he time that it takes to bring a block from disk into main memory

DATABASE SYSTEM CONCEPTS, SIXTH EDITION

There is a trade-off that the system designer must make between access time
and space overhead. Although the decision regarding this trade-off depends on
the specific application, a good compromise is to have a sparse index with one
index entry per block. The reason this design is a good trade-off is that the
dominant cost in processing a database request is the time that it takes to bring
a block from disk into main memory. Once we have brought in the block, the
time to scan the entire block is negligible. Using this sparse index, we locate the
block containing the record that we are seeking. Thus, unless the record is on an
overflow block (see Section 10.6.1), we minimize block accesses while keeping
the size of the index (and thus our space overhead) as small as possible.