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From: Masahiko Sawada <sawada.mshk@gmail.com>
Date: Mon, 27 Feb 2023 23:11:53 +0900
Message-ID: <CAD21AoC=b09pzWNyPYuti0M2DQwy57CGF9UWWb_+v=gdWeNG=A@mail.gmail.com>
Subject: Re: Should vacuum process config file reload more often
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>
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Hi,

On Fri, Feb 24, 2023 at 7:08 AM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> Hi,
>
> Users may wish to speed up long-running vacuum of a large table by
> decreasing autovacuum_vacuum_cost_delay/vacuum_cost_delay, however the
> config file is only reloaded between tables (for autovacuum) or after
> the statement (for explicit vacuum). This has been brought up for
> autovacuum in [1].
>
> Andres suggested that it might be possible to check ConfigReloadPending
> in vacuum_delay_point(), so I thought I would draft a rough patch and
> start a discussion.

In vacuum_delay_point(), we need to update VacuumCostActive too if necessary.

> Apart from this, one higher level question I have is if there are other
> gucs whose modification would make reloading the configuration file
> during vacuum/analyze unsafe.

As far as I know there are not such GUC parameters in the core but
there might be in third-party table AM and index AM extensions. Also,
I'm concerned that allowing to change any GUC parameters during
vacuum/analyze could be a foot-gun in the future. When modifying
vacuum/analyze-related codes, we have to consider the case where any
GUC parameters could be changed during vacuum/analyze. I guess it
would be better to apply the parameter changes for only vacuum delay
related parameters. For example, autovacuum launcher advertises the
values of the vacuum delay parameters on the shared memory not only
for autovacuum processes but also for manual vacuum/analyze processes.
Both processes can update them accordingly in vacuum_delay_point().

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com