A non-hierarchical query 

SELECT
    comment_id,
    thread_id,
    parent_comment_id AS parent,
    score,
    text
FROM comment;

comment_id | thread_id | parent | score |                  text
-----------+-----------+--------+-------+----------------------------------------
         1 |         1 |        |     4 | Comment from earlier thread
         2 |         1 |      1 |     2 | Reply to comment from earlier thread
         3 |         2 |        |     0 | Crap comment that nobody wants to read
         4 |         2 |        |    19 | Top comment
         5 |         2 |      4 |     6 | Earliest reply to top comment
         6 |         3 |        |     2 | Comment from later thread
         7 |         2 |        |     3 | Less highly rated comment
         8 |         2 |      4 |    22 | Top reply to top comment
         9 |         2 |      8 |     1 | Meh response to top reply
        10 |         2 |      7 |     5 | Reply to less highly rated comment
        11 |         2 |      8 |    12 | Top response to top reply