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Re: Rule Based Optimization

From: Terry Coccoli <request_at_ifneeded.com>
Date: Mon, 05 Apr 2004 22:09:59 GMT
Message-ID: <XOkcc.12498199$Of.2086398@news.easynews.com>

Daniel Roy wrote:

>>What happens if you throw in a hint like FIRST_ROWS?  Would the 
>>optimizer accept the hint ?

>
>
> If you throw any hint to a query (except "RULE"), cost-based
> optimizing is triggered. In that case, since you probably don't have
> any stats for your tables, Oracle will try to make up these stats by
> itself, using criteria such as number of extents used by a table (to
> estimate its size), ... Oracle usually does a decent job at making up
> these stats (from what I've seen), but of course it would be much
> better to have real up-to-date stats if you intend to use CBO for some
> queries. If you decide to go that route (use hints and gather the
> stats), make sure that OPTIMIZER_MODE (for the instance running
> Siebel) is RULE (and not CHOOSE), or otherwise CBO will be used for
> all the queries (including the ones from the Siebel application)
> running against the database, and your Siebel TAM will slap your
> fingers.
>
> Daniel Roy
> IBM
> Siebel/Oracle Consultant

Thanks for the headsup, Roy. I think you've given me a couple of questions that I need to discuss with the DBA. Received on Mon Apr 05 2004 - 17:09:59 CDT

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