When the optimizer estimates costs for the query, the two factors it considers are the cost of physical I/O, reading pages from disk, and the cost of logical I/O, finding pages in the data cache. The optimizer assigns 18 as the cost of a physical I/O and 2 as the cost of a logical I/O. These are relative units of cost and do not represent time units such as milliseconds or clock ticks. These units are used in the formulas in this chapter, with the physical I/O costs first, then the logical I/O costs. The total cost of accessing a table can be expressed as:
Cost = All physical IOs * 18 + All logical IOs * 2
