To: Robert Haas <robertmhaas@gmail.com>
cc: 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 "Wed, 25 May 2011 13:15:59 -0400"
Date: Wed, 25 May 2011 17:54:30 -0400
Message-ID: <28141.1306360470@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, May 25, 2011 at 1:04 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Because the problem is not specific to TOAST tables.  As things
>> currently stand, we will accept the word of an ANALYZE as gospel even if
>> it scanned 1% of the table, and completely ignore the results from a
>> VACUUM even if it scanned 99% of the table.  This is not sane.

> I agree that if VACUUM scanned 99% of the table, it's probably fine to
> use its numbers.  It's also fine to use the numbers from ANALYZE,
> because those pages are chosen randomly.  What bothers me is the idea
> of using a small *non-random* sample, and I'm not sure that
> incorporating possibly-bogus results slowly is any better than
> incorporating them quickly.

The above is simply fuzzy thinking.  The fact that ANALYZE looked at a
random subset of pages is *no guarantee whatsoever* that its results are
highly accurate.  They might be more trustworthy than VACUUM's nonrandom
sample of a similar number of pages, but it doesn't hold even a little
bit of water to claim that we should believe ANALYZE completely and
VACUUM not at all even when the latter has looked at a significantly
larger sample of pages.

In any case, your line of thought doesn't help us for fixing the problem
with toast tables, because we aren't going to start doing ANALYZEs on
toast tables.

The bottom line here is that making use of stats we have is a lot better
than not making use of them, even if they aren't entirely trustworthy.

			regards, tom lane