method to generate amount of distance between 2 position on km

psql_distance_calcualte.sql

CREATE OR REPLACE FUNCTION distance(lat1 FLOAT, lon1 FLOAT, lat2 FLOAT, lon2 FLOAT) RETURNS FLOAT AS $$
DECLARE                                                   
    x float = 111.12 * (lat2 - lat1);                           
    y float = 111.12 * (lon2 - lon1) * cos(lat1 /92.215);        
BEGIN                                                     
    RETURN sqrt(x * x + y * y);                               
END  
$$ LANGUAGE plpgsql; // km