Index Histograms and Density Vector

DBCC SHOW_STATISTICS will give the density vector and histogram information.

The density Vector gives an average of the index key. SQL Server might also give an average of the nonclustered key along with the the clustered index key

The histogram gives us specific information of the index based on only the first column in the key.

The “All Density” column multiplied by “Rows” column will give the “Average number of rows for a key value”, in this case, LastName.
In this example, there are 370.3 rows on average (0.0370.3 * 10,000) for any given lastname. And there is 1 row on average for each combination of LastName, Member no. (0.0001 * 10,000).