Re: Collect statistics about conflicts in logical replication

Hi Hou-San. Here are my review comments for v4-0001.

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1. Add links in the docs

IMO it would be good for all these confl_* descriptions (in
doc/src/sgml/monitoring.sgml) to include links back to where each of
those conflict types was defined [1].

Indeed, when links are included to the original conflict type
information then I think you should remove mentioning
"track_commit_timestamp":
+ counted only when the
+ <link linkend="guc-track-commit-timestamp"><varname>track_commit_timestamp</varname></link>
+ option is enabled on the subscriber.

It should be obvious that you cannot count a conflict if the conflict
does not happen, but I don't think we should scatter/duplicate those
rules in different places saying when certain conflicts can/can't
happen -- we should just link everywhere back to the original
description for those rules.

~~~

2. Arrange all the counts into an intuitive/natural order

There is an intuitive/natural ordering for these counts. For example,
the 'confl_*' count fields are in the order insert -> update ->
delete, which LGTM.

Meanwhile, the 'apply_error_count' and the 'sync_error_count' are not
in a good order.

IMO it makes more sense if everything is ordered as:
'sync_error_count', then 'apply_error_count', then all the 'confl_*'
counts.

This comment applies to lots of places, e.g.:
- docs (doc/src/sgml/monitoring.sgml)
- function pg_stat_get_subscription_stats (pg_proc.dat)
- view pg_stat_subscription_stats (src/backend/catalog/system_views.sql)
- TAP test SELECTs (test/subscription/t/026_stats.pl)

As all those places are already impacted by this patch, I think it
would be good if (in passing) we (if possible) also swapped the
sync/apply counts so everything is ordered intuitively top-to-bottom
or left-to-right.

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[1] https://www.postgresql.org/docs/devel/logical-replication-conflicts.html#LOGICAL-REPLICATION-CONFLICTS

Kind Regards,
Peter Smith.
Fujitsu Australia