Query optimization

Query optimization is a function of many relational database management systems in which multiple query plans for satisfying a query are examined and a good query plan is identified. Cost based query optimizers evaluate the resource footprint of various query plans and use this as the basis for plan selection. Typically the resources which are costed are CPU path length, disk storage service time, and interconnect usage between units of parallelism. The set of query plans examined is formed by examining possible access paths (e.g., primary index access, secondary index access, full file scan) and various relational table join techniques (e.g, merge join, hash join, product join). The search space can become quite large depending on the complexity of the SQL query.