Postgres Routine to calculates distance between two points (latitude/longitude)

calculate_distance.sql

/*::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::    lat1, lon1 = Latitude and Longitude of point 1 (in decimal degrees)  :*/
/*::    lat2, lon2 = Latitude and Longitude of point 2 (in decimal degrees)  :*/
/*::    unit = the unit you desire for results                               :*/
/*::           where: 'M' is statute miles (default)                         :*/
/*::                  'K' is kilometers                                      :*/
/*::                  'N' is nautical miles                                  :*/
/*::                                                                         :*/
/*::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/

CREATE OR REPLACE FUNCTION calculate_distance(lat1 float, lon1 float, lat2 float, lon2 float, units varchar)
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;

            IF units = 'K' THEN dist = dist * 1.609344; END IF;
            IF units = 'N' THEN dist = dist * 0.8684; END IF;

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

SELECT calculate_distance(32.9697, -96.80322, 29.46786, -98.53506, 'M');
SELECT calculate_distance(32.9697, -96.80322, 29.46786, -98.53506, 'K');
SELECT calculate_distance(32.9697, -96.80322, 29.46786, -98.53506, 'N');