This does not solve the fundamental problem of an upsert. This rule allows the possibility of raising an exception on INSERT if two Pages with the same Url are

This does not solve the fundamental problem of an upsert. This rule allows the possibility of raising an exception on INSERT if two Pages with the same Url are "upserted" in parallel. You would still require a table level lock to ensure you don't get an exception within the statement as a whole. Also: Rules are easy to get wrong. Try not to reach for them unless absolutely necessary.