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Message Posted: Thu, 11 Jan 2001 @ 13:57:22 GMT


     
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Subj:   Re: Limited Select
 
From:   Adam DeYoung

We use the times in EXPLAINS as a very rough estimate, and look much more closely at the steps in the EXPLAIN. If the EXPLAIN says a query will take 200 hours, 5 minutes and 12 seconds to complete but it appears that the parser is building the query to take advantage of joins and indexes "correctly", we will execute the query and monitor its run time and spool consumption. 99 times out of 100 the query completes in less than 20% of the estimated time.

If the parser gives the same time estimate as above but indicates it will do one or more product joins where we think it should not (or returns a truly enormous time estimate in the thousands of hours), we examine and modify the joins in the WHERE clause of the query and re- explain. In no case do we hold Teradata to its EXPLAIN time and do not get concerned if a query runs over (or under) the EXPLAIN estimate. The steps in the EXPLAIN are a much better indicator of whether the query is working well or not.

Adam DeYoung



     
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