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From: =?ISO-8859-1?Q?C=E9dric_Villemain?= <cedric.villemain.debian@gmail.com>
Date: Wed, 25 May 2011 21:14:06 +0200
Message-ID: <BANLkTinKDYoBo8ZpreagS3E5wkfL5zYNcg@mail.gmail.com>
Subject: Re: [ADMIN] pg_class reltuples/relpages not updated by
	autovacuum/vacuum
To: Alvaro Herrera <alvherre@commandprompt.com>
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>
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2011/5/25 Alvaro Herrera <alvherre@commandprompt.com>:
> Excerpts from C=E9dric Villemain's message of mi=E9 may 25 13:24:01 -0400=
 2011:
>
>> > Well, we only actually need to store one number, because you can figur=
e
>> > 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.

Okay! I just read that part. Interesting.
(If I dive correctly, we search our last segment and then use a
fileseek to the end of this segment to get our information)

make more sense, suddendly :)

>
> --
> =C1lvaro Herrera <alvherre@commandprompt.com>
> The PostgreSQL Company - Command Prompt, Inc.
> PostgreSQL Replication, Consulting, Custom Development, 24x7 support
>


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