Calculate distance between two point on earth in PostgreSQL

CREATE OR REPLACE 
FUNCTION calculate_distance(lat1 float8, lon1 float8, lat2 float8, lon2 float8)
RETURNS float8 AS
$$
	SELECT ACOS(SIN(RADIANS($1)) * SIN(RADIANS($3)) 
		 + COS(RADIANS($1)) * COS(RADIANS($3)) 
		 * COS(RADIANS($4) - RADIANS($2))) * 6371;

$$ LANGUAGE 'sql' IMMUTABLE;