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Re: Musings on tuning and the optimizer

From: Jared Still <jkstill_at_cybcon.com>
Date: Mon, 28 May 2001 10:12:35 -0700
Message-ID: <F001.003111A9.20010528092546@fatcity.com>

On Monday 28 May 2001 04:45, Nuno Souto wrote:

> > Always thought that the RBO joins your tables in the order found in
>> the FROM clause? Think again.
>
> Actually, I found one in Metalink that says with RBO, it's the reverse
> order of the FROM clause. Which I was aware of. But it also
> says: if there are NO STATS whatsoever and you use a hint that forces
> the CBO to be used or it is the default, the order of tables is
> left to right. Like what you get when using the hint ORDERED.
>

The RBO may consider them in reverse order of how they are listed in the FROm clause, but the docs are quite clear that this is not necessarily the order they will be joined in.

> And a few others. I found out the problem I reported
> a while ago with CBO suddenly going South on hash scan joins and
> completely ignoring nested loops or indexes is actually an introduced
> problem due to a change in CBO rules after 8.0.4. It first
> affected SAP users. It used to be fixed by a couple of events
> which due to their usefulness, became the two "new" optimizer cost
> adjust parameters in later versions of 8.0 and some of the 8.1.

I think a lot of us have been bitten by that. It has caused a lot of extra work for me when upgrading.

Jared

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Author: Jared Still
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Received on Mon May 28 2001 - 12:12:35 CDT

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