RE: speed up a logical replica setup

Dear Amit,

> On Fri, Jan 5, 2024 at 3:36 PM Hayato Kuroda (Fujitsu)
> <kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
> >
> > I love your proposal, so I want to join the review. Here are my first comments.
> >
> > 01.
> > Should we restrict that `--subscriber-conninfo` must not have hostname or IP?
> > We want users to execute pg_subscriber on the target, right?
> >
>
> I don't see any harm in users giving those information but we should
> have some checks to ensure that the server is in standby mode and is
> running locally. The other related point is do we need to take input
> for the target cluster directory from the user? Can't we fetch that
> information once we are connected to standby?

I think that functions like inet_client_addr() may be able to use, but it returns
NULL only when the connection is via a Unix-domain socket. Can we restrict
pg_subscriber to use such a socket?

> >
> > 05.
> > I found that the connection string for each subscriptions have a setting
> > "fallback_application_name=pg_subscriber". Can we remove it?
> >
> > ```
> > postgres=# SELECT subconninfo FROM pg_subscription;
> > subconninfo
> >
> ---------------------------------------------------------------------------------
> > user=postgres port=5431 fallback_application_name=pg_subscriber
> dbname=postgres
> > (1 row)
> > ```
>
> Can that help distinguish the pg_subscriber connection on the publisher?
>

Note that this connection string is used between the publisher instance and the
subscriber instance (not pg_subscriber client application). Also, the
fallback_application_name would be replaced to the name of subscriber in
run_apply_worker()->walrcv_connect(). Actually the value would not be used.
See below output on publisher.

```
publisher=# SELECT application_name, backend_type FROM pg_stat_activity where backend_type = 'walsender';
application_name | backend_type
----------------------+--------------
pg_subscriber_5_9411 | walsender
(1 row)
```

Or, if you mean to say that this can distinguish whether the subscription is used
by pg_subscriber or not. I think it is sufficient the current format of name.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED