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Oracle9i - Database Managed Undo Segments

We found automatic undo management to be pretty reliable during our initial beta testing of Oracle9i. The key to success is to allocate sufficient disk storage to the undo tablespace and to set AUTOEXTEND on to allow the tablespace datafiles to grow automatically. During our beta testing, numerous heavy batch update jobs were simultaneously run to simulate heavy work loads. We found that the system managed undo segments worked as advertised. During performance comparisons, we did find that system managed undo segments did seem to add some extra processing time to the batch loads. We found that the rollback segment tablespace used in our comparison testing auto extended sooner than its system managed undo tablespace counterpart. One possible explanation is that the overhead can be attributed to the system managed undo segments performing additional actions to squeeze more undo data in the tablespace before giving up and auto expanding the undo tablespace datafile.

All of the Oracle certification tests will have numerous questions on undo segments.  Test candidates should have a strong knowledge of undo segments including:

• What rollback segments are used for

  • Transaction rollback
     

  • Transaction recovery
     

  • Read consistency
     

• The parameters used to activate automatic undo management

  • undo_management
     

  • undo_tablespace
     

  • undo_retention
     

• The differences between automatic undo management and manual undo management
   

• What “snapshot too old” error messages and how to avoid them
   

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