Should every SQL Server foreign key have a matching index?

Question

Is it good practice to create an index for every foreign key in a SQL Server database?

Answer 1

Every foreign key? No. Where the selectivity is low (i.e. many values are duplicated), an index may be more costly than a table scan. Also, in a high activity environment (much more insert/update/delete activity than querying) the cost of maintaining the indexes may affect the overall performance of the system.