CREATE TYPE CIR_TYPE AS
(
    ID integer,
    path text,
    cycle boolean
);

CREATE OR REPLACE FUNCTION circular_ref() RETURNS setof CIR_TYPE AS $body$

DECLARE
    r CIR_TYPE;

BEGIN
    For r in WITH RECURSIVE graph(ID, path, cycle) AS (
                SELECT id AS id
                     , ARRAY[parentid, id] AS path
                     , (parentid = id) AS cycle
                FROM  mytable

                UNION ALL

                SELECT d.id, d.parentid ||path, d.parentid = ANY(path)
                FROM   graph g
                JOIN   mytable d ON d.id = g.path[1]
                WHERE  NOT g.cycle
             )
             SELECT DISTINCT *
             FROM   graph
    Loop

        If r.cycle= TRUE then
        Return NEXT r;
        End if;

    End loop;

END;
$body$ LANGUAGE plpgsql;

circular_ref
----------------------------
(0,"{1,0,1}",t)
(1,"{0,1,0}",t)

ID     |    Path
----       ------
0      |   1,0,1
1      |   0,1,0

SELECT * FROM circular_ref();

SELECT circular_ref();

SELECT id, path FROM circular_ref();