9-9. Monitoring and Administering Undo

In order to minimize the impact to the users, proactively monitor the alert log for the below errors and resize the undo tablespace accordingly:

• ORA-01650 unable to extend rollback segment error message: happens when the undo tablespace runs out of room for undo data. Make sure users commit large transactions periodically.
  
• ORA-01555 snapshot too old error message: occurs when the query needs to access undo information that has already been overwritten. This may happen in a long-running or flashback query. Configure an appropriate undo retention interval by setting UNDO_RETENTION in seconds and altering tablespace to have RETENTION GUARANTEE. This parameter UNDO_RETENTION is only used when:
  - the undo tablespace has the AUTOEXTEND option enabled or
  - you want to set undo retention for LOBS or
  - you want to guarantee retention.

The recommendation is to set the AUTOEXTEND to true until we determined the space requirements for the undo tablespace. After that, we should disable it.