Subject Doubt about RDB$GET_CONTEXT

Regarding the database optimizer and query performance.
Does RDB$GET_CONTEXT on a where clause would be treated like a literal value? 
Or should I consider it a select on an one record table?

I'm trying to emulate database constants for some known defined values on my application.
Or the following three statements are equivalent and I shouldn't worry about it?

select .. from .. where FIELD = 500;
select .. from .. where FIELD = RDB$GET_CONTEXT('USER_SESSION', 'MY_VALUE');
select .. from .. where FIELD = (select MY_VALUE from CFG_TABLE) -- CFG_TABLE will ever have only one record;