错误的参数的PostgreSQL调用函数

我声明了postgresql函数，如下所示，该函数仅计算kmeter中2个点之间的距离

CREATE OR REPLACE FUNCTION show_places(ref refcursor,lat float8,lon float8, radius double precision) RETURNS refcursor AS $$
BEGIN
  OPEN ref FOR SELECT * FROM place p where ST_Distance(ST_Point(lon,lat)::geography,ST_Point(p.lon,p.lat)::geography) <= radius;    -- Open a cursor
  RETURN ref;                                                        
END;
$$ LANGUAGE plpgsql;

调用如下：

BEGIN;
SELECT show_places('cities_cur',44.379,-79.703,255621.82229418);
FETCH ALL IN "cities_cur";
COMMIT;

但是我遇到了这个SQL异常：