From: Tom Lane <tgl@sss.pgh.pa.us>
To: Josh Berkus <josh@agliodbs.com>
Cc: postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: Shortcutting too-large offsets?
Date: Fri, 30 Sep 2011 10:36:50 -0400
Message-ID: <6843.1317393410@sss.pgh.pa.us> (raw)
In-Reply-To: <4E850FC0.509@agliodbs.com>
References: <4E850FC0.509@agliodbs.com>
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
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, josh@agliodbs.com
Subject: Re: Shortcutting too-large offsets?
In-Reply-To: <6843.1317393410@sss.pgh.pa.us>
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