Issue
I want to install an extension/change a PostgreSQL setting/check system tables and this requires having superuser permissions.
Resolution
Unfortunately, we don't give you a superuser role on any of our databases for the security and stability of all our customers.
The default user that comes with your Postgres database is a permissive role that is one step below the superuser and allows you to GRANT certain privileges to other users, but not all privileges to the same extent as a superuser. For example, you can use the default user to grant SELECT, INSERT, UPDATE and DELETE privileges to other users.
The following DevCenter article contains more information to know more about credentials and managing permissions: https://devcenter.heroku.com/articles/heroku-postgresql-credentials#the-default-credential