Undo segments can exist only in a specialized form of tablespace called an undo tablespace. (You cannot create other segment types, such as tables, in the undo tablespace.)
The installation process automatically creates a “smallfile” undo
tablespace. You can also create a “bigfile” undo tablespace. However, in a
high-volume online transaction processing (OLTP) environment with many short
concurrent transactions, contention could occur on the file header. An undo
tablespace, stored in multiple data files, can resolve this potential issue.
Although a database may have many undo tablespaces, only one of them at a
time can be designated as the current undo tablespace for any instance in the
database.
Undo segments are automatically created and always owned by SYS. Because
the undo segments act as a circular buffer, each segment has a minimum of two
extents. The default maximum number of extents depends on the database block
size but is very high (32,765 for an 8 KB block size).
Undo tablespaces are permanent, locally managed tablespaces with
automatic extent allocation. They are automatically managed by the database.
Because undo data is required to recover from failed transactions (such
as those that may occur when an instance crashes), undo tablespaces can be
recovered only while the instance is in the MOUNT state.
