how to check someones permission on a redhisft external schema
SELECT schemaname, objectname, usename,
       has_schema_privilege(usrs.usename, schemaname, 'usage')  AS usage
FROM(
    SELECT schemaname, tablename AS objectname, schemaname + '.' + tablename AS fullobj
    FROM SVV_EXTERNAL_TABLES
    ) AS objs,(SELECT * FROM pg_user) AS usrs
