The query plan

The query optimizer evaluates the different ways in which a query might be performed and determines the best way to select the requested data. During this evaluation, the optimizer formulates a query plan to fetch the data rows that are required to process a query.

For example, when evaluating the different ways in which a query might be performed, the optimizer must determine whether indexes should be used. If the query includes a join, the optimizer must determine the join plan (hash or nested loop) and the order in which tables are evaluated or joined.

The following topics describe the components of a query plan and show examples of query plans.