Fastest way to get current user's OID in Postgres?
The fastest way I know of is a lookup in the system catalog view pg_roles:

SELECT * FROM pg_roles WHERE rolname = 'postgres';


Strictly speaking, it would be even slightly faster to use the underlying table pg_authid, but access to it is restricted to superusers for good reasons.

There is no object identifier type like for tables or types, which would allow a simple cast like 'mytable'::regclass.

Be aware though, that OIDs are not stable across a dump / restore cycle. So, the OID is no good for the use case!

In some databases I have a separate login table with a serial primary key that I use for similar purposes. Maintained manually. And functions using it are prepared to occasionally not find a user in this table. A very basic and fast table:

CREATE TABLE users.login (
  login_id serial PRIMARY KEY 
 ,username text NOT NULL UNIQUE
);


When creating new users I use a plpgsql function that creates the new user in the system and enters it into my table at the same time. And I use this login_id in many places. For instance I keep track of who made the last change to a row in most tables. I use this simple function:

CREATE OR REPLACE FUNCTION public.f_login_id()
  RETURNS int AS
$func$
SELECT COALESCE((SELECT login_id FROM users.login
                 WHERE  username = session_user::text), 0::int)
$func$ LANGUAGE sql STABLE;


No foreign key constraints so to keep things fast and simple. Obviously, I don't need strict referential integrity ...