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 ---------------------------(end of broadcast)--------------------------- TIP 4: Have you searched our list archives? http://archives.postgresql.org