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From: John Naylor <john.naylor@enterprisedb.com>
Date: Tue, 21 Dec 2021 17:35:05 -0400
Message-ID: 
 <CAFBsxsE1TWidnMs=RMx1eb2QxqmNqdDaLLN5E7SP_v5_g2ZRwA@mail.gmail.com>
Subject: Re: do only critical work during single-user vacuum?
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Andres Freund <andres@anarazel.de>,
	"Bossart, Nathan" <bossartn@amazon.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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Precedence: bulk

I wrote:

> Instead of a boolean, it seems like the new option should specify some
> age below which VACUUM will skip the table entirely, and above which
> will enter fail-safe mode. As mentioned earlier, the shutdown hint
> could spell out the exact command. With this design, it would specify
> the fail-safe default, or something else, to use with the option.

On second thought, we don't really need another number here. We could
simply go by the existing failsafe parameter, and if the admin wants a
different value, it's already possible to specify
vacuum_failsafe_age/vacuum_multixact_failsafe_age in a session,
including in single-user mode. Perhaps a new boolean called
FAILSAFE_ONLY. If no table is specified, then when generating the list
of tables, include only those with relfrozenxid/relminmxid greater
than their failsafe thresholds.

-- 
John Naylor
EDB: http://www.enterprisedb.com