 10000 not null;

Table altered.

Elapsed: 00:00:00.34

速度飞快

这里其实只是在数据字典中做了一个标记,并不会对块中行的实际数据进行修改,所以速度飞快

我们查询的时候,会对新列做一个类似nvl()之类的操作,如果是null,则返回默认的数值。

在设置完成以后,如果是新插入的行,在默认值列,会将数据真正的插入到行

总结一下,这个特性类似于 给表设了一个断点, 断点之前的,如果该列是null,则对其转换为 默认值,而断点之后的,,该列会插入真正的值

这篇文章  对该特性有dump过程,可以参考阅读。

如果想让新列,之前的都为null,而之后的数据则有默认值,也可以,分两步

alter table t add MrDai number ;

alter table t modify MrDai number default 10000;

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