8 Oracle Database 11g DBA Handbook
Temporary Segment
When a user??™s SQL statement needs disk space to complete an operation, such as a sorting
operation that cannot fit in memory, Oracle allocates a temporary segment. Temporary segments
exist only for the duration of the SQL statement.
Rollback Segment
As of Oracle 10g, rollback segments only exist in the SYSTEM tablespace, and typically the DBA
does not need to maintain the SYSTEM rollback segment. In previous Oracle releases, a rollback
segment was created to save the previous values of a database DML operation in case the
transaction was rolled back, and to maintain the ???before??? image data to provide read-consistent
views of table data for other users accessing the table. Rollback segments were also used during
database recovery for rolling back uncommitted transactions that were active when the database
instance crashed or terminated unexpectedly.
Automatic Undo Management handles the automatic allocation and management of rollback
segments within an undo tablespace. Within an undo tablespace, the undo segments are structured
similarly to rollback segments, except that the details of how these segments are managed is under
control of Oracle, instead of being managed (often inefficiently) by the DBA. Automatic undo
segments were available staring with Oracle9i, but manually managed rollback segments are
still available in Oracle 10g. However, this functionality is deprecated as of Oracle 10g, and will
no longer be available in future releases.
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