Maybe you should explain why you want to do such a thing first. There isn't really any practice, let alone a best one, to do exactly what you say. Most system variables are changeable by users. There are some that can be changed that could be abused but the general thinking is that while doing so maliciously is possible there are lots of others ways a user with access to a database session can cause you grief too and that solutions to this attack vector are social, not technical, in nature.
What I try to do is performing a row level security by setting a variable such that RLS function can only return the rows base on the variable. I have two way to do this one is using SET SESSION AUTHORIZATION the other is set a variable by using set_config. If I use SET SESSION AUTHORIZATION it increase administration overhead. If I use set_config the potential wrong access of RLS is possible. Do you have any suggestion?
