From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: Mladen Gogala <mladen.gogala@vmsinfo.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: anti-join chosen even when slower than old plan
Date: Thu, 11 Nov 2010 13:23:03 -0500
Message-ID: <18450.1289499783@sss.pgh.pa.us> (raw)
In-Reply-To: <AANLkTi=dzhRTTNm1LkyNCAC5+PWbp1jWtKFh4T1Bu4nj@mail.gmail.com>
References: <4CD9663502000025000374B1@gw.wicourts.gov>
<20961.1289347680@sss.pgh.pa.us>
<4CDA62C60200002500037512@gw.wicourts.gov>
<AANLkTi=eNXKEBo5dXJDd4+aHhfmkibH++fqjj12+DA3o@mail.gmail.com>
<4CDACBBF020000250003755B@gw.wicourts.gov>
<4CDB7BAA.7020506@vmsinfo.com>
<4CDBB0A40200002500037571@gw.wicourts.gov>
<AANLkTi=dzhRTTNm1LkyNCAC5+PWbp1jWtKFh4T1Bu4nj@mail.gmail.com>
Robert Haas <robertmhaas@gmail.com> writes:
> Let's back up a moment and talk about what the overall goal is, here.
> Ideally, we would like PostgreSQL to have excellent performance at all
> times, under all circumstances, with minimal tuning. Therefore, we do
> NOT want to add variables that will, by design, need constant manual
> adjustment. That is why I suggested that Tom's idea of an
> assume_cached GUC is probably not what we really want to do. On the
> other hand, what I understand Mladen to be suggesting is something
> completely different. He's basically saying that, of course, he wants
> it to work out of the box most of the time, but since there are
> guaranteed to be cases where it doesn't, how about providing some
> knobs that aren't intended to be routinely twaddled but which are
> available in case of emergency? Bravo, I say!
Um ... those are exactly the same thing. You're just making different
assumptions about how often you will need to twiddle the setting.
Neither assumption is based on any visible evidence, unfortunately.
I was thinking of assume_cached as something that could be
set-and-forget most of the time, and you're entirely right to criticize
it on the grounds that maybe it wouldn't. But to support a proposal
that doesn't even exist yet on the grounds that it *would* be
set-and-forget seems a tad inconsistent. We can't make that judgment
without a whole lot more details than have been provided yet for any
idea in this thread.
I do think that something based around a settable-per-table caching
percentage might be a reasonable way to proceed. But the devil is in
the details, and we don't have those yet.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, robertmhaas@gmail.com, Kevin.Grittner@wicourts.gov, mladen.gogala@vmsinfo.com
Subject: Re: anti-join chosen even when slower than old plan
In-Reply-To: <18450.1289499783@sss.pgh.pa.us>
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