In my presentations on indexing I have always said that you should explore the advantages of covering your clustered index with a secondary index. In circumstances where you might want to just return values form the PK ( assuming it's your clustered index ) a secondary index will be more efficient especially when the row size is wide. Any operation on a clustered index will always return the entire row, so select ID from dbo.mytable where ID is the clustered PK integer will return not just the...(read more)