What is Query Optimization

Is a function of many relational database management systems to find a good (not necessarily the best) query plan from multiple query plans satisfying a query. Its goal is to eliminate as many unneeded tuples, or rows as possible. Query optimization may be based on heuristic or cost.

The process whereby a query written in SQL (and, in general, in a declarative query language) is transformed into a query plan that will actually produce the results of running the query. Under the assumption that more than one query plan can be produced for a given query, the query optimizer chooses the plan that it deems better performing.