A query plan (or query execution plan) is an ordered set of steps used to access or modify information in a SQL relational database management system. This is a specific case of the relational model concept of access plans.
Since SQL is declarative, there are typically a large number of alternative ways to execute a given query, with widely varying performance. When a query is submitted to the database, the query optimizer evaluates some of the different, correct possible plans for executing the query and returns what it considers the best alternative. Because query optimizers are imperfect, database users and administrators sometimes need to manually examine and tune the plans produced by the optimizer to get better performance.
Read more about Query Plan: Generating Query Plans, Database Tuning, Query Tuning
Famous quotes containing the words query and/or plan:
“Such condition of suspended judgment indeed, in its more genial development and under felicitous culture, is but the expectation, the receptivity, of the faithful scholar, determined not to foreclose what is still a question—the “philosophic temper,” in short, for which a survival of query will be still the salt of truth, even in the most absolutely ascertained knowledge.”
—Walter Pater (1839–1894)
“...I believed passionately that Communists were a race of horned men who divided their time equally between the burning of Nancy Drew books and the devising of a plan of nuclear attack that would land the largest and most lethal bomb squarely upon the third-grade class of Thomas Jefferson School in Morristown, New Jersey.”
—Fran Lebowitz (b. 1950)