RE: Time delayed LR (WAS Re: logical replication restrictions)

> Yeah, min_send_delay and max_slots_wal_keep_size should be easily tunable
> because
> the appropriate value depends on the enviroment and workload.
> However, pg_replication_slots.pg_replication_slots cannot show the exact amout
> of WALs,
> so it may not suitable for tuning. I think user can compare the value
> pg_replication_slots.restart_lsn (or pg_stat_replication.sent_lsn) and
> pg_current_wal_lsn() to calclate number of WALs to be delayed, like
>
> ```
> postgres=# select pg_current_wal_lsn() - pg_replication_slots.restart_lsn as
> delayed from pg_replication_slots;
> delayed
> ------------
> 1689153760
> (1 row)
> ```
>
> > I think it would be better to tell about this in the docs along with
> > the 'min_send_delay' description. The key point is whether this would
> > be an acceptable trade-off for users who want to use this feature. I
> > think it can harm only if users use this without understanding the
> > corresponding trade-off. As we kept the default to no delay, it is
> > expected from users using this have an understanding of the trade-off.
>
> Yes, the trade-off should be emphasized.

Based on the understanding, I added them to the doc in new version patch.
Please see [1].

[1]: https://www.postgresql.org/message-id/flat/TYAPR01MB586606CF3B585B6F8BE13A9CF5B29%40TYAPR01MB5866.jpnprd01.prod.outlook.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED