When is a full table scan better than index scan?

What are some scenarios (in SQL Server) in which a full table scan is better than an index scan?

When the

• table is small enough there is no practical difference

• statistically, you'd return most rows anyway

The 2nd case needs qualifying

• An index scan will replace an index seek if an index is covering

• An index seek or scan with many rows that requires key/bookmark lookups will be expensive and a table scan could be better

Finally

• An index scan and a table scan are pretty much the same for clustered indexes

StackExchange Database Administrators Q#11679, answer score: 11