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From: Laurenz Albe <[email protected]>
To: Marcelo Fernandes <[email protected]>
To: [email protected]
Subject: Re: Do stuck replication slots prevent autovacuum of running entirely?
Date: Wed, 21 May 2025 08:05:19 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAM2F1VP11Pv7QcVjTzM_X_uNDvd_qPYvGAnneU-Fn8hW2umFPA@mail.gmail.com>
References: <CAM2F1VP11Pv7QcVjTzM_X_uNDvd_qPYvGAnneU-Fn8hW2umFPA@mail.gmail.com>

On Wed, 2025-05-21 at 16:34 +1200, Marcelo Fernandes wrote:
> I am trying to understand if a stuck replication slot would be sufficient to
> stop an autovacuum of even starting.
> 
> Couldn't the autovacuum process start, but fail to remove dead tuples that
> are still necessary by the replication slot? Why would it prevent autovacuum
> of even starting instead?

I cannot think of a reason why an abandoned replication slot (not sure what
you mean with "stuck") would keep an autovacuum worker from starting.

Typical reasons why autovacuum doesn't start running on a table are:

- there are already "autovacuum_max_workers" worker processes running

- the thresholds for dead or inserted tuples have not been crossed

- the statistics collector has a problem and doesn't gather statistics;
  this applies mostly to v14 and older, see
  https://www.cybertec-postgresql.com/en/stale-statistics-cause-table-bloat/

- the parameter "track_activities" was disabled, so that PostgreSQL doesn't
  collect statistics

- the functions "pg_stat_reset" or "pg_stat_reset_single_table_counters"
  are called repeatedly and wipe out table statistics

Yours,
Laurenz Albe

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