Date: Thu, 3 Dec 2020 10:28:09 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: yuzuko <yuzukohosoya@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	Justin Pryzby <pryzby@telsasoft.com>,
	Daniel Gustafsson <daniel@yesql.se>,
	Amit Langote <amitlangote09@gmail.com>,
	Masahiko Sawada <masahiko.sawada@2ndquadrant.com>,
	Laurenz Albe <laurenz.albe@cybertec.at>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Greg Stark <stark@mit.edu>
Subject: Re: Autovacuum on partitioned table (autoanalyze)
Message-ID: <20201203132809.GA23362@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: 
 <CAKkQ509GJpRHRSYkrvcAnLbO3KEs4=5Nbydpv54Gu2wOQtZPJw@mail.gmail.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

Hello Yuzuko,

On 2020-Dec-02, yuzuko wrote:

> The problem Horiguchi-san mentioned is as follows:
> [explanation]

Hmm, I see.  So the problem is that if some ancestor is analyzed first,
then analyze of one of its partition will cause a redundant analyze of
the ancestor, because the number of tuples that is propagated from the
partition represents a set that had already been included in the
ancestor's analysis.

If the problem was just that, then I think it would be very simple to
solve: just make sure to sort the tables to vacuum so that all leaves
are vacuumed first, and then all ancestors, sorted from the bottom up.
Problem solved.

But I'm not sure that that's the whole story, for two reasons: one, two
workers can run simultaneously, where one analyzes the partition and the
other analyzes the ancestor.  Then the order is not guaranteed (and
each process will get no effect from remembering whether it did that one
or not).  Second, manual analyzes can occur in any order.

Maybe it's more useful to think about this in terms of rememebering that
partition P had changed_tuples set to N when we analyzed ancestor A.
Then, when we analyze partition P, we send the message listing A as
ancestor; on receipt of that message, we see M+N changed tuples in P,
but we know that we had already seen N, so we only record M.

I'm not sure how to implement this idea however, since on analyze of
ancestor A we don't have the list of partitions, so we can't know the N
for each partition.