CREATE OR REPLACE FUNCTION calculate_distance_kilometers(lat1 float, lon1 float, lat2 float, lon2 float)
RETURNS float AS $dist$
    DECLARE
        dist float = 0;
        radlat1 float;
        radlat2 float;
        theta float;
        radtheta float;
    BEGIN
        IF lat1 = lat2 OR lon1 = lon2
            THEN RETURN dist;
        ELSE
            radlat1 = pi() * lat1 / 180;
            radlat2 = pi() * lat2 / 180;
            theta = lon1 - lon2;
            radtheta = pi() * theta / 180;
            dist = sin(radlat1) * sin(radlat2) + cos(radlat1) * cos(radlat2) * cos(radtheta);

            IF dist > 1 THEN dist = 1; END IF;

            dist = acos(dist);
            dist = dist * 180 / pi();
            dist = dist * 60 * 1.1515 * 1.609344;

            RETURN dist;
        END IF;
    END;
    $dist$
    LANGUAGE plpgsql;