Re: BUG #19549: Physical replication slot xmin value stuck - Mailing list pgsql-bugs

Hi,

On Thu, Jul 9, 2026 at 3:04 AM PG Bug reporting form
<noreply@postgresql.org> wrote:
>
> The following bug has been logged on the website:
>
> Bug reference:      19549
> Logged by:          nitin mangnale
> Email address:      nmangnale2@gmail.com
> PostgreSQL version: Unsupported/Unknown
> Operating system:   Linux (ubuntu 22.04 LTS)
> Description:
>
> Hello PostgreSQL Community,
>
> I am looking for assistance in investigating what appears to be an unusual
> replication/xmin behavior on PostgreSQL 13.20.

PostgreSQL 13 reached end of life in November 2025 [1]. The community
no longer provides bug fixes or investigates issues for EOL versions,
so getting help here will be difficult. If possible, I would recommend
upgrading to a supported version and checking if the issue still
reproduces there.

[1] https://www.postgresql.org/support/versioning/

> Problem Description
>
> One of our standby servers periodically causes a physical replication slot
> on the primary to retain a very old "xmin". The "age(xmin)" continuously
> increases over time, preventing vacuum from advancing normally on the
> primary.
>
> The unusual part is that we cannot identify any backend or query responsible
> for holding the snapshot.
>
> The issue only affects one standby. The second standby remains healthy.
>
> Restarting the affected standby immediately clears the problem, but after
> several days the same behavior returns.

Thanks for reporting this.

> Observed Behavior
>
> On the primary:
>
> slot_name      : oseu2012766
> slot_type      : physical
> active         : true
> xmin           : 788928341
> age(xmin)      : 5419501 (continuously increasing)
> restart_lsn    : 1835DD/EB000000

Did you check on the standby if there is any backend holding xmin =
788928341 or closer somewhere?

> Questions
>
> 1. Is this a known issue or edge case in PostgreSQL 13.20 involving physical
> replication slots or hot_standby_feedback?
>
> 2. Is it expected for "pg_replication_slots.xmin" to remain pinned while
> "pg_stat_replication.backend_xmin" is NULL?

Yes. Check the docs [2]. The field backend_xmin shows the standby's
xmin horizon reported by hot_standby_feedback. This field will be null
if a replication slot is used (in that case, the standby's xmin is
shown in pg_replication_slots). So in your case, this is expected.

[2] https://www.postgresql.org/docs/devel/monitoring-stats.html#MONITORING-PG-STAT-REPLICATION-VIEW

> 3. Are there any known bugs in PostgreSQL 13.x related to stale standby
> feedback or snapshot lifecycle that match this behavior?

Without a reproducer or additional details or extensive checks on the
standby, it is hard to treat this as a bug.

> 4. Are there any additional internal views, debugging techniques, or GUC
> parameters that would help identify the source of the pinned xmin?

Maybe it is worth looking at what the standby reports as xmins via
hot_standby_feedback [3] and whether it matches what is being reported
in the primary's pg_replication_slots.xmin.

[3] https://github.com/postgres/postgres/blob/REL_13_STABLE/src/backend/replication/walreceiver.c#L1252

> 5. Would upgrading to a newer PostgreSQL major version be expected to
> resolve this behavior if it is related to snapshot or hot standby feedback
> handling?

Without understanding the issue and query patterns on the standby, it
is hard to say. However, since PostgreSQL 13 is no longer supported by
the community, upgrading to a newer version in a sandboxed environment
and testing whether the issue recurs would be a reasonable next step.

> At this point we are considering rebuilding the affected standby, but we
> would like to understand the root cause before doing so.

Why do you think rebuilding the standby would resolve the issue?

--
Bharath Rupireddy
Amazon Web Services: https://aws.amazon.com