Horton

Re: Fix lag columns in pg_stat_replication not advancing when replay LSN stalls

Xuneng Zhou <xunengzhou@gmail.com>

View thread View on postgr.es
From: Xuneng Zhou <xunengzhou@gmail.com>
To: Fujii Masao <masao.fujii@gmail.com>
Cc: Shinya Kato <shinya11.kato@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Chao Li <li.evan.chao@gmail.com>
Date: 2025-10-22T15:25:51Z
Lists: pgsql-hackers 
Hi,

On Wed, Oct 22, 2025 at 10:45 PM Fujii Masao <masao.fujii@gmail.com> wrote:
>
> On Wed, Oct 22, 2025 at 4:49 PM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> > How about something like:
> >
> > /*
> >  * Overflow entries for read heads that collide with the write head.
> >  *
> >  * When the cyclic buffer fills (write head is about to collide with a read
> >  * head), we save that read head's current sample here and mark it as using
> >  * overflow (read_heads[i] = -1). This allows the write head to continue
> >  * advancing while the overflowed mode continues lag computation using the
> >  * saved sample.
> >  *
> >  * Once the standby's reported LSN advances past the overflow entry's LSN,
> >  * we transition back to normal buffer-based tracking.
> >  */
>
> LGTM. Thanks!
>
> I've created a patch adding your suggested comments (attached).
> Since this is a follow-up to commit 883a95646a8, which was recently applied
> and backpatched to all supported branches, I think we should backpatch
> this one as well. Thought?
>

LGTM. Thanks for the patch!

Best,
Xuneng

Commits

  1. Add comments explaining overflow entries in the replication lag tracker.

  2. Fix stalled lag columns in pg_stat_replication when replay LSN stops advancing.