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Message Posted: Wed, 26 Apr 2006 @ 20:34:35 GMT
| Subj: | | Re: How good is Explain on a SQL ? |
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| From: | | Anomy Anom |
<-- Anonymously Posted: Wednesday, April 26, 2006 15:44 -->
Original:
| | When I run the Explain on a SQL few steps shows High Confidence and few as no confidence. | |
| | When and why are the Confidence high, low and No ? | |
The confidence level is related to how accurate the estimate of the optimizer process your request. It had to do the freshness of stats on the
join condition. This is a cost based optimizer so more relevent stats will enable optimizer to process your query with higher level of confidence
in creating the join plan which we extract using explain modifier for review and troubleshooting.
| | Why it the Actual run time(30 mins) a lot more then the Estimated time(2 mins) ? | |
Depend on the confidence level of the explain for that estimate, it could be more or less than what was estimated. If the estimate was to
process 1000 records and actual process required 1000,000,000 record processing then the time it take will deviate greatly. You can use PMON to
see the processing explain and SQL while the query is active on the system to get better understanding of how the system is processing your
queries.
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