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Rollback Segments

A transaction uses a rollback segment to record before images of data it intends to change. If the transaction fails before committing, Oracle uses the before images to rollback or undo the uncommitted data changes. Oracle also uses rollback segments for statement-level read consistency. Read consistency ensures that all data returned by a query comes from the same point-in-time (query start time). Lastly, rollback segments provide before images of data to help the instance roll back failed transactions during instance recovery.

When is the System Rollback Segment Used?

When you create a database, Oracle creates a rollback segment called system in the system tablespace. The system rollback segment is only used for transactions that occur inside the system tablespace. The system rollback segment’s main purpose is to handle the rollback generated for DDL statements. DDL statements cause updates to be applied to the data dictionary tables in the system tablespace. It is possible for the system rollback segment to be used for non-data dictionary tables, but only if those tables are created inside the system tablespace (which is very bad development practice).

Rollback Segments and Transaction Processing Workloads

In releases prior to Oracle9i, sizing rollback segments is more of an art than a science. The key to good rollback segment performance is monitoring the statistics associated with rollback segment waits, sizing and activity. The queries below provide information that can be used to tune rollback segments:
 

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