A hierarchical query 

WITH RECURSIVE
    comment_tree (comment_id) AS (
        SELECT comment_id
        FROM comment
        WHERE thread_id = 2
        AND parent_comment_id IS NULL

        UNION

        SELECT c.comment_id
        FROM comment c, comment_tree ct
        WHERE c.parent_comment_id = ct.comment_id
    )
SELECT c.*
FROM comment_tree ct
JOIN comment c USING (comment_id);