find the nearest location by latitude and longitude in postgresql

PostGIS

Don't store lat and long on a table like that. Instead use an PostGIS geometry or geography type.

CREATE EXTENSION postgis;

CREATE TABLE foo (
  geog geography;
);

CREATE INDEX ON foo USING gist(geog);

INSERT INTO foo (geog)
  VALUES (ST_MakePoint(x,y));

Now when you need to query it, you can use KNN (<->) which will actually do this on an index.

SELECT *
FROM foo
ORDER BY foo.geog <-> ST_MakePoint(x,y)::geography;

In your query, you explicitly have HAVING distance < 5. You can do that on the index too.

SELECT *
FROM foo
WHERE ST_DWithin(foo.geog, ST_MakePoint(x,y)::geography, distance_in_meters)
ORDER BY foo.geog <-> ST_MakePoint(x,y)::geography;

This ensure that nothing is returned if all points lie outside of distance_in_meters.

Furthermore x and y are decimal numbers ST_MakePoint(46.06, 14.505)

select * from (
SELECT  *,( 3959 * acos( cos( radians(6.414478) ) * cos( radians( lat ) ) * cos( radians( lng ) - radians(12.466646) ) + sin( radians(6.414478) ) * sin( radians( lat ) ) ) ) AS distance 
FROM station_location
) al
where distance < 5
ORDER BY distance
LIMIT 20;