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Re: Query optimization problem

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Yeb Havinga <yebhavinga(at)gmail(dot)com>
Cc: Dimitri Fontaine <dfontaine(at)hi-media(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Zotov <zotov(at)oe-it(dot)ru>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Query optimization problem
Date: 2010-07-28 11:30:48
Message-ID: AANLkTi=UTjEsnjcfz=C2kkf65ufq7-6+fQ97yMKeMWyk@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Jul 28, 2010 at 7:24 AM, Yeb Havinga <yebhavinga(at)gmail(dot)com> wrote:
>>> 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)

Yeah, that could be done, but it's not necessarily a win from a
performance standpoint.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company

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