Re: Query optimization problem

Robert Haas wrote:
> 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.
>
Wouldn't it be relatively easy, to rewrite the filter expression by
adding expressions, instead of replacing constants, in the disjunctive
case, so the example at hand would become:

WHERE (d1.ID=234409763) or (d2.ID=234409763)
AND (d2.BasedOnID=234409763) or (d2.ID=234409763)

regards,
Yeb Havinga