During normal database operation, the database does not allow the SYSAUX tablespace to be dropped or renamed. If the SYSAUX tablespace becomes unavailable, then core database functionality remains operational. The database features that use the SYSAUX tablespace could fail, or function with limited capability.

Undo Tablespaces

An undo tablespace is a locally managed tablespace reserved for system-managed undo data (see "Undo Segments" on page 12-24). Like other permanent tablespaces, undo tablespaces contain data files. Undo blocks in these files are grouped in extents.

Automatic Undo Management Mode

Undo tablespaces 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 tablespace is automatically created with a new installation of Oracle Database. Earlier versions of Oracle Database may not include an undo tablespace 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 tablespace. Oracle Database contains an Undo Advisor that provides advice on and helps automate your undo environment.

A database can contain multiple undo tablespaces, 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 tablespace. If no undo tablespace is available, then the instance starts without an undo tablespace and stores undo data in the SYSTEM tablespace. Storing undo data in SYSTEM is not recommended.

Automatic Undo Retention

The undo retention period is the minimum amount of time that Oracle Database attempts to retain old undo data before overwriting it. Undo retention is important because long-running queries may require older block images to supply read consistency. Also, some Oracle Flashback features can depend on undo availability.

In general, it is desirable to retain old undo data as long as possible. After a transaction commits, undo data is no longer needed for rollback or transaction recovery. The database can retain old undo data if the undo tablespace has space for new transactions. When available space is low, the database begins to overwrite old undo data for committed transactions.

Oracle Database automatically provides the best possible undo retention for the current undo tablespace. The database collects usage statistics and tunes the retention period based on these statistics and the undo tablespace size. If the undo tablespace is configured with the AUTOEXTEND option, and if the maximum size is not specified, then undo retention tuning is different. In this case, the database tunes the undo retention period to be slightly longer than the longest-running query, if space allows.

Temporary Tablespaces