Date: Sun, 4 Mar 2018 19:19:18 -0800
From: David Gould <daveg@sonic.net>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
        Alina Alexeeva
 <alexeeva@adobe.com>,
        Ullas Lakkur Raghavendra <lakkurra@adobe.com>
Subject: Re: [patch] BUG #15005: ANALYZE can make pg_class.reltuples
 inaccurate.
Message-ID: <20180304191918.39e95382@engels>
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 <CAMkU=1yQz4OEmoWSnY_wE03KdpoQ4zpzmjpyPDUQwz+tbsU21g@mail.gmail.com>
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	<CAMkU=1yQz4OEmoWSnY_wE03KdpoQ4zpzmjpyPDUQwz+tbsU21g@mail.gmail.com>
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On Sun, 4 Mar 2018 07:49:46 -0800
Jeff Janes <jeff.janes@gmail.com> wrote:

> I don't see how it could have caused the problem in the first place.  In
> your demonstration case, you had to turn off autovac in order to get it to
> happen, and then when autovac is turned back on, it is all primed for an
> autovac to launch, go through, touch almost all of the pages, and fix it
> for you.  How did your original table get into a state where this wouldn't
> happen?

One more way for this to happen, vacuum was including the dead tuples in the
estimate in addition to the live tuples. This is a separate bug that tends
to aggravate the one I'm trying to fix. See the thread re BUG #15005 at:

https://www.postgresql.org/message-id/16db4468-edfa-830a-f921-39a50498e77e%402ndquadrant.com
> It seems to me that VACUUM and ANALYZE somewhat disagree on what exactly 
> reltuples means. VACUUM seems to be thinking that
> 
>      reltuples = live + dead
> 
> while ANALYZE apparently believes that
> 
>      reltuples = live

There is a patch for this one from Tomas Vondra/Tom Lane that I hope it will
land in the next set of releases.

-dg

-- 
David Gould                                   daveg@sonic.net
If simplicity worked, the world would be overrun with insects.