Access path selection

During query processing, your database server takes a nonprocedural query and produces a procedural plan for satisfying it; this process is called making an access path selection.

Queries are nonprocedural because they describe only the records of interest and what operations to perform on them. They do not prescribe an algorithm (procedure) for locating records on disk or the order in which to process them.

Your database server evaluates a collection of possible query plans that can execute the query correctly. The server estimates the cost of running each plan and chooses the one with the smallest cost estimate. Cost estimates are a combination of the number of expected disk I/Os, the expected number of records that must be processed, and the cost of invoking each of the routines in the query on each candidate row.