Re: Experimental patch for terminating VACUUM freeze blockers

On Wed, May 13, 2026 at 07:56:43PM +0800, wenhui qiu wrote:
> I have an experimental patch to explore handling this situation. The patch
> adds a GUC, vacuum_freeze_terminate_blockers_pid, which allows VACUUM to
> terminate regular client backends whose transaction horizon blocks VACUUM
> from advancing its freeze cutoff. The intended targets are
> idle-in-transaction sessions and long-running active transactions that are
> holding an old xmin or assigned XID.

Thanks for sharing. I certainly agree that this area has room for
improvement in Postgres.

> The patch deliberately does not try to handle other causes of freeze
> horizon retention, such as replication slots, hot standby feedback, or
> prepared transactions.

My experience is a bit dated, but I remember the two main issues being
replication slots and temporary tables. We now have
idle_replication_slot_timeout, and there's a somewhat active thread on an
XID version of that parameter [0], but I'm not aware of any recent ideas
about how to deal with stranded temporary tables. I'd encourage you to
think about these problems, too.

[0] https://postgr.es/m/CA%2B-JvFsMHckBMzsu5Ov9HCG3AFbMh056hHy1FiXazBRtZ9pFBg%40mail.gmail.com

--
nathan