ESTIMATE_SOURCE( 
column-name
[, value  
[, relation-string ] ] 
)

The following list shows the selectivity estimate sources that ESTIMATE_SOURCE returns. For more information about the sources, see Selectivity estimate sources.

This function returns the source of the selectivity estimate for the predicate column-name relation-string value. If value is NULL and the relation string is '=', the selectivity source is for the predicate column-name IS NULL. If value is NULL and the relation string is '!=' or '<>', the selectivity source is for the predicate column-name IS NOT NULL.

• Selectivity estimate sources
• ESTIMATE function [Miscellaneous]
• INDEX_ESTIMATE function [Miscellaneous]

The following statement returns the selectivity source Index for evaluating whether the first value in the EmployeeID column is greater than 200. Returning Index means that the query optimizer used an index to estimate the selectivity.

SELECT FIRST ESTIMATE_SOURCE( EmployeeID, 200, '>' )
   FROM Employees
   ORDER BY 1;