SQL Server creates a histogram on the leading key but not on the other columns

the leading key should be the most selective column—number in our case. Allegedly (according to the recommendation), SQL Server creates a histogram on the leading key but not on the other columns; so if you follow the recommendation, the optimizer can produce better cardinality estimates, which tend to lead to more optimal choices. http://sqlmag.com/t-sql/optimization-tips-multiple-range-predicates-part-1