Undo Table spaces
An undo table space is a locally managed table space reserved for system-managed undo data. Like other permanent table spaces, undo table spaces contain data files. Undo blocks in these files are grouped in extents. Automatic Undo Management Mode Undo table spaces require the database to be in the default automatic undo management mode. This mode eliminates the complexities of manually administering undo segments. The database automatically tunes itself to provide the best possible retention of undo data to satisfy long-running queries that may require this data.
An undo table space is automatically created with a new installation of Oracle Database. Earlier versions of Oracle Database may not include an undo table space and use legacy rollback segments instead, known as manual undo management mode. When upgrading to Oracle Database 11g, you can enable automatic undo management mode and create an undo table space. A database can contain multiple undo table spaces, but only one can be in use at a time. When an instance attempts to open a database, Oracle Database automatically selects the first available undo table space. If no undo table space is available, then the instance starts without an undo table space and stores undo data in the SYSTEM table space. Storing undo data in SYSTEM is not recommended.