How does one define the most limited role/user possible in PostgreSQL?
Ideally, this role would not be able to do *anything* at all. In particular, this role would not be able to query meta-information about existing tables, functions, etc. with backslash commands such as \dt, \df.
(Of course, in practice such a role would not correspond to any real role. Its purpose, rather, is to serve as the starting point for defining more realistic roles by selectively adding the fewest privileges possible).
