12_Query_Processing

12_Query_Processing - Distributed Database Systems Overview...

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Distributed Database Systems Overview of Query Processing 605.741 David Silberberg
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D. Silberberg Distributed Database Systems Overview of Query Processing 2 Distributed Query Processing • Language of Access is SQL • Non-procedural - no program is written to tell the DBMS how to get the data • Structure of SQL is asking for attributes, constraints, and joins • The order and the process of solving the query is done by a query processor • Query processor efficiency is crucial to the success of RDBMSs • Very complicated issue - many factors to consider
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D. Silberberg Distributed Database Systems Overview of Query Processing 3 Distributed Databases • More complicated - many more factors to consider • Distributed fragments • More communication • Account for replication • More sites increases query response time
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D. Silberberg Distributed Database Systems Overview of Query Processing 4 Query Processing Purpose • Execute SQL query onto a distributed database Decompose the query to localized queries of distributed fragments • Optimize the performance of the query – Minimize the cost – I/O, communication, joining, etc.
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D. Silberberg Distributed Database Systems Overview of Query Processing 5 Query Processing Example • Translate SQL query to relational algebra query • Optimize this translation and determine the best query plan • There are several choices, which lead to different efficiencies • Consider the following schema EMP(ENO, ENAME, TITLE) ASG(ENO, PNO, RESP, DUR)
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D. Silberberg Distributed Database Systems Overview of Query Processing 6 Example Continued • Find employee names who manage projects SELECT ENAME FROM EMP, ASG WHERE EMP.ENO = ASG.ENO AND RESP = 'Manager'
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D. Silberberg Distributed Database Systems Overview of Query Processing 7 Optimization Via Relational Algebra • Here is the query expressed in relational algebra Π ENAME ( σ RESP='Manager' EMP.ENO = ASG.ENO (EMP × ASG)) Π ENAME ( EMP.ENO = ASG.ENO (EMP ×σ RESP='Manager' (ASG))) Π ENAME (EMP EMP.ENO = ASG.ENO σ RESP='Manager' (ASG))
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D. Silberberg Distributed Database Systems Overview of Query Processing 8 Central vs. Distributed Databases • In central systems, the problem is finding the best relational algebra expression – In general, this process is intractable – But, we find something that is close to best using rules and heuristics • In distributed system, this becomes more complicated – Communication time for exchanging data between sites – Advantages of parallel processing – Choosing the right site for replicated data – The solution space is larger
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D. Silberberg Distributed Database Systems Overview of Query Processing 9 Query Processing Strategies • Different processing strategies are have different
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This note was uploaded on 12/23/2009 for the course DBST 663 taught by Professor Tba during the Spring '09 term at MD University College.

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12_Query_Processing - Distributed Database Systems Overview...

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