If there is no join index that handles all of the reference tables involved in a query, the query is resolved with an ad hoc join. Because you cannot create a join index to represent a many-to-many join relationship, you can only issue ad hoc queries against such a relationship. Ad hoc queries provide flexibility, but in some situations this flexibility comes at the expense of performance. If you have sufficient space for the join indexes, and you do not require many-to-many relationships or multilevel star join indexes, you may find it helpful to create join indexes where performance is critical.
