Consider the relation r(A, B, C) with the attribute A as the index. Give an example of a query that can be answered only by looking at the index, rather than the tuples in the relationship....

Consider the relation r(A, B, C) with the attribute A as the index. Give an example of a query that can be answered only by looking at the index, rather than the tuples in the relationship. (Index-only plans are query plans that just utilise the index and do not access the actual relation.)