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