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From: Peter Geoghegan <pg@bowt.ie>
Date: Mon, 14 Feb 2022 20:21:41 -0800
Message-ID: <CAH2-WzkEStBgnk0buJYAKw5jpRYUE+-tuLYn5U+vDyhXNfu+DQ@mail.gmail.com>
Subject: Re: do only critical work during single-user vacuum?
To: John Naylor <john.naylor@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Mon, Feb 14, 2022 at 8:04 PM John Naylor
<john.naylor@enterprisedb.com> wrote:
> The failsafe mode does disable truncation as of v14:
>
> commit 60f1f09ff44308667ef6c72fbafd68235e55ae27
> Author: Peter Geoghegan <pg@bowt.ie>
> Date:   Tue Apr 13 12:58:31 2021 -0700
>
> Don't truncate heap when VACUUM's failsafe is in effect.

That's true, but bear in mind that it only does so when the specific
table being vacuumed actually triggers the failsafe. I believe that
VACUUM(EMERGENCY) doesn't just limit itself to vacuuming tables where
this is guaranteed (or even likely). If I'm not mistaken, it's
possible (even likely) that there will be a table whose
age(relfrozenxid) is high enough for VACUUM(EMERGENCY) to target the
table, and yet not so high that the failsafe will kick in at the
earliest opportunity.

> To demonstrate to myself, I tried a few vacuums in a debugger session
> with a breakpoint at GetNewTransactionId(). I've only seen it reach
> here when heap truncation happens (or the not relevant for wraparound
> situations FULL and ANALYZE).

It's possible for a manually issued VACUUM to directly disable
truncation (same with index_cleanup). Without getting into the
question of what the ideal behavior might be right now, I can say for
sure that it wouldn't be difficult to teach VACUUM(EMERGENCY) to pass
down the same options.

The failsafe is essentially a mechanism that dynamically changes these
options for an ongoing vacuum, once age(relfrozenxid) crosses a
certain threshold. There is nothing fundamentally special about that.

-- 
Peter Geoghegan