<P> Indices can be implemented using a variety of data structures . Popular indices include balanced trees, B+ trees and hashes . </P> <P> In Microsoft SQL Server, the leaf node of the clustered index corresponds to the actual data, not simply a pointer to data that resides elsewhere, as is the case with a non-clustered index . Each relation can have a single clustered index and many unclustered indices . </P> <P> An index is typically being accessed concurrently by several transactions and processes, and thus needs concurrency control . While in principle indexes can utilize the common database concurrency control methods, specialized concurrency control methods for indexes exist, which are applied in conjunction with the common methods for a substantial performance gain . </P> <P> In most cases, an index is used to quickly locate the data record (s) from which the required data is read . In other words, the index is only used to locate data records in the table and not to return data . </P>

Which type of clustering index are defined on the fields