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.
Other articles related to "query plan, query":
... Some packages feature tools which will generate a graphical representation of a query plan ... to cause the DBMS to return a textual description of the query plan ... Another mechanism for retrieving the query plan involves querying a virtual database table after executing the query to be examined ...
... The query optimizer will not always choose the best query plan for a given query ... In some databases the query plan can be reviewed, problems found, and then the query optimizer given hints on how to improve it ... alternatives to express the same query (other queries that return the same results) can be tried ...
... The query optimizer will not always choose the best query plan for a given query ... In some databases the query plan can be reviewed, problems found, and then the query optimizer given hints on how to improve it ... In other databases alternatives to express the same query (other queries that return the same results) can be tried ...
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