To: Josh Berkus <josh@agliodbs.com>
cc: postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: Shortcutting too-large offsets? 
In-reply-to: <4E860E09.7090107@agliodbs.com> 
References: <4E850FC0.509@agliodbs.com> <6843.1317393410@sss.pgh.pa.us>
	<4E860E09.7090107@agliodbs.com>
Comments: In-reply-to Josh Berkus <josh@agliodbs.com>
	message dated "Fri, 30 Sep 2011 11:44:25 -0700"
Date: Fri, 30 Sep 2011 14:56:42 -0400
Message-ID: <11059.1317409002@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Josh Berkus <josh@agliodbs.com> writes:
>> 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.

> It's not that uncommon of a corner case though; it's one which happens
> all the time in webapps which paginate.  People just have to ask for a
> page after the end.  It's really a question of how simple the code to
> make the optimization would be; if it would be a 5-line patch, then it's
> worth it; if it would be a 110-line refactor, no.

No, it's really a question of whether it's worth any lines at all,
and I remain unconvinced.  If someone asks for the last page, or any
page near the end, it'll take just about the same amount of time as
asking for a page past the end.  So any app like this is going to have
sucky performance, and kluging the corner case isn't going to help much.

			regards, tom lane