oracle10g Undo tablespaces

undo表空间的用途：
Undo tablespaces are special tablespaces used solely for storing undo information

约束：
You cannot create any other segment types (for example, tables or indexes) in undo
tablespaces

数量：
Each database contains zero or more undo tablespaces.
In automatic undo management mode, each Oracle instance is assigned one (and only one) undo tablespace

创建的时机：
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.

没有指定undo的后果：
In rare circumstances, if the instance does not have a designated
undo tablespace, the transaction binds to the system undo segment.

undo表空间的创建、修改、删除与其他表空间一样。

指定undo表空间的两种方式：

Assignment of Undo Tablespaces
You assign an undo tablespace to an instance in one of two ways:

At instance startup. You can specify the undo tablespace in the initialization file
or let the system choose an available undo tablespace.

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 used.