From: Tom Lane <tgl@sss.pgh.pa.us>
To: Matt Silverlock <matt@eatsleeprepeat.net>
cc: pgsql-performance@postgresql.org
Subject: Re: OFFSET/LIMIT - Disparate Performance w/ Go application
In-reply-to: <1D92FBC7-76DD-4B96-86E5-4989878C2847@eatsleprepeat.net>
References: <1D92FBC7-76DD-4B96-86E5-4989878C2847@eatsleprepeat.net>
Comments: In-reply-to Matt Silverlock <matt@eatsleeprepeat.net>
 message dated "Thu, 12 Jun 2014 15:08:27 +0800"
Date: Thu, 12 Jun 2014 10:58:06 -0400
Message-ID: <24297.1402585086@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

Matt Silverlock <matt@eatsleeprepeat.net> writes:
> Hi all. This might be tricky in so much as there’s a few moving parts (when isn’t there?), but I’ve tried to test the postgres side as much as possible.
> Trying to work out a potential database bottleneck with a HTTP application (written in Go):
> Pages that render HTML templates but don’t perform DB queries can hit ~36k+ req/s
> Pages that perform a SELECT on a single row net about ~6.6k req/s: db.Get(l, "SELECT * FROM listings WHERE id = $1 AND expiry_date > current_date", l.Id)
> Pages that SELECT multiple rows with OFFSET and LIMIT conditions struggle to top 1.3k req/s

You don't show us exactly what you're doing with OFFSET/LIMIT, but I'm
going to guess that you're using it to paginate large query results.
That's basically always going to suck: Postgres has no way to implement
OFFSET except to generate and then throw away that number of initial rows.
If you do the same query over again N times with different OFFSETs, it's
going to cost you N times as much as the base query would.

If the application's interaction with the database is stateless then you
may not have much choice, but if you do have a choice I'd suggest doing
pagination by means of fetching from a cursor rather than independent
queries.

			regards, tom lane


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