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Lecture Database system concepts (6/e): Chapter 12 - Silberschatz, Korth, Sudarshan

This chapter describes the process by which queries are executed efficiently by a database system. Query processing refers to the range of activities involved in extracting data from a database. The activities include translation of queries in high-level database languages into expressions that can be used at the physical level of the file system, a variety of query-optimizing transformations, and actual evaluation of queries. | Chapter 12: Query Processing Database System Concepts, 6th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 12: Query Processing Overview Measures of Query Cost Selection Operation Sorting Join Operation Other Operations Evaluation of Expressions Database System Concepts - 6th Edition 12.2 ©Silberschatz, Korth and Sudarshan Basic Steps in Query Processing 1. Parsing and translation 2. Optimization 3. Evaluation Database System Concepts - 6th Edition 12.3 ©Silberschatz, Korth and Sudarshan Basic Steps in Query Processing (Cont.) Parsing and translation translate the query into its internal form. This is then translated into relational algebra. Parser checks syntax, verifies relations Evaluation The query-execution engine takes a query-evaluation plan, executes that plan, and returns the answers to the query. Database System Concepts - 6th Edition 12.4 ©Silberschatz, Korth and Sudarshan Basic Steps in Query Processing : Optimization A relational algebra expression may have many equivalent expressions E.g., σsalary<75000(∏salary(instructor)) is equivalent to ∏salary(σsalary<75000(instructor)) Each relational algebra operation can be evaluated using one of several different algorithms Correspondingly, a relational-algebra expression can be evaluated in many ways. Annotated expression specifying detailed evaluation strategy is called an evaluation-plan. E.g., can use an index on salary to find instructors with salary < 75000, or can perform complete relation scan and discard instructors with salary ≥ 75000 Database System Concepts - 6th Edition 12.5 ©Silberschatz, Korth and .