Are circular references acceptable in database?

Question

When are circular references acceptable in database?

Theoretical and practical, any help is appreciated.

Answer 1

I have seen circular references done for performance reasons. It looks ugly though, and the performance might be negligible.

Example: some bulletin boards (I think phpBB does this) have a lastpostid in the category table that is a shortcut to the last post in the thread.

This creates a circle, where the last post has a FK to the category table and the category table has a FK back to the last post.

Like I said, I don't really like it, but I've seen it done.