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

> On Oct 17, 2025, at 22:28, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>
> On Fri, Oct 17, 2025 at 5:11 PM Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
>> It took me some time to understand this fix. My most confusing was that once overwrite happens, how a reader head to catch up again? Finally I figured it out:
>>
>> ```
>> + lag_tracker->read_heads[head] =
>> + (lag_tracker->write_head + 1) % LAG_TRACKER_BUFFER_SIZE;
>> ```
>>
>> "(lag_tracker->write_head + 1) % LAG_TRACKER_BUFFER_SIZE” points to the oldest LSN in the ring, from where an overflowed reader head starts to catch up.
>>
>> I have no comment on the code change. Nice patch!
>
> Thanks for the review!
>
> I've updated the source comment to make the code easier to understand.
> The updated patch is attached.
>
> <v2-0001-Fix-stalled-lag-columns-in-pg_stat_replication-wh.patch>

Thanks for adding the comment.

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 “>=“?

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/