Chapter13 - Database Management Systems R Ramakrishnan and...

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Unformatted text preview: Database Management Systems, R. Ramakrishnan and J. Gehrke Introduction to Query Optimization Chapter 13 Database Management Systems, R. Ramakrishnan and J. Gehrke Overview of Query Optimization ❖ Plan : Tree of R.A. ops, with choice of alg for each op. – Each operator typically implemented using a `pull’ interface: when an operator is `pulled’ for the next output tuples, it `pulls’ on its inputs and computes them. ❖ Two main issues: – For a given query, what plans are considered ? ◆ Algorithm to search plan space for cheapest (estimated) plan. – How is the cost of a plan estimated ? ❖ Ideally: Want to find best plan. Practically: Avoid worst plans! ❖ We will study the System R approach. Database Management Systems, R. Ramakrishnan and J. Gehrke Highlights of System R Optimizer ❖ Impact: – Most widely used; works well for < 10 joins. ❖ Cost estimation: Approximate art at best. – Statistics, maintained in system catalogs, used to estimate cost of operations and result sizes. – Considers combination of CPU and I/O costs. ❖ Plan Space: Too large, must be pruned. – Only the space of left-deep plans is considered. ◆ Left-deep plans allow output of each operator to be pipelined into the next operator without storing it in a temporary relation. – Cartesian products avoided. Database Management Systems, R. Ramakrishnan and J. Gehrke Schema for Examples ❖ Similar to old schema; rname added for variations....
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This note was uploaded on 04/06/2010 for the course CS COP 6371 taught by Professor Dr.hua during the Spring '10 term at University of Central Florida.

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Chapter13 - Database Management Systems R Ramakrishnan and...

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