Maybe I have done too much SQL, but for me it is trivial and easier to do than in most other languages. PostgreSQL will execute the query below in O(n) if there is no index and O(1) if there is an index on num.

Does any other language implement this in a better way? While also still giving a O(n) time complexity and O(1) space? I may have to implement my own top-n heapsort then, or my own top-n insertion sort.

    SELECT * FROM (
      SELECT
        row_number() AS n, num
      FROM t
      ORDER BY num DESC
      LIMIT 5
    ) top5 WHERE n IN (1, 3, 5);

This is assuming we do not care about ties. If we care about ties it gets a bit messier but not that bad.