Re: ORACLE query optimiser question

From: Bruce Horrocks <bh_at_granby.demon.co.uk>
Date: Sat, 25 Feb 1995 20:48:12 +0000
Message-ID: <793745292snz_at_granby.demon.co.uk>


In article <199502241524.PAA19201_at_helios>

           cs5gr_at_scms.rgu.ac.uk "Graeme Raffan" writes:

>
>Does anyone know what optimization algorithm the ORACLE
>query optimizer is based on.
>

Oracle uses statistics gathered on the data to estimate the number of rows in each table that would be affected by the query. The values are then used to determine the optimum join order.

A shortcoming of Oracle's approach is that it only computes statistics for columns that are indexed. Ideally statistics should be maintained for all columns that appear in the where clause.

Regards,

-- 
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Bruce Horrocks
Hampshire, England                   Object Oriented programmers only
                                     need two methods - Create and Scoop-Up
bh_at_granby.demon.co.uk
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Received on Sat Feb 25 1995 - 21:48:12 CET

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