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CHAPTER 9 REDO AND UNDO
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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 rollback segment in your system, not the largest and not the average. Adding one big rollback segment will not make this problem go away. It only takes the smallest rollback segment to wrap around while a query is processing, and this query stands a chance of an ORA-01555 error. This is why I was a big fan of equi-sized rollback segments when using the legacy rollback segments. In this fashion, each rollback segment is both the smallest and the largest. This is also why I avoid using optimally sized rollback segments. If you shrink a rollback 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 rollback data when it does this, minimizing the risk, but still the risk is there. I prefer to manually shrink rollback 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 be moving 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 fault, but it is your problem since you hit it. It is the same as if you run out of temporary space during a query. You either configure sufficient temporary space for the system, or you rewrite the queries so they use a plan that does not require temporary space. To demonstrate this effect, we can set up a small, but somewhat artificial test. We ll create a very small undo tablespace with one session that will generate many small transactions, virtually assuring us that it will wrap around and reuse its allocated space many times regardless of the UNDO_RETENTION setting, since we are not permitting the undo tablespace to grow. The session that uses this undo segment will be modifying a table, T. It will use a full scan of T and read it from top to bottom. In another session, we will execute a query that will read the table T via an index. In this fashion, it will read the table somewhat randomly: it will read row 1, then row 1,000, then row 500, then row 20,001, and so on. In this way, we will tend to visit blocks very randomly and perhaps many times during the processing of our query. The odds of getting an ORA-01555 error in this case are virtually 100 percent. So, in one session we start with the following: ops$tkyte@ORA10G> create undo tablespace undo_small 2 datafile size 2m 3 autoextend off 4 / Tablespace created. ops$tkyte@ORA10G> alter system set undo_tablespace = undo_small; System altered. Now, we ll set up the table T used to query and modify. Note that we are ordering the data randomly in this table. The CREATE TABLE AS SELECT tends to put the rows in the blocks in the order it fetches them from the query. We ll just scramble the rows up so they are not artificially sorted in any order, randomizing their distribution:
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