There is so much that could affect the answers to this question. > Does the table have a **clustered** index already? > How many other indexes are there on the table? > Do you need to add this index or alter an existing one? > What datatypes are going to be in this index? > How many columns will be in it, and how many included columns will be in it? > How often does the query run that needs this index? > What are the details of the table - How many rows, How many columns? Generally however, adding an index will speed up data retrieval (SELECT) operations and slow down data updates(UPDATE and DELETE) operations - just as @scot hauder says in his comment.