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ch12_query_processing

ch12_query_processing - Chapter 12 Query Processing Chapter...

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Chapter 12: Query Processing Chapter 12: Query Processing
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©Silberschatz, Korth and Sudarshan 12.2 Database System Concepts - 6 th Edition Chapter 12: Query Processing Chapter 12: 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 12.3 Database System Concepts - 6 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 12.4 Database System Concepts - 6 th Edition Basic Steps in Query Processing (Cont.) (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.
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©Silberschatz, Korth and Sudarshan 12.5 Database System Concepts - 6 th Edition Basic Steps in Query Processing : Optimization Optimization A relational algebra expression may have many equivalent expressions E.g., σ salary < 75000 ( salary ( instructor)) is equivalent to salary ( σ salary < 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
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©Silberschatz, Korth and Sudarshan 12.6 Database System Concepts - 6 th Edition Basic Steps: Optimization (Cont.) Basic Steps: Optimization (Cont.) Query Optimization : Amongst all equivalent evaluation plans choose the one with lowest cost. 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 How to measure query costs Algorithms for evaluating relational algebra operations How to combine algorithms for individual operations in order to evaluate a complete expression In Chapter 14 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 12.7 Database System Concepts - 6 th Edition Measures of Query Cost Measures of Query Cost Cost is generally measured as total elapsed time for answering query Many factors contribute to time cost disk accesses, CPU , or even network communication Typically disk access is the predominant cost, and is also relatively easy to estimate. Measured by taking into account Number of seeks * average-seek-cost Number of blocks read * average-block-read-cost Number of blocks written * average-block-write-cost Cost to write a block is greater than cost to read a block data is read back after being written to ensure that the write was successful
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