defining and using constants in postgreSQL

Hello,

I am looking for a way that I can define a constant in PostgreSQL (so that
I end up with a constant similar to one that I could have in Oracle). I have
searched the archives and it seems that the solutions that were suggested in
include creating a table of constants or alternatively creating a function
that returns a constant. However, I would like to have a predefined constant
in the global scope which I can access without the round brackets used to
call a function. That is, I would like to be able to define a constant
named foo that I can then access with the name foo, not foo().

Is this possible at all with PostgreSQL?

I know PostgreSQL has some predefined constants that don't take the round
brackets so in principle it should be possible to define more. Is there a
way to do such thing in PostgreSQL?

Thanks,

Neil