I once built a Reddit clone and used recursive ctes for the comment trees. Imagine each comment has a parent comment foreign key which might be null if at top level. If you use a recursive cte you can simply start with top level comments with null parents and recursively get all of the subtrees and end up with a nice flat list of all your comments in the order you want. The other way to do this is to just fetch all the comments and assemble the tree in memory and then flatten it, which is what I believe many sites do.
A third way to do this is described in https://en.wikipedia.org/wiki/Nested_set_model. Its performance for the common case of heavy reads and few writes is better than either of the two that you describe.
