Re: Adding REPACK [concurrently]

> On Apr 10, 2026, at 18:53, Zhijie Hou (Fujitsu) <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> Hi,
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> Thoughts ?
>
> 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/