What is meant by cost-based query optimization?

16.3: Define the following terms: disk, disk pack, track, block, cylinder, sector, interblock gap, and read/write head.

17.1: Define the following terms: indexing field, primary key field, clustering field, secondary key field, block anchor, dense index, and nondense (sparse) index.

 19.5: What is meant by cost-based query optimization?