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: Chao Li <li.evan.chao@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-10-20T01:12:07Z
Lists: pgsql-hackers 
On Sat, Oct 18, 2025 at 9:16 AM Chao Li <li.evan.chao@gmail.com> wrote:
> I think I put all concentration on the big picture yesterday, so I ignored this implementation detail:
>
> ```
> +               if (lag_tracker->overflowed[head].lsn > lsn)
> +                       return now - lag_tracker->overflowed[head].time;
> ```
>
> Should this “>” be “>=“?

I think either ">" or ">=" would work. However, the following loop
in LagTrackerRead() advances the read head when the LSN
in the current buffer entry is equal to the given LSN, so I followed
the same logic for the overflow entry and used ">" there.

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.