Active or unexpired is undo data that is still needed for read consistency, even after a
transaction has been committed. Once all queries needing the active undo data have completed
and the undo retention period is reached, the active undo data becomes expired. Expired undo
data may still be used to support other Oracle features, such as the Flashback features, but it is
no longer needed to support read consistency for long-running transactions. Unused undo data
is space in the undo tablespace that has never been used.
As a result, the minimum size for an undo tablespace is enough space to hold the before-image
versions of all data from all active transactions that have not yet been committed or rolled back.
If the space allocated to the undo tablespace cannot even support the changes to uncommitted
transactions to support a rollback operation, the user will get the error message ???ORA-30036:
unable to extend segment by space_qty in undo tablespace tablespace_name.??? In this situation,
the DBA must increase the size of the undo tablespace, or as a stopgap measure the user can split
up a larger transaction into smaller ones while still maintaining any required business rules.
Manual Methods
The DBA can use a number of manual methods to correctly size the undo tablespace. As
demonstrated in Chapter 6, we can review the contents of the dynamic performance view
V$UNDOSTAT to see the undo segment usage at ten-minute intervals.
Pages:
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379