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CHAPTER 9 REDO AND UNDO
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With Oracle9i and above, there are two methods to manage undo in the system: Automatic undo management: Here, Oracle is told how long to retain undo for, via the UNDO_RETENTION parameter. Oracle will determine how many undo segments to create based on concurrent workload and how big each should be. The database can even reallocate extents between individual undo segments at runtime to meet the UNDO_RETENTION goal set by the DBA. This is the recommended approach for undo management. Manual undo management: Here, the DBA does the work. The DBA determines how many undo segments to manually create, based on the estimated or observed workload. The DBA determines how big the segments should be based on transaction volume (how much undo is generated) and the length of the longrunning queries.
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Manual undo management, where a DBA figures out how many undo segments to have and how big each should be, is where one of the points of confusion comes into play. People say, Well, we have XMB of undo configured, but this can grow. We have MAXEXTENTS set at 500 and each extent is 1MB, so the undo can get quite large. The problem is that the manually managed undo segments will never grow due to a query; they will grow only due to INSERTs, UPDATEs, and DELETEs. The fact that a long-running query is executing does not cause Oracle to grow a manual undo segment to retain the data in case it might need it. Only a long-running UPDATE transaction would do this. In the preceding example, even if the manual undo segments had the potential to grow, they will not. What you need to do for this system is have manual undo segments that are already big. You need to permanently allocate space to the undo segments, not give them the opportunity to grow on their own. The only solutions to this problem are to either make it so that the manual undo segments are sized so that they wrap only every six to ten minutes, or make it so your queries never take more than two to three minutes to execute. The first suggestion is based on the fact that you have queries that take five minutes to execute. In this case, the DBA needs to make the amount of permanently allocated undo two to three times larger. The second (perfectly valid) suggestion is equally appropriate. Any time you can make the queries go faster, you should. If the undo generated since the time your query began is never overwritten, you ll avoid ORA-01555. Under automatic undo management, things are much easier from the ORA-01555 perspective. Rather than having to figure out how big the undo space needs to be and then pre-allocating it, the DBA tells the database how long the longest-running query is and sets that value in the UNDO_RETENTION parameter. Oracle will attempt to preserve undo for at least that duration of time. If sufficient space to grow has been allocated, Oracle will extend an undo segment and not wrap around in trying to obey the UNDO_RETENTION period. This is in direct contrast to manually managed undo, which will wrap around and reuse undo space as soon as it can. It is primarily for this reason, the support of the UNDO_RETENTION parameter, that I highly recommend automatic undo management whenever possible. That single parameter reduces the possibility of an ORA-01555 error greatly (when it is set appropriately!). When using manual undo management, it is also important to remember that the probability of an ORA-01555 error is dictated by the smallest undo segment in your system, not the largest and not the average. Adding one big undo segment will not make this problem go away. It only takes the smallest undo segment to wrap around while a query is processing, and this query stands a chance of an ORA01555 error. This is why I was a big fan of equi-sized rollback segments when using the legacy rollback segments. In this fashion, each undo segment is both the smallest and the largest. This is also why I avoid using optimally sized undo segments. If you shrink an undo segment that was forced to grow, you are throwing away a lot of undo that may be needed right after that. It discards the oldest undo data when it does this, minimizing the risk, but still the risk is there. I prefer to manually shrink undo segments during off-peak times if at all. I am getting a little too deep into the DBA role at this point, so we ll move on to the next case. It s just important that you understand that the ORA-01555 error in this case is due to the system not being sized correctly for your workload. The only solution is to size correctly for your workload. It is not your
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