Nonclustered indexes help query performance in all manner of ways. However, unlike clustered indexes, the data isn’t stored with nonclustered indexes. Therefore, a common occurrence is the lookup, going back to the clustered index or the heap to retrieve the data not stored with the nonclustered index. In some cases, this is benign behavior. In other cases, this becomes a performance problem. Having ways to deal with this common issue can help your systems.