Re: Add REPACK progress phases for logical decoding setup

Hi,

On Fri, Sep 18, 2026 at 2:19 PM Sami Imseih <samimseih(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,
>
> While stress testing REPACK (CONCURRENTLY), I noticed that the current
> "initializing" phase contains multiple operations that can wait indefinitely.
> The transaction wait can be prolonged by long-running or prepared
> transactions, or by another REPACK (CONCURRENTLY) operation that already has
> an XID. Describing the whole phase as "expected to be very brief" [1] is
> misleading.

Yeah, for REPACK (CONCURRENTLY), it could take a time before the scan starts.

>
> To make these waits visible and distinguish the work immediately before and
> after the old transaction wait, three progress phases between
> "initializing" and
> the heap scan should be added.
>
> 1/ enabling logical decoding
>
> When wal_level is logical, or logical decoding is already enabled, this phase
> is effectively instantaneous. Otherwise, REPACK waits for every running
> process to acknowledge a ProcSignalBarrier. This normally finishes quickly,
> but the barrier has no timeout and can take longer if a process does not
> respond promptly.
>
> 2/ waiting for old transactions
>
> The snapshot builder waits for transactions with assigned XIDs to finish
> before it can reach a consistent point.

Right, these steps could take a time. Please note that if there is one
logical slot or wal_level is set to 'logical', (1) is no-op.

>
> 3/ building initial snapshot
>
> REPACK obtains the initial snapshot from the decoding worker and prepares to
> start copying the table. This separates the end of the transaction wait
> from the start of the heap scan.

While it's true that the logical decoding might need to read WAL
records until the snapshot reaches a consistent state, I'm not sure
that it can justify having its own phase. I think we can merge it to
(2) and have something like a "initializing logical decoding" phase.

>
> This gives the following sequence:
>
> initializing (default, expected to be brief)
> enabling logical decoding
> waiting for old transactions
> building initial snapshot
> seq scanning heap / index scanning heap
>
> The attached patch has the worker publish its setup stage through shared
> memory and signal the existing condition variable.
>
> Thoughts?

I've not looked at the patch in depth yet but I agree with the general
idea. Just to be clear, I think adding new phases would be a v20 item
rather than an open item for v19. For v19, we can revise the
documentation part only.

Regards,

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