From: Noah Misch <noah@leadboat.com>
To: Michael Meskes <meskes@postgresql.org>
Cc: Boszormenyi Zoltan <zb@cybertec.at>
Cc: PG Hackers <pgsql-hackers@postgresql.org>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: Bruce Momjian <bruce@momjian.us>
Subject: Re: ECPG FETCH readahead
Date: Mon, 5 Mar 2012 13:50:04 -0500
Message-ID: <20120305185004.GD13348@tornado.leadboat.com> (raw)
In-Reply-To: <20120304161606.GA4706@feivel.credativ.lan>
References: <201006232042.o5NKgb503695@momjian.us>
<4C2308FD.1090300@cybertec.at>
<4C231F9E.6000102@enterprisedb.com>
<20100624121922.GD24137@feivel.credativ.lan>
<AANLkTinij9ULnS54zKsdAJnhvuj-07-8-T-D=Nwk6f14@mail.gmail.com>
<4CB6D3C8.3020709@cybertec.at>
<4EC41434.7010603@cybertec.at>
<4EFC36EF.1060308@cybertec.at>
<20120302164105.GD23100@tornado.leadboat.com>
<20120304161606.GA4706@feivel.credativ.lan>
On Sun, Mar 04, 2012 at 05:16:06PM +0100, Michael Meskes wrote:
> On Fri, Mar 02, 2012 at 11:41:05AM -0500, Noah Misch wrote:
> > I suggest enabling the feature by default but drastically reducing the default
> > readahead chunk size from 256 to, say, 5. That still reduces the FETCH round
> > trip overhead by 80%, but it's small enough not to attract pathological
> > behavior on a workload where each row is a 10 MiB document. I would not offer
> > an ecpg-time option to disable the feature per se. Instead, let the user set
> > the default chunk size at ecpg time. A setting of 1 effectively disables the
> > feature, though one could later re-enable it with ECPGFETCHSZ.
>
> Using 1 to effectively disable the feature is fine with me, but I strongly
> object any default enabling this feature. It's farily easy to create cases with
> pathological behaviour and this features is not standard by any means. I figure
> a normal programmer would expect only one row being transfered when fetching
> one.
On further reflection, I agree with you here. The prospect for queries that
call volatile functions changed my mind; they would exhibit different
functional behavior under readahead. We mustn't silently give affected
programs different semantics.
Thanks,
nm
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To: pgsql-hackers@postgresql.org
Cc: noah@leadboat.com, meskes@postgresql.org, zb@cybertec.at, robertmhaas@gmail.com, heikki.linnakangas@enterprisedb.com, bruce@momjian.us
Subject: Re: ECPG FETCH readahead
In-Reply-To: <20120305185004.GD13348@tornado.leadboat.com>
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