ch7 - Distributed Database Systems Chapter 7 Overview of...

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Unformatted text preview: Distributed Database Systems Chapter 7 Overview of Query Processing Chapter 7 Overview of Query Processing SQL - nonprocedural language of RDB Tuple calculus { | ( )} t F t where t: tuple variable, F(t): wff, Example: get the no. and name of all managers { ( , )| ( ) " "} t ENO ENAME t E t TITLE MANAGER = Domain caculus { , , . . . | ( , , . . . , )} x x x F x x x n n 1 2 1 2 where xs: domain variables, F(x 1 ,,x n ): wff Example: )} " " , , ( | , { m a n a g e r y x E y x SQL is a tuple calculus language select ENO, ENAME from E where TITLE=manager End user uses non-procedural languages to express queries. 1 Query Processor (optimization) Distributed Database Systems Chapter 7 Overview of Query Processing Query processor transforms queries into procedural operations to access data. Calculus Relational formula algebra operations Distributed query processor has to deal with D query decomposition, and q data localization 71 QUERY PROCESSING PROBLEMS Centralized query processor must- transform calculus query into algebra operation- choose the best execution plan Example: SELECT ENAME FROM E,G WHERE RESP = Manager Relational algebra 1: ENAME RESP Manager E EMO G ENO E G ( ( )) " " . . = = 2 Distributed Database Systems Chapter 7 Overview of Query Processing Relational algebra 2: ))) ( ( ( " " G E Manager RESP ENO ENAME = Execution plan 2 is better for consuming less resources! In DDB, the query processor must consider the communication cost and select the best site!...
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ch7 - Distributed Database Systems Chapter 7 Overview of...

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