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Re: Cost estimates for parameterized paths

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Cost estimates for parameterized paths
Date: 2010-09-03 21:53:53
Message-ID: 536.1283550833@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Fri, Sep 3, 2010 at 2:04 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> On reflection I think that for parameterized paths the problem won't be
>> too bad, because (a) we'll ignore parameterized paths except when
>> considering a join to the right outer rel, so their presence in the
>> rel's pathlist won't cost much otherwise,

> Hmm.  Maybe they should go into a separate path list, and perhaps we
> could do the min/max calculations only with that pathlist (at least
> for now), thus avoiding taking a generalized penalty to handle this
> specific case.  IIUC, a parameterized path should never cause an
> unparamaterized path to be thrown out,

Yeah, but the converse isn't true.  I had considered the idea of keeping
parameterized paths in a separate list, but you'd still have to examine
the main list to look for unparameterized paths that might dominate them.

			regards, tom lane

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