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Subject: Re: Adding REPACK [concurrently]
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: 
 <TYRPR01MB14195633567DA00ABD42570B794592@TYRPR01MB14195.jpnprd01.prod.outlook.com>
Date: Tue, 21 Apr 2026 15:24:25 +0800
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
 Amit Kapila <amit.kapila16@gmail.com>,
 "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>,
 Antonin Houska <ah@cybertec.at>,
 Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
 Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
 Pg Hackers <pgsql-hackers@lists.postgresql.org>,
 Robert Treat <rob@xzilla.net>
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To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
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> On Apr 10, 2026, at 18:53, Zhijie Hou (Fujitsu) =
<houzj.fnst@fujitsu.com> wrote:
>=20
> Hi,
>=20
> When testing REPACK concurrently, I noticed that all WALs are retained =
from
> the moment REPACK begins copying data to the new table until the =
command
> finishes replaying concurrent changes on the new table and stops the =
repack
> decoding worker.
>=20
> I understand the reason: the REPACK command itself starts a =
long-running
> transaction, and logical decoding does not advance restart_lsn beyond =
the
> oldest running transaction's start position. As a result, =
slot.restart_lsn
> remains unchanged, preventing the checkpointer from recycling WALs.
>=20
> However, since REPACK can run for a long time (hours or even days), =
I'd like
> to confirm whether this is expected behavior or if we plan to improve =
it
> in the future ? And additionally, IIUC, REPACK without using =
concurrent option
> does not have this issue.
>=20
> Given that we do not restart a REPACK, I think the repack decoding =
worker
> should be able to advance restart_lsn each time after writing changes
> (similar to how a physical slot behaves). To illustrate this, I've =
written
> a patch (attached) that implements this approach, and it works fine =
for me.
>=20
> BTW, catalog_xmin also won't advance, but that seems not a big issue =
as
> the REPACK transaction itself also holds a snapshot that retains =
catalog tuples,
> so advancing catalog_xmin wouldn't change the situation anyway.
>=20
> Thoughts ?
>=20
> Best Regards,
> Hou zj
> <v1-0001-Allow-old-WALs-to-be-removed-during-REPACK-CONCUR.patch>

I found the same problem with LogicalConfirmReceivedLocation and posted =
a fix in a separate thread [1]. So I would withdraw my patch.

Looking at this patch, the change is exactly the same as what I did in =
[1], but I think the code comment should be updated as well. For the =
comment change, please see my patch in [1].

[1] =
https://www.postgresql.org/message-id/D8D9F770-DAA2-482C-A7E0-F87E5104C13E=
%40gmail.com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/