In database, index architectures can be classified as clustered or non-clustered. Non-clustered: A non-clustered index is structured in a way that doesn't correspond to the order of the actual data records. It resembles the words index at the back of a book. For this reason, it will perform worse than clustered indexes on ranged searches where the result set is large, since each result could an additional I/O operations to get the actual data record. Clustered: Clustering re-orders the data block in the same order as the index, hence it is also an operation on the data storage blocks as well as on the index. A phone book ordered by last name resemples a clustered index in it structure and purpose.