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HomeDatabaseMysql TutorialOracle 11g对大表中添加DEFAULT值的NOT NULL字段速度有大幅度的提升

Oracle11g中,在添加一个包含DEFAULT值的NOT NULL字段,Oracle不会去更新现有的数据,Oracle需要做的不过是将默认值以及对应的表

今天同事问我一个问题他说在一张2000万的表上增加了一个字段并字段一个默认值,,执行这条语句(alter table tablename add new_col default ‘col’)一个小时没有执行完,问我有没有其他解决方法

我查了一下资料发现

Oracle11g中,在添加一个包含DEFAULT值的NOT NULL字段,Oracle不会去更新现有的数据,Oracle需要做的不过是将默认值以及对应的表信息、列信息一起存储在一个新增数据字典表ecol$中。这张表利用BLOB字段存储ALTER TABLE添加的DEFAULT值

然后我测试一下情况(在11g中测试alter table tablename add new_col default ‘col’ 后代not null和不带not null的区别)

1.新建一张表test

SQL> create table test (id number,name varchar(10));

表已创建。

2.向test表中插入500000记录

SQL> begin

  2  for i in 1..500000 loop

  3  insert into test values(i,'jack');

  4  end loop;

  5  end;

  6  /

PL/SQL 过程已成功完成。

SQL> select count(1) from test;

  COUNT(1)

----------

    500000

SQL>

SQL> set timing on

3.向表中添加一个新的字段NEW_COL1带有默认值如下语句不带NOT NULL

ALTER TABLE t ADD NEW_COL1 CHAR(10) DEFAULT 'TESTCOLUMN'

SQL> ALTER TABLE test ADD NEW_COL1 CHAR(10) DEFAULT 'TESTCOLUMN';

表已更改。

已用时间:  00: 00: 43.23

SQL>

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