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Automatic Undo Retention

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 table space 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 table space. The database collects usage statistics and tunes the retention period based on these statistics and the undo table space size. If the undo table space is configured with the AUTO EXTEND 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.
View More:
Overview of Data Blocks
Introduction to Logical Storage Structures
Archived Redo Log Files
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