In Postgres 11, the
CREATE DATABASE command that makes a new catalog has an option
ALLOW_CONNECTIONS. The documentation says this option can be used to block all connections to the database:
[ ALLOW_CONNECTIONS [=] allowconn ]
If false then no one can connect to this database. The default is true, allowing connections (except as restricted by other mechanisms, such as GRANT/REVOKE CONNECT).
➥ Why would anyone want to create a database that nobody can connect to?
➥ If you cannot connect, how can you ever change this setting to
true to allow connections?