ch13 - Chapter 13: Query Processing Chapter Aug 10, 2006...

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Database System Concepts, 5th Ed . ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Chapter 13: Query Processing Chapter 13: Query Processing Aug 10, 2006
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©Silberschatz, Korth and Sudarshan 13.2 Database System Concepts - 5 th Edition. Chapter 13: Query Processing Chapter 13: Query Processing ± Overview ± Measures of Query Cost ± Selection Operation ± Sorting ± Join Operation ± Other Operations ± Evaluation of Expressions
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©Silberschatz, Korth and Sudarshan 13.3 Database System Concepts - 5 th Edition. Basic Steps in Query Processing Basic Steps in Query Processing 1. Parsing and translation 2. Optimization 3. Evaluation
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©Silberschatz, Korth and Sudarshan 13.4 Database System Concepts - 5 th Edition. Basic Steps in Query Processing Basic Steps in Query Processing (Cont.) (Cont.) ± Parsing and translation z translate the query into its internal form. This is then translated into relational algebra. z Parser checks syntax, verifies relations ± Evaluation z The query-execution engine takes a query-evaluation plan, executes that plan, and returns the answers to the query.
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©Silberschatz, Korth and Sudarshan 13.5 Database System Concepts - 5 th Edition. Basic Steps in Query Processing : Basic Steps in Query Processing : Optimization Optimization ± A relational algebra expression may have many equivalent expressions z E.g., σ balance < 2500 ( balance ( account)) is equivalent to balance ( σ balance < 2500 ( account)) ± Each relational algebra operation can be evaluated using one of several different algorithms z Correspondingly, a relational-algebra expression can be evaluated in many ways. ± Annotated expression specifying detailed evaluation strategy is called an evaluation-plan . z E.g., can use an index on balance to find accounts with balance < 2500, z or can perform complete relation scan and discard accounts with balance 2500
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©Silberschatz, Korth and Sudarshan 13.6 Database System Concepts - 5 th Edition. Basic Steps: Optimization (Cont.) Basic Steps: Optimization (Cont.) ± Query Optimization : Amongst all equivalent evaluation plans choose the one with lowest cost. z Cost is estimated using statistical information from the database catalog ± e.g. number of tuples in each relation, size of tuples, etc. ± In this chapter we study z How to measure query costs z Algorithms for evaluating relational algebra operations z How to combine algorithms for individual operations in order to evaluate a complete expression ± In Chapter 14 z We study how to optimize queries, that is, how to find an evaluation plan with lowest estimated cost
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©Silberschatz, Korth and Sudarshan 13.7 Database System Concepts - 5 th Edition. Measures of Query Cost
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This note was uploaded on 02/17/2010 for the course CAS 6M03 taught by Professor Franek during the Spring '10 term at McMaster University.

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ch13 - Chapter 13: Query Processing Chapter Aug 10, 2006...

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