SET ROLE Admin; DECLARE @secure_variable INTEGER RESTRICT; -- only accessible to Admin
Why introduce another security system?
That is a good question.
I would prefer to avoid it and just have simple session variables... but this is not what you want, so I'm trying to find a compromise which both gives you the feature you are seeking and would keep session variables as inexpensive, i.e. without catalog costs.
A simplistic PUBLIC/PRIVATE permissions on simple session variable can be done with nothing (no additional data structures): (store: hash_put(<owner id if private or nothing> || '@varname', value); consult: if exists(<owner id> || '@varname') then return it else if exists('@varname') then return it else "error variable does not exist").
Now if you can offer an inexpensive GRANT/REVOKE on simple session variables, i.e. without catalog changes, then I think I would agree to it, even if I would not like it much.
The reason I "do not like much" is subjective. I associate SQL permission commands (GRANT, REVOKE...) to real SQL "objects" (i.e. persistent, transactional, secured things subject to CREATE ALTER DROP...). However light-weight session variables are not really like that.
Also if you can have inexpensive GRANT/REVOKE then probably I would also have to accept "CREATE SESSION VARIABLE @foo", because it would be consistent to have it with GRANT/REVOKE. I would "not like it much" either to have CREATE for an non persistant object, but that is life...
However I understood that for permissions you do need "pg_class", which means catalog changes on session variable creation, which means expensive for simple session variables, so not desirable.
I don't understand to "expensive" word. How much often you create/drop these variables? The deployment cycle is similar to functions. I don't propose any feature, that can enforce bloating of system catalogue.
The variable metadata will be persistent across sessions. I can imagine the local (temporal) session variable where life cycle of metadata will be related to session like our current temporary tables. But it is not in this proposal.