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From: Matthias van de Meent <boekewurm+postgres@gmail.com>
Date: Mon, 16 Mar 2026 20:24:45 +0100
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Subject: Re: Adding REPACK [concurrently]
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Antonin Houska <ah@cybertec.at>,
 Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
	Pg Hackers <pgsql-hackers@lists.postgresql.org>,
 Robert Treat <rob@xzilla.net>
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On Mon, 16 Mar 2026 at 16:10, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> On 2026-Mar-16, Matthias van de Meent wrote:
>
> > Note that most of my argument hinges on the impact on other, unrelated
> > databases/tables/sessions. Replication slots have a hard cap defined
> > at startup, and effective_wal_level increases the WAL generated by
> > practically all backends.
>
> I'd rather have a new GUC to declare a bunch of additional slots that
> are reserved exclusively for repack, set its default to something like
> 3, and call it a day.  If all repack slots are in use, you don't get to
> run repack, period.
>
> A slot costs nothing if unused, and we really don't want to make the
> interaction with regular replication more complicated than it is today.

There are a few overheads even for unused slots: each slot uses some
shared memory, and many (most) functions that operate on the shared
slot state have O(n_slots) overhead through e.g.
ReplicationSlotsComputeRequiredXmin(); so having a 10k slot limit
whilst using only 1 will be slower for that one active slot than if
the limit was just 10. Granted, that's not a lot of overhead, but it's
not free.

> > However, we don't live in that world, so I am opposed to allowing
> > table owners without REPLICATION to take any/all replication slots.
>
> I think requiring REPACK users to have the REPLICATION priv is rather
> user unfriendly.  Some potential REPACK users might not have any other
> use for replication at all.

I agree it's not user-friendly, but that's the point of limiting
permissions. Users can't install c-functions without SUPERUSER,
because it can cause cluster instability and crashes. Users can't
create slots without REPLICATION, because they'll be able to
negatively impact the whole cluster's performance, and possibly,
stability, when taking up replication slots that otherwise would be
used for critical HA purposes.

Kind regards,

Matthias van de Meent
Databricks (https://www.databricks.com)