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Re: LBO/CBO

From: Richard Foote <richard.foote_at_bigpond.com>
Date: Fri, 27 Sep 2002 22:44:12 +1000
Message-ID: <GdYk9.41123$g9.118181@newsfeeds.bigpond.com>


Hi Marc,

See Below.

"Marc Zinzen" <mzinzen_at_genese.de> wrote in message news:3D944739.9DB6C71E_at_genese.de...
> Hi.
>
> Sorry to bother again.
> Two more questions came up.
> 1. If the Rulebase Optimizer executes a query how does it read its
predicates.
> Left to right or right to left.

To understand the RBO, may I suggest reading the Performance Tuning Manual (Chaper 8 using the RBO). The answers are all in there and is worth a read if you plan to use RBO in any way.

Note though that the RBO is doomed, with 9i probably being it's last gasp.

> 2. The CBO. I have 2 tables. One very big. One very small. What happens
when the
> Selectivity value is the same. Will the optimizer acces the table with the
least
> results first?

Not quite sure what you mean here. Are you saying that you wish to join these two tables, what will the CBO do ? It depends on the selectiviy or the percentage of rows you need to return from each table. It also depends very much on which optimizer you are referring to. It also depends on how these tables are joined, whether there are indexes on the join columns and what other predicates and hence what other indexes may come into play.

In answer, it really does depend ...

But the optimizer will try and select an execution plan that is "less costly" based on the number of rows you wish retrieved.

Cheers

Richard
>
> Marc
>
Received on Fri Sep 27 2002 - 07:44:12 CDT

Original text of this message

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