Index does not support range queries

Generally, clustered indexes and covering indexes provide good performance for range queries and for search arguments (SARG) that match many rows. Range queries that reference the keys of noncovering indexes use the index for ranges that return a limited number of rows.

As the number of rows the query returns increases, however, using a nonclustered index or a clustered index on a data-only-locked table can cost more than a table scan.