[chousuke]

> I've had people say "I installed postgres - here's the password". Then... I can't log in. Because I can't switch to the postgres user.

You don't need to switch to the postgres user if you have another database user and password.

Are you talking about a situation where someone has installed a PostgreSQL server but hasn't configured PostgreSQL to allow password authentication? The server admin needs to allow that explicitly, because some distributions don't allow password authentication even on localhost by default, but honestly, it's all very well documented.

> If someone uses common default package managers to set up mysql/Maria, and also for postgres, you'll be able to connect to mysql/Maria from any account.

This is not the case on Fedora at least, since fresh out of the package the MySQL root user has no password; the only way to connect is via local system authentication as the root user.