May 20, 2007 Chapter 13: Query Processing Overview Measures of Query Cost Selection Operation Sorting Join Operation Other Operations Evaluation of Expressions Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. Query Decomposition The query decomposition is the first phase of query processing whose aims are to transform a high-level query into a relational algebra query and to check. Selection from Database Systems: Concepts, Design and Applications Book.
The optimizer tries to generate the almost all optimal delivery plan for a SQL declaration.
The optimizer choose the plan with the minimum cost among all considered candidate plans. The optimizer uses available statistics to compute cost. For a particular query in a given environment, the price computation accounts for elements of query setup like as I/U, Central processing unit, and conversation.
For instance, a query might demand details about workers who are supervisors. If the optimizer data show that 80% of employees are managers, then the optimizer may choose that a complete table check is most efficient. Nevertheless, if figures suggest that quite few workers are administrators, then reading an list followed by a desk accessibility by rowid may end up being more efficient than a full table check out.
Because the data source has numerous internal data and tools at its convenience, the optimizer can be generally in a much better position than the user to figure out the optimal method of declaration setup. For this cause, all SQL claims use the optimizer.