PostgreSQL - Convertir procedimiento Firebird a PostgreSQL

CREATE OR ALTER PROCEDURE DIAS_SIN RETURNS ("RESULT" INTEGER)

AS

DECLARE VARIABLE AUX INTEGER;

BEGIN

  AUX    = 0;

  RESULT = 0;

  FOR

    SELECT max(DATEDIFF(DAY FROM DT2.FECHA TO DT1.FECHA))

  FROM DATOS DT1, DATOS DT2

  WHERE DT1.SALIDA_NUM = DT2.SALIDA_NUM + 1

    INTO :AUX

  DO

    IF(AUX > RESULT) THEN RESULT = AUX;

  SUSPEND;

END