Re: COPY with no WAL, in certain circumstances

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: <pgsql-hackers(at)postgresql(dot)org>, "Robert Treat" <xzilla(at)users(dot)sourceforge(dot)net>, <pgsql-patches(at)postgresql(dot)org>, "Bruce Momjian" <bruce(at)momjian(dot)us>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>
Subject: Re: COPY with no WAL, in certain circumstances
Date: 2007-01-07 08:28:17
Message-ID: 1168158497.3951.28.camel@silverbirch.site
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On Sat, 2007-01-06 at 21:32 -0500, Tom Lane wrote:
> Robert Treat <xzilla(at)users(dot)sourceforge(dot)net> writes:
> > On Saturday 06 January 2007 16:36, Simon Riggs wrote:
> > <snip>
> >> BEGIN;
> >> CREATE TABLE foo...
> >> INSERT INTO foo --uses WAL
> >> COPY foo.. --no WAL
> >> INSERT INTO foo --uses WAL
> >> COPY foo.. --no WAL
> >> INSERT INTO foo --uses WAL
> >> COPY foo... --no WAL
> >> COMMIT;
>
> > Is there some technical reason that the INSERT statements need to use WAL in
> > these scenarios?
>
> First, there's enough other overhead to an INSERT that you'd not save
> much percentagewise. Second, not using WAL doesn't come for free: the
> cost is having to fsync the whole table afterwards. So it really only
> makes sense for commands that one can expect are writing pretty much
> all of the table. I could easily see it being a net loss for individual
> INSERTs.

Agreed. We agreed that before, on the original design thread.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com

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