Re: Query optimization problem

On Wed, Jul 28, 2010 at 6:55 AM, Dimitri Fontaine
<dfontaine(at)hi-media(dot)com> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>  But here you want to have different paths for
>> the same relation that generate *different output*, and the planner
>> doesn't understand that concept.
>
> Sorry? I though what Equivalence Class provides is the "proving" that
> using this qualification or another will *not* affect the output.

In a query like...

SELECT d1.ID, d2.ID
FROM DocPrimary d1
JOIN DocPrimary d2 ON d2.BasedOn=d1.ID
WHERE (d1.ID=234409763) or (d2.ID=234409763)

...you're going to scan d1, scan d2, and then join the results. The
scan of d1 is going to produce different results depending on whether
you evaluate or not d1.ID=234409763, and the scan of d2 is going to
produce different results depending on whether or not you evaluate
d2.BasedOn=234409763.

--
Robert Haas
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