As mentioned earlier, the selectivity of a key is an important factor that determines whether an index will be used to retrieve the data rows that satisfy a query. SQL Server stores the selectivity and a histogram of sample values of the key; based on the statistics stored for the key columns for the index and the SARGs specified for the query, the Query Optimizer decides which index to use.
To see the statistical information stored for an index, use the
DBCC SHOW_STATISTICS command, which returns the following pieces of information:
A histogram that contains an even sampling of the values for the first column in the index key. ...