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Re: [PATCHES] Proposed patch: synchronized_scanningGUCvariable

From: Kenneth Marshall <ktm(at)rice(dot)edu>
To: Zeugswetter Andreas ADI SD <Andreas(dot)Zeugswetter(at)s-itsolutions(dot)at>
Cc: Guillaume Smet <guillaume(dot)smet(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>,Euler Taveira de Oliveira <euler(at)timbira(dot)com>,Simon Riggs <simon(at)2ndquadrant(dot)com>,Heikki Linnakangas <heikki(at)enterprisedb(dot)com>,Neil Conway <neilc(at)samurai(dot)com>,Gregory Stark <stark(at)enterprisedb(dot)com>,"Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: [PATCHES] Proposed patch: synchronized_scanningGUCvariable
Date: 2008-01-30 13:25:18
Message-ID: 20080130132518.GS4201@it.is.rice.edu (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
On Wed, Jan 30, 2008 at 10:56:47AM +0100, Zeugswetter Andreas ADI SD wrote:
> 
> > > The plural seems better to me; there's no such thing as a solitary
> > > synchronized scan, no?  The whole point of the feature is to affect
> > > the behavior of multiple scans.
> > 
> > +1. The plural is important IMHO.
> 
> ok, good.
> 
> > As I stated earlier, I don't really like this argument (we already
> > broke badly designed applications a few times in the past) but we
> > really need a way to guarantee that the execution of a query is stable
> > and doesn't depend on external factors. And the original problem was
> > to guarantee that pg_dump builds a dump as identical as possible to
> > the existing data by ignoring external factors. It's now the case with
> > your patch.
> > The fact that it allows us not to break existing applications relying
> > too much on physical ordering is a nice side effect though :).
> 
> One more question. It would be possible that a session that turned off
> the synchronized_seqscans still be a pack leader for other later
> sessions.
> Do/should we consider that ?
> 
> The procedure would be:
> start from page 0
> iff no other pack is present fill the current scan position for others
> 

I think that allowing other scans to use the scan started by a query that
disabled the sync scans would have value. It would prevent these types
of queries from completely tanking the I/O.

+1

Ken

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