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In the context of a database system operating in a client/server model, the database server is responsible for processing the queries presented by the client and returning the results accordingly. This has been termed query shipping, where the query is shipped to the server and a payload (data) is returned. The benefits of query shipping are a reduction of communication time for queries and the ability to exploit server resources rather than using the more limited resources of the client to conduct the query. This model also permits a separation of how the data is stored and retrieved on the server from the way the data is used on the client. In other words, the client/server model supports data independence. Data independence is one of the principal advantages of the relational model introduced by Codd in 1970: the separation of the physical implementation from the logical model. According to Codd,8 Users of large data banks must be protected from having to know how the data is organized in the machine Activities of users at terminals and most application programs should remain unaffected when the internal representation of data is changed. This separation allows a powerful set of logical semantics to be developed, independent of a particular physical implementation. The goal of data independence (called physical data independence by Elmasri and Navathe9), is that each of the logical elements is independent of all of the physical elements (see Table 2-1). For example, the logical layout of the data into relations
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8. C. J. Date, The Database Relational Model: A Retrospective Review and Analysis (Reading, MA: Addison-Wesley, 2001). 9. R. Elmasri and S. B. Navathe, Fundamentals of Database Systems, 4th ed. (Boston: Addison-Wesley, 2003).
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CHAPTER 2 THE ANATOMY OF A DATABASE SYSTEM
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(tables) with attributes (fields) arranged by tuples (rows) is completely independent of how the data is stored on the storage medium.
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Table 2-1. The Logical and Physical Models of Database Design
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Logical Model
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Query language Relational Algebra Relational Calculus Relvars
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Sorting algorithms Storage mechanisms Indexing mechanisms Data representation
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One of the challenges of data independence is that database programming becomes a two-part process. First, there is the writing of the logical query describing what the query is supposed to do. Second, there is the writing of the physical plan, which shows how to implement the logical query. The logical query can be written, in general, in many different forms, such as a high-level language like SQL or as an algebraic query tree.10 For example, in the traditional relational model a logical query can be described in relational calculus or relational algebra. The relational calculus is better in terms of focusing on what needs to be computed. The relational algebra is closer to providing an algorithm that lets you find what you are querying for, but still leaves out many details involved in the evaluation of a query. The physical plan is a query tree implemented in a way that it can be understood and processed by the database system s query execution engine. A query tree is a tree structure in which each node contains a query operator and has a number of children that correspond to the number of tables involved in the operation. The query tree can be transformed via the optimizer into a plan for execution. This plan can be thought of as a program that the query execution engine can execute. A query statement goes through several phases before it is executed; parsing, validation, optimization, plan generation/compilation, and execution. Figure 2-2 depicts the query processing steps that a typical database system would employ. Each query statement is parsed for validity and checked for correct syntax and for identification of the query operations. The parser then outputs the query in an intermediate form to allow the optimizer to form an efficient query execution plan. The execution engine then executes the query and the results are returned to the client. This progression is shown in Figure 2-2, where once parsing is completed the query is validated for errors, then optimized; a plan is chosen and compiled; and finally the query is executed.
10. A. B. Tucker, Computer Science Handbook, 2nd ed. (Boca Raton, FL: CRC Press, 2004).
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