《循序渐进ORACLE》学习笔记连载02
时间：2010-03-08 来源：q3dx
今天的话题是：
ORACLE闪回特性所用到的关键结构——UNDO TABLESPACE 简介
Undo tablespaces are special tablespaces
Undo 表空间是特殊的表空间
for storing undo information
是用来存储撤销内容的
You cannot create any other segment types (for example, tables or indexes) in undo tablespaces.
不能够在Undo表空间中创建任何其他的段类型
Each database contains zero or more undo tablespaces.
一个数据库包括0个或者多个Undo 表空间
In automatic undo management mode, each Oracle instance is assigned one (and only one) undo tablespace.
在自动回滚段管理模式下，每一个oracle实例被分配仅仅一个Undo 表空间
Undo data is managed within an undo tablespace using undo segments that are automatically created and maintained by Oracle.
oracle自动在undo表空间中创建和管理Undo段，用来储存undo 数据
When the first DML operation is run within a transaction, the transaction is bound (assigned) to an undo segment (and therefore to a transaction table) in the current undo tablespace.
NOTE:
Do not run any user transactions before creating the first undo tablespace and taking it online.
An undo tablespace cannot be dropped if it is being used by any instance or contains any undo information needed to recover transactions.
You assign an undo tablespace to an instance in one of two ways:
1.At instance startup. You can specify the undo tablespace in the initialization file or let the system choose an available undo tablespace.
2.While the instance is running. Use ALTER SYSTEM SET UNDO_TABLESPACE to replace the active undo tablespace with another undo tablespace. This method is rarely(很少) used.
You can add more space to an undo tablespace by adding more data files to the undo tablespace with the ALTER TABLESPACE statement.
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