=> select 'ex.tt'::regclass::text;
 text
-------
 ex.tt
(1 row)

=> select 'public.tt'::regclass::text;
 text
------
 tt
(1 row)

SET LOCAL search_path = '';
SELECT 'public.tt'::regclass::text;

SELECT relnamespace::regnamespace::text
FROM   pg_catalog.pg_class
WHERE  oid = 'public.tt'::regclass;