This record contains search key value and a pointer to the actual record. To locate a record, we find the index record with the largest search key value less than or equal to the search key value we are looking for. Notice how we would find records for Perryridge branch using both methods. Dense indices are faster in general, but sparse indices require less space and impose less maintenance for insertions and deletions.
Biggest cost is in bringing a block into main memory.
Index size still small.