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Message Posted: Thu, 27 Apr 2006 @ 17:55:58 GMT

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Subj:   Re: How good is Explain on a SQL ?
From:   David Wellman

I completely agree with what Harold and Frank.C.Martinez have said about using the 'times' as comparitive numbers, however I would add one additional factor.

In my opinion you also need to consider the rough accuracy of the row estimates (where they're shown - and they're not always shown). My standard line when I'm teaching about Explains is:

Use the times as comparative ** providing that ** the row estimates are in the correct order of magnitude.

Most people can make a reasonable guess-estimate as to how many rows meet some selection criteria, particularly when dealing with the larger tables.



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