Cost is expressed as follows:
Elapsed time is a fairly simple measure, but it is not comparable across different system configurations, nor is it independent of other workloads running on a system. It has the advantages of conceptual simplicity and ease of measurement. With respect to join index creation, it is a measure of how long it takes to create the index. With respect to maintenance, it is a measure of how long it takes to update a join index table beyond the time required to perform the update of the base table. In this sense, the term update refers generically to the data manipulation operations insert, update, and delete performed by any of the following SQL statements:
CPU path per transaction is a sophisticated measure that is comparable across different system configurations. It has the disadvantages of being both difficult to measure without the proper tools and difficult to understand conceptually.
Whether creation and maintenance cost is measured as elapsed time or as CPU path per transaction, the same proportional relationship holds: the smaller the number, the better.
The total cost for a join index is the sum of its creation and maintenance costs.