Undo tablespaces (41)

Undo tablespaces are special tablespaces used solely for storing undo information.
You cannot create any other segment types (for example, tables or indexes) in undo
tablespaces. Each database contains zero or more undo tablespaces. In automatic undo
management mode, each Oracle instance is assigned one (and only one) undo
tablespace. Undo data is managed within an undo tablespace using undo segments
that are automatically created and maintained by Oracle.
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. In rare circumstances, if the instance does not have a designated
undo tablespace, the transaction binds to the system undo segment.

Each undo tablespace is composed of a set of undo files and is locally managed. Like
other types of tablespaces, undo blocks are grouped in extents and the status of each
extent is represented in the bitmap. At any point in time, an extent is either allocated to
(and used by) a transaction table, or it is free.
You can create a bigfile undo tablespace.

1. 保存回滚信息
2. 不能创建其他任务类型的segment
3. 在aum模式下 ， 每个实例只能拥有一个回滚表空间
4. aum ： Oracle自动创建回滚段并且维护

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