Re: pg_upgrade and logical replication

I've been looking into optimizing pg_upgrade's once-in-each-database steps
[0], and I noticed that we are opening a connection to every database in
the cluster and running a query like

    SELECT count(*) FROM pg_catalog.pg_subscription WHERE subdbid = %d;

Then, later on, we combine all of these values in
count_old_cluster_subscriptions() to verify that max_replication_slots is
set high enough.  AFAICT these per-database subscription counts aren't used
for anything else.

This is an extremely expensive way to perform that check, and so I'm
wondering why we don't just do

    SELECT count(*) FROM pg_catalog.pg_subscription;

once in count_old_cluster_subscriptions().

[0] https://commitfest.postgresql.org/48/4995/

-- 
nathan