Horton

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

Xuneng Zhou <xunengzhou@gmail.com>

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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: 2025-10-22T07:49:18Z
Lists: pgsql-hackers 
Hi,

On Wed, Oct 22, 2025 at 10:34 AM Fujii Masao <masao.fujii@gmail.com> wrote:
>
> On Tue, Oct 21, 2025 at 11:52 PM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> > The patch LGTM.
>
> Thanks for the review!
> I've pushed the patch and backpatched it to all supported versions.
>
>
> > I am wondering whether it is helpful to add some
> > comments for this overflowed array
>
> Yes, do you have any specific suggestions?
>

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.
 */


> > and replacing literal zeros with the constant InvalidXLogRecPtr for
> > better readability.
> >
> > /* InvalidXLogRecPtr means no overflow yet */
> > if (lag_tracker->overflowed[i].lsn == InvalidXLogRecPtr)
>
> I couldn't find any code like "lag_tracker->overflowed[i].lsn == 0",
> so I'm not sure which part should be replaced with InvalidXLogRecPtr.
> Could you point me to the exact location?
>

Sorry for the noise here, I mean
if (lag_tracker->read_heads[head] == InvalidXLogRecPtr) before.
But with this change, position 0 would be treated as overflow mode,
which is clearly wrong...

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.