Evaluating Index Usefulness
SQL Server provides indexes for two primary reasons: as a method to enforce the uniqueness of the data in the database tables and to provide faster access to data in the tables. Creating the appropriate indexes for a database is one of the most important aspects of your physical database design. Because you can't have an unlimited number of indexes on a table, and it wouldn't be feasible anyway, you'll want to create indexes on columns that have high selectivity so that the index will be used by your queries. The selectivity of an index can be defined as follows:
Selectivity ratio = (Number of unique index values)/ (Total number of rows in the table)
If the selectivity ratio is high—that is, a large number of rows ...
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