Given a query, there are many access plans that a database management system (DBMS) can follow to process it and produce its answer. All plans are equivalent in terms of their final output but vary in their cost, that is, the amount of time that they need to run. This cost difference can be several orders of magnitude large. Thus all DBMSs have a module that examines “all” alternatives and chooses the plan that needs the least amount of time. This module is called the query optimizer.