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LIGHTLY TECHNICAL OBSERVATIONS ON PEOPLESOFT, SQR, AND RELATED AND UNRELATED TOPICS

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Dropping the User IndexKEVIN RESCHENBERG
02-21-2005

Last week I talked about adding a user index on JOB.EMPL_STATUS that resulted in an 80% reduction in run time for a query. Well, bad news on that one—I have now dropped the new index. It did, in fact, save 80% of the run time for that query, but only for that one day!

The person testing these queries sent me another similar one that was now running seemingly forever. The query plan showed that it was using the new index. I dropped the index and the query plan showed a major reshuffling of the various access paths. Now the SELECT was running slowly, but at least it was finishing within a reasonable period of time. I recreated the index and now the query ran quickly. What was going on? This situation should have been the same as what I started with, but instead of taking "forever", the query was now finishing within seconds.

Updating the statistics on JOB revealed the problem. We update the statistics in our database every night. During this operation, the database (Oracle, in this case) analyzes each table and index to understand the types of data it contains. This theoretically helps in developing the best query plan.

When I created the index, Oracle didn't look closely at the data and just made some generic assumptions about the index. But when the statistics were eventually updated, Oracle noticed that this index contains very few unique values (mostly "A" and "T"), and this changed its opinion of how the index should be used. That, unfortunately, destroyed its usefulness as an index for our queries. This is why this particular index was a good idea—for one day!

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