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Re: Find a better plan than that computed by the optimizer ??

From: Spendius <spendius_at_muchomail.com>
Date: 23 Feb 2005 01:11:04 -0800
Message-ID: <aba30b75.0502230111.48f4cca1@posting.google.com>


So how can you manage to find out -by looking at the result of a 10053 trace- that the optimizer did miss something ?!? (as long as of course you have factorial(N) join combinations, with N as the number of tables in your FROM clause) This is what I can't figure out...

Thanks.

"Jonathan Lewis" <jonathan_at_jlcomp.demon.co.uk> wrote...
> First of all, the optimizer is programmed to
> stop checking after a (variable) time limit,
> so it may never reach the best plan.
>
> Secondly, the optimizer has an algorithm
> to generate a set of join orders for testing,
> and may therefore skip over an order that
> would be better than any order it will otherwise
> generate. (This is, in part, a side-effect of the
> time-limit above).
>
> Third, the optimizer has a number of assumptions
> built in that may not be true for your system, so
> the best cost path it finds may be totally inappropriate
> for the system you are running.
>
> Finally, you know the business better than the optimizer,
> so there may be some information available to you that
> allows you to select a path that is technically unsuitable
> as far as the optimizer is concerned.
>
Received on Wed Feb 23 2005 - 03:11:04 CST

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