To: Josh Berkus <josh@agliodbs.com>
cc: postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: Shortcutting too-large offsets? 
In-reply-to: <4E850FC0.509@agliodbs.com> 
References: <4E850FC0.509@agliodbs.com>
Comments: In-reply-to Josh Berkus <josh@agliodbs.com>
	message dated "Thu, 29 Sep 2011 17:39:28 -0700"
Date: Fri, 30 Sep 2011 10:36:50 -0400
Message-ID: <6843.1317393410@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Josh Berkus <josh@agliodbs.com> writes:
> Here's a case which it seems like we ought to be able to optimize for:
> [ offset skips all the output of a sort node ]
> Is there some non-obvious reason which would make this kind of
> optimization difficult?  Doesn't the executor know at that point how
> many rows it has?

In principle, yeah, we could make it do that, but it seems like a likely
source of maintenance headaches.  This example is not exactly compelling
enough to make me want to do it.  Large OFFSETs are always going to be
problematic from a performance standpoint, and the fact that we could
short-circuit this one corner case isn't really going to make them much
more usable.

			regards, tom lane