CREATE TABLE filling
(
  id_filling serial NOT NULL,
  start_date date,
  end_date date,
  CONSTRAINT filling_pkey PRIMARY KEY (id_filling)
) 
WITHOUT OIDS;
ALTER TABLE filling OWNER TO postgres;

   SELECT INTO cnt COUNT(*) FROM filling
    WHERE new.start_date <= filling.end_date
      AND new.end_date >= filling.start_date;

CREATE OR REPLACE FUNCTION check_shipment_addition()
  RETURNS "trigger" AS
$BODY$  
DECLARE 
-- Объявление переменной для количества 
cnt integer; 

BEGIN 
   SELECT INTO cnt COUNT(*) FROM filling
    WHERE new.start_date <= filling.end_date
      AND new.end_date >= filling.start_date;
   IF cnt >  1  THEN    
      RAISE EXCEPTION 'Invalid record' ;
   END IF;   

RETURN NEW; 
END; 
$BODY$
  LANGUAGE 'plpgsql' VOLATILE;
ALTER FUNCTION check_shipment_addition() OWNER TO postgres;

CREATE CONSTRAINT  TRIGGER filling_i
    AFTER INSERT OR UPDATE
    ON filling
    FOR EACH ROW
    EXECUTE PROCEDURE check_shipment_addition ();