Horton

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

Fujii Masao <masao.fujii@gmail.com>

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From: Fujii Masao <masao.fujii@gmail.com>
To: Shinya Kato <shinya11.kato@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Chao Li <li.evan.chao@gmail.com>
Date: 2025-10-20T01:17:21Z
Lists: pgsql-hackers

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On Sun, Oct 19, 2025 at 2:04 AM Shinya Kato <shinya11.kato@gmail.com> wrote:
> Thank you for the patch. I have one comment.
>
> +       if (lag_tracker->overflowed[head].lsn > lsn)
> +           return now - lag_tracker->overflowed[head].time;
>
> Could this return a negative value if the clock somehow went
> backwards? The original code returns -1 in this case, so I'm curious
> about this.

Thanks for the review!

Yes, you're right. So I've updated the patch so that -1 is returned
when the current time is earlier than the time in the overflow entry,
treating it as "no new sample found".

Regards,

-- 
Fujii Masao

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.