table thread:
(id, parent_id, content FT, title FT)


a main thread:
(1, 0, "some content", "the title")


a reply in the main thread:
(2, 1, "more content", null)



Because thread(id=2) is a part of the main thread(id=1), I want it to be a part
of the search condition using fulltext.
I only want "main threads" back in the result-set.
The content in the "main thread" may not MATCH in search conditions, but the replies may...
Therefore, the "main thread" should be a part of the resultset.