Hello, Guix Help! I think GuixSD is cute, but manual is so incomprehensible https://www.gnu.org/software/guix/manual/en/guix.html <https://www.gnu.org/software/guix/manual/en/guix.html> 

I want to clear all possible options when I install packages that work as services.
For example, can get postgresql by typing by non-root user `$ guix package -i postgresql` and run it? How to run it?

Can I install postgresql and run it only when I need?

Do I need install it only from defining service in config.scm and typing `# guix system reconfigure config.scm` ?

Manual "6.2.15 Defining Services" has lot of variables and I do not know where to find them definitions.