To: Robert Haas <robertmhaas@gmail.com>
cc: Greg Stark <gsstark@mit.edu>, Florian Helmberger <fh@25th-floor.com>,
	pgsql-hackers@postgresql.org, Alvaro Herrera <alvherre@commandprompt.com>
Subject: Re: [ADMIN] pg_class reltuples/relpages not updated by
	autovacuum/vacuum
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Thu, 26 May 2011 11:55:58 -0400"
Date: Thu, 26 May 2011 12:23:02 -0400
Message-ID: <25097.1306426982@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Robert Haas <robertmhaas@gmail.com> writes:
> I would feel a lot better about something that is deterministic, like,
> I dunno, if VACUUM visits more than 25% of the table, we use its
> estimate.  And we always use ANALYZE's estimate.  Or something.

This argument seems to rather miss the point.  The data we are working
from is fundamentally not deterministic, and you can't make it so by
deciding to ignore what data we do have.  That leads to a less useful
estimate, not a more useful estimate.

> Another thought: Couldn't relation_needs_vacanalyze() just scale up
> reltuples by the ratio of the current number of pages in the relation
> to relpages, just as the query planner does?

Hmm ... that would fix Florian's immediate issue, and it does seem like
a good change on its own merits.  But it does nothing for the problem
that we're failing to put the best available information into pg_class.

Possibly we could compromise on doing just that much in the back
branches, and the larger change for 9.1?

			regards, tom lane