Content-Type: text/plain; charset=UTF-8
Cc: Kevin Grittner <kevin.grittner@wicourts.gov>,
 Tom Lane <tgl@sss.pgh.pa.us>,
	Florian Helmberger <fh@25th-floor.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: [ADMIN] pg_class reltuples/relpages not updated by
	autovacuum/vacuum
From: Alvaro Herrera <alvherre@commandprompt.com>
To: =?utf-8?q?C=C3=A9dric_Villemain?= <cedric.villemain.debian@gmail.com>
In-reply-to: <BANLkTimyfvQXsLswzBCV5PFi+NN_TeDZ+g@mail.gmail.com>
References: <4DD28F73.6010108@25th-floor.com> <25017.1306291669@sss.pgh.pa.us>
	<4DDCA801.70805@25th-floor.com> <19738.1306338472@sss.pgh.pa.us>
	<1306339028-sup-6948@alvh.no-ip.org>
	<22173.1306340636@sss.pgh.pa.us>
	<4DDCE9F4020000250003DC48@gw.wicourts.gov>
	<1306342228-sup-2964@alvh.no-ip.org>
	<BANLkTimyfvQXsLswzBCV5PFi+NN_TeDZ+g@mail.gmail.com>
Date: Wed, 25 May 2011 13:37:07 -0400
Message-Id: <1306344946-sup-417@alvh.no-ip.org>
User-Agent: Sup/git
Content-Transfer-Encoding: 8bit

Excerpts from Cédric Villemain's message of mié may 25 13:24:01 -0400 2011:

> > Well, we only actually need to store one number, because you can figure
> > out a much more precise number-of-pages figure with pg_relation_size()
> > divided by configured page size.

> I may miss something but we need relation size in costsize.c even if
> we have a reldensity (or we need a reltuples). Else what values should
> be used to estimate the relation size ? (pg_relation_size() goes down
> to kernel/fs to ask the stat.st_size, is it really what we want?)

Actually yes, the planner does go to kernel to determine the current
relation size, and then multiplies by density as computed from catalog
data to figure out current reasonably accurate number of tuples.

-- 
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support