If the problem occurs gradually (like leaking 20 connections per hour during
ETL), you can check pg_stat_activity every hour or so to try to observe the
problem before all the connection slots are used up, to collect diagnostic
information.
Alternately, leave a connection opened to the DB and wait until all connection
slots *are* used up, and then check pg_stat_activity. That will take longer,
and you'll have more information to weed through.
What messages are in the server's log ?
v11.2 is years old and hundreds of bugfixes behind. Since you ran into this
problem anyway, why not run 11.16, which was released today ?
How did you install postgres 11 ? From source or from packages ? Which
packages ? The semi-official PGDG RPM packages are available here:
https://yum.postgresql.org/
--
Justin