Writing Postgres function for Hasura

Question

I am learning Hasura and Postgres for Postgis support in my project. During learning hasura i encountered Postgres Functions which Hasura can track and i can use them as regular mutation.

Now after apart from Postgres i know about Hasura how it works it is simple wrt to Postgres which different field for me right now. But i tried to create Postgres Function but always failed as i dont know what i am doing. so i head over to hasura so that i can find out something which can help me so i found out this.

CREATE FUNCTION search_landmarks_near_user(userid integer, distance_kms integer)
RETURNS SETOF user_landmarks AS $$
  SELECT  A.user_id, A.location,
  (SELECT json_agg(row_to_json(B)) FROM landmark B
   WHERE (
     ST_Distance(
       ST_Transform(B.location::Geometry, 3857),
       ST_Transform(A.location::Geometry, 3857)
     ) /1000) < distance_kms
   ) AS nearby_landmarks
  FROM user_location A where A.user_id = userid
$$ LANGUAGE sql STABLE;

Now here outer body is much elaborative but after $$ confusion starts to take over me. I have table like this in postgres which contains point(Location) stamp every second. Location Table

What could be the function defination which can return Locations(More than one and single one) near given point in table.