UNDO_TABLESPACE

Property	Description
Parameter type	String
Syntax	`UNDO_TABLESPACE =` `undoname`
Default value	The first available undo tablespace in the database.
Modifiable	`ALTER SYSTEM`
Range of values	Legal name of an existing undo tablespace
Basic	Yes
Oracle RAC	Each instance must have a unique value for this parameter, when it is set.

UNDO_TABLESPACE specifies the undo tablespace to be used when an instance starts up. If this parameter is specified when the instance is in manual undo management mode, then an error will occur and startup will fail.

If the UNDO_TABLESPACE parameter is omitted, the first available undo tablespace in the database is chosen. If no undo tablespace is available, the instance will start without an undo tablespace. In such cases, user transactions will be executed using the SYSTEM rollback segment. You should avoid running in this mode under normal circumstances.

You can replace an undo tablespace with another undo tablespace while the instance is running.