[HACKERS] is possible cache tupledesc templates in execution plan? significantperformance issue, maybe bug?

I have a function

CREATE OR REPLACE FUNCTION ides_funcs.najdatsplt_cislo_exekuce(mid_najdatsplt bigint)
 RETURNS character varying
 LANGUAGE plpgsql
 STABLE SECURITY DEFINER COST 1000
AS $function$
DECLARE

 Result varchar(200);

  --mZALOBCISLO NAJZALOBCE.ZALOBCISLO%TYPE;
  mAdra varchar(200);


BEGIN
   BEGIN

-- there are only tables

     select CISLOEXEKUCE INTO STRICT mADRA
      from najzalobpr MT, najvzallok A1,
                    NAJZALOBST A2, NAJZALOBCE A3 where
                    MT.ID_NAJVZALLOK= A1.ID_NAJVZALLOK AND
                    A1.ID_NAJZALOBST=A2.ID_NAJZALOBST AND
                    A2.ID_NAJZALOBCE= A3.ID_NAJZALOBCE AND
                    MT.ID_NAJDATSPLT = mID_NAJDATSPLT  LIMIT 1;
    EXCEPTION
      WHEN OTHERS THEN
        mADRA := NULL;
    END;



  Result:=mADRA;
  return(Result);
end;
$function$