Implementing "Related posts" in CI?

[eluser]aquary[/eluser]
[quote author="jedd" date="1248886721"]
As you've no doubt discovered, this is a pretty painful way of doing it.
[/quote]

At that time, I'm too lazy to think of a way to do it properly. Maybe I should remake the system again

About using tags with forums, if it is not too difficult to implement, I'd like to do so. However, almost none of forum members would fill in the tags for their threads anyway...