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
