A query plan (or query execution plan) is a 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.
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- A query plan (or query execution plan) is a 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.
- クエリ実行計画（くえりじっこうけいかく）とは、ユーザが発行した問い合わせに基づきDBMSが内部的に生成する情報であり、これによりDBMSの行うデータ処理がプログラム的に表わされる。 DBMSはクエリ実行計画の生成にあたりクエリ最適化の処理を行い、最も効果的に処理できる(と判断された)クエリ実行計画を問い合わせから導き出す。 クエリ実行計画はDBMSがその機能を実現するための内部的な情報に過ぎないが、ユーザがチューニングを行うとき手がかりとなる情報を提供するために、多くのDBMSが実行計画の表示機能を提供する。 例としてApache Derbyの実行計画を以下に示す。 Statement Name: null Statement Text: SELECT Country FROM Countries WHERE Region = 'Central America' Parse Time: 10 Bind Time: 0 Optimize Time: 370 Generate Time: 10 Compile Time: 390 Execute Time: 0 Begin Compilation Timestamp : 2005-05-25 09:20:41.274 End Compilation Timestamp : 2005-05-25 09:20:41.664 Begin Execution Timestamp : 2005-05-25 09:20:41.674 End Execution Timestamp : 2005-05-25 09:20:41.674 Statement Execution Plan Text: Project-Restrict ResultSet (2): Number of opens = 1 Rows seen = 6 Rows filtered = 0 restriction = false projection = true constructor time (milliseconds) = 0 open time (milliseconds) = 0 next time (milliseconds) = 0 close time (milliseconds) = 0 restriction time (milliseconds) = 0 projection time (milliseconds) = 0 optimizer estimated row count: 11.90 optimizer estimated cost: 69.35 Source result set: Table Scan ResultSet for COUNTRIES at read committed isolation level using instantaneous share row locking chosen by the optimizer Number of opens = 1 Rows seen = 6 Rows filtered = 0 Fetch Size = 16 constructor time (milliseconds) = 0 open time (milliseconds) = 10 next time (milliseconds) = 0 close time (milliseconds) = 0 next time in milliseconds/row = 0 scan information: Bit set of columns fetched={0, 2} Number of columns fetched=2 Number of pages visited=3 Number of rows qualified=6 Number of rows visited=114 Scan type=heap start position: null stop position: null qualifiers: Column[0][0] Id: 2 Operator: = Ordered nulls: false Unknown return value: false Negate comparison result: false optimizer estimated row count: 11.90 optimizer estimated cost: 69.35
- План выполне́ния запро́са — последовательность операций, необходимых для получения результата SQL-запроса в реляционной СУБД. План в целом разделяется на две стадии: Выборка результатов; Сортировка и группировка, выполнение агрегаций. Сортировка и группировка — это опциональная стадия, которая выполняется, если не найдено путей доступа для получения результата в запрошенном порядке. Выборка результатов выполняется следующими способами: Вложенные циклы; Слияние.
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- A query plan (or query execution plan) is a 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.
- План выполне́ния запро́са — последовательность операций, необходимых для получения результата SQL-запроса в реляционной СУБД. План в целом разделяется на две стадии: Выборка результатов; Сортировка и группировка, выполнение агрегаций.
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- Query plan
- クエリ実行計画
- План выполнения запроса
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