To: Stephan Szabo <sszabo@megazone.bigpanda.com>
Cc: Rod Taylor <rbt@rbt.ca>, ow <oneway_111@yahoo.com>,
	pgsql-sql@postgresql.org
Subject: Re: 7.4 - FK constraint performance 
In-reply-to: <20040213073852.K20792@megazone.bigpanda.com> 
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	<20040213065530.M19832@megazone.bigpanda.com>
	<2455.1076686047@sss.pgh.pa.us>
	<20040213073852.K20792@megazone.bigpanda.com>
Comments: In-reply-to Stephan Szabo <sszabo@megazone.bigpanda.com>
	message dated "Fri, 13 Feb 2004 07:49:59 -0800"
Date: Fri, 13 Feb 2004 10:56:44 -0500
Message-ID: <2662.1076687804@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Stephan Szabo <sszabo@megazone.bigpanda.com> writes:
> On Fri, 13 Feb 2004, Tom Lane wrote:
>> I was looking at that last night.  It seems like we could add a LIMIT at
>> least in some contexts.  In the case at hand, we're just going to error
>> out immediately if we find a matching row, and so there's no need for
>> FOR UPDATE, is there?

> I think there still is, because a not yet committed transaction could have
> deleted them all in which case I think the correct behavior is to wait and
> if that transaction commits allow the action and if it rolls back to
> error.

Good point.  Okay, we can't put in a LIMIT.  But we could still hack the
planner to prefer a fast-start plan by passing an out-of-band tuple
fraction, for those RI plans where it's appropriate.  That would not
affect correctness.

>> However, I'm not sure it would help the OP anyway.  With the stats he
>> had, the planner would still take a seqscan, because it's going to
>> expect that it can find a match by probing the first ten or so rows of
>> the first page.  With anything close to the normal cost parameters,
>> that's going to look more expensive than an index probe.

s/more expensive/less expensive/ ... need more caffeine obviously ...

			regards, tom lane