In a terminal I do all, with all permissions by this simple command and no extra credentials:
psql postgres://postgres@localhost/myDB # No password!! No other credentials!
That is: host=localhost, user=postgres, database=myDB.
So, how can I keep QGIS simple to connect with my local PostgreSQL instance?
Can I connect to QGIS using only it? Only host/user/database.
I suppose that QGIS's community is mature (since 2002), and it needs and pushes for easy to use and simplicity...
I suppose that QGIS was created to be used with PostGIS... So I suppose that what I need is valid for all versions... But, if you need a reference, I am using QGIS v3+ with PostgreSQL v12+,
psqlversion 12+ and PostGIS 3+.
Please, any simple solution, by terminal or by menu "Add Layer/PostgreSQL", with no password and no extra-credentials. This question/answer use password, so it is not a solution.