RE: NOTIFY and pg_notify performance when deduplicating notifications

> Indeed, I have the same and am very interested in this.
>
> > I hope this patch can be reviewed and included in PostgreSQL.
>
> I added this to the next Commitfest and added myself as a reviewer.
> Will try to a review beginning of next week.
> https://commitfest.postgresql.org/20/1820/

Thank you for reviewing.

I just caught an error in my patch, it's fixed in the attachment. The 'never' and 'maybe' collapse modes were mixed up in one location.

I can't find a reasonable way to build a regression test that checks whether notifications are effectively deduplicated. The output of the LISTEN command lists the PID of the notifying backend for each notification, e.g. : 'Asynchronous notification "foobar" received from server process with PID 24917'. I can't just add this to async.out. I did test manually for all eight combinations : four collapse mode values (missing, empty string, 'maybe' and 'never'), both with NOTIFY and pg_notify().