To: Matthew <matthew@flymine.org>
cc: pgsql-performance@postgresql.org
Subject: Re: TB-sized databases 
In-reply-to: <Pine.LNX.4.58.0712061626370.3731@aragorn.flymine.org> 
References: <4544e0330711260904g45d893a8ub1731e245aa8fc84@mail.gmail.com>
	<13851.1196351131@sss.pgh.pa.us>
	<1196352866.4246.1304.camel@ebony.site>
	<200712051507.22745.xzilla@users.sourceforge.net>
	<1196933896.4255.340.camel@ebony.site>
	<20071206154212.GP5294@mathom.us> <15753.1196957598@sss.pgh.pa.us>
	<Pine.LNX.4.58.0712061626370.3731@aragorn.flymine.org>
Comments: In-reply-to Matthew <matthew@flymine.org>
	message dated "Thu, 06 Dec 2007 17:46:35 +0000"
Date: Thu, 06 Dec 2007 12:55:38 -0500
Message-ID: <17456.1196963738@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Matthew <matthew@flymine.org> writes:
> ... For this query, Postgres would perform a nested loop,
> iterating over all rows in the small table, and doing a hundred index
> lookups in the big table. This completed very quickly. However, adding the
> LIMIT meant that suddenly a merge join was very attractive to the planner,
> as it estimated the first row to be returned within milliseconds, without
> needing to sort either table.

> The problem is that Postgres didn't know that the first hit in the big
> table would be about half-way through, after doing a index sequential scan
> for half a bazillion rows.

Hmm.  IIRC, there are smarts in there about whether a mergejoin can
terminate early because of disparate ranges of the two join variables.
Seems like it should be straightforward to fix it to also consider
whether the time-to-return-first-row will be bloated because of
disparate ranges.  I'll take a look --- but it's probably too late
to consider this for 8.3.

			regards, tom lane