query13.txt

WITH RECURSIVE search_path(id, link, depth, route, cycle) AS (
  SELECT k.k_person1id, k.k_person2id, 1,
    ARRAY[k.k_person1id],
    false
  FROM knows k where k.k_person1id = @Person1@

 UNION ALL

 SELECT k.k_person1id, k.k_person2id,  sp.depth+1,
    route || k.k_person1id,
    k.k_person1id = ANY(route)
 FROM knows k, search_path sp
 WHERE k.k_person1id = sp.link AND NOT cycle AND sp.depth < 4
)
SELECT sp.depth AS route 
FROM search_path AS sp 
WHERE link = @Person2@ AND NOT cycle
LIMIT 1;