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Re: [PATCHES] Proposed patch: synchronized_scanning GUCvariable

From: "Zeugswetter Andreas ADI SD" <Andreas(dot)Zeugswetter(at)s-itsolutions(dot)at>
To: "Guillaume Smet" <guillaume(dot)smet(at)gmail(dot)com>,"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "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_scanning GUCvariable
Date: 2008-01-30 09:56:47
Message-ID: E1539E0ED7043848906A8FF995BDA57902C23FC1@m0143.s-mxs.net (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
> > 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

Andreas

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