日期：2011-01-25 22:55:00 来源：本站整理

关于Oracle的Dual表-入门底子[Oracle防范]

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有朋友问到关于Oracle的Dual表问题,并且提到了Tom的一个链接:http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:1562813956388

很多人关心的是Dual的内部实现,这Oracle自然是不会表露的,不过我们可以从一些有限的资料得到关于Dual的印象:

There is internalized code that makes this happen. Code checks that ensure that a table scan of SYS.DUAL only returns one row. Svrmgrl behaviour is incorrect but this is now an obsolete product.

The base issue you should always remember and keep is: DUAL table should always have 1 ROW. Dual is a normal table with one dummy column of varchar2(1).
This is basically used from several applications as a pseudo table for getting results from a select statement that use functions like sysdate or other prebuilt or application functions.
If DUAL has no rows at all some applications (that use DUAL) may fail with NO_DATA_FOUND exception. If DUAL has more than 1 row then applications (that use DUAL) may fail with TOO_MANY_ROWS exception.

So DUAL should ALWAYS have 1 and only 1 row.

前两句话最为关键,实际上我们也简单猜到,Oracle通过内部代码来实现关于DUAL的拜候和掌握,并且通过Internal Code使得这个表与众差别.

Tom提到在Close了Database之后,可以看到这个表的内存地址及展示,这阐明这个表的构造并不但纯:

SQL> select * from dual;

D
-
X

SQL> alter database close;

Database altered.

SQL> select * from dual;

ADDR INDX INST_ID D
-------- ---------- ---------- -
0A4F8858 0 1 X

注意不要更改Dual表的内容,不然大概惹起数据库的问题.

假如该表不测删除,可以通过设置初始化参数replication_dependency_tracking = FALSE,重启数据库来重建该表. 　　以上是“关于Oracle的Dual表-入门底子[Oracle防范]”的内容，如果你对以上该文章内容感兴趣，你可以看看七道奇为您推荐以下文章：

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