To: "Pavan Deolasee" <pavan.deolasee@gmail.com>
cc: "Heikki Linnakangas" <heikki@enterprisedb.com>,
	"Craig Ringer" <craig@postnewspapers.com.au>,
	pgsql-patches <pgsql-patches@postgresql.org>
Subject: Re: [PERFORM] Very slow (2 tuples/second) sequential scan after bulk
	insert; speed returns to ~500 tuples/second after commit
In-reply-to: <2e78013d0803121002o2b7a8531y1553ce66cb6d1537@mail.gmail.com> 
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Comments: In-reply-to "Pavan Deolasee" <pavan.deolasee@gmail.com>
	message dated "Wed, 12 Mar 2008 22:32:37 +0530"
Date: Wed, 12 Mar 2008 13:22:13 -0400
Message-ID: <23603.1205342533@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Pavan Deolasee" <pavan.deolasee@gmail.com> writes:
> On Wed, Mar 12, 2008 at 9:27 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> and it would have problems with a slow transaction
>> generating a sparse set of subtransaction XIDs.

> I agree thats the worst case. But is that common ? Thats what I
> was thinking when I proposed the alternate solution. I thought that can
> happen only if most of the subtransactions abort, which again I thought
> is not a normal case.

No, I was thinking of the case where other sessions are chewing up XIDs
while the lots-of-subtransactions transaction runs.  If it's slow
enough, there could be very large gaps between the XIDs it acquires for
its subtransactions.  So you'd have a situation where the exact same
transaction processing might or might not run out of memory depending
on what else happened meanwhile.  Not a very pleasant property.

			regards, tom lane