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From: John Naylor <john.naylor@enterprisedb.com>
Date: Thu, 3 Feb 2022 16:18:27 -0500
Message-ID: <CAFBsxsHrkSrFBg+w27xWGSgVrTJ4WeSuseoKW3FHhOkgYSsWDQ@mail.gmail.com>
Subject: Re: do only critical work during single-user vacuum?
To: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Peter Geoghegan <pg@bowt.ie>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Thu, Feb 3, 2022 at 1:42 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Thu, Feb 3, 2022 at 1:34 PM John Naylor <john.naylor@enterprisedb.com> wrote:
> > The word "advice" sounds like people have a choice, rather than the
> > system not accepting commands anymore. It would be much less painful
> > if the system closed connections and forbade all but superusers to
> > connect, but that sounds like a lot of work. (happy to be proven
> > otherwise)
>
> They *do* have a choice. They can continue to operate the system in
> multi-user mode, they can have read access to their data, and they can
> run VACUUM and other non-XID-allocating commands to fix the issue.
> Sure, their application can't run commands that allocate XIDs, but
> it's not going to be able to do that if they go to single-user mode
> either.

I just checked some client case notes where they tried just that
before getting outside help, and both SELECT and VACUUM FREEZE
commands were rejected. The failure is clearly indicated in the log.
-- 
John Naylor
EDB: http://www.enterprisedb.com