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1974 Join Strategies that Exploit Parallelism
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Consider a join of four relations: r1
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where relation ri is stored at site Si Assume that the result must be presented at site S1 There are many possible strategies for parallel evaluation (We study the issue of parallel processing of queries in detail in 20) In one such strategy, r1 is shipped to S2 , and r1 1 r2 computed at S2 At the same time, r3 is shipped to S4 , and r3 1 r4 computed at S4 Site S2 can ship tuples of (r1 1 r2 ) to S1 as they are produced, rather than wait for the entire join to be computed Similarly, S4 can ship tuples of (r3 1 r4 ) to S1 Once tuples of (r1 1 r2 ) and (r3 1 r4 ) arrive at S1 , the computation of (r1 1 r2 ) 1 (r3 1 r4 ) can begin, with the pipelined join technique of Section 13722 Thus, computation of the nal join result at S1 can be done in parallel with the computation of (r1 1 r2 ) at S2 , and with the computation of (r3 1 r4 ) at S4
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Many new database applications require data from a variety of preexisting databases located in a heterogeneous collection of hardware and software environments Manipulation of information located in a heterogeneous distributed database requires an additional software layer on top of existing database systems This software layer is called a multidatabase system The local database systems may employ different logical models and data-de nition and data-manipulation languages, and may differ in their concurrency-control and transaction-management mechanisms A multidatabase system creates the illusion of logical database integration without requiring physical database integration
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19 Distributed Databases
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Full integration of heterogeneous systems into a homogeneous distributed database is often dif cult or impossible: Technical dif culties The investment in application programs based on existing database systems may be huge, and the cost of converting these applications may be prohibitive Organizational dif culties Even if integration is technically possible, it may not be politically possible, because the existing database systems belong to different corporations or organizations In such cases, it is important for a multidatabase system to allow the local database systems to retain a high degree of autonomy over the local database and transactions running against that data For these reasons, multidatabase systems offer signi cant advantages that outweigh their overhead In this section, we provide an overview of the challenges faced in constructing a multidatabase environment from the standpoint of data de nition and query processing Section 246 provides an overview of transaction management issues in multidatabases
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Each local database management system may use a different data model For instance, some may employ the relational model, whereas others may employ older data models, such as the network model (see Appendix A) or the hierarchical model (see Appendix B) Since the multidatabase system is supposed to provide the illusion of a single, integrated database system, a common data model must be used A commonly used choice is the relational model, with SQL as the common query language Indeed, there are several systems available today that allow SQL queries to a nonrelational database management system Another dif culty is the provision of a common conceptual schema Each local system provides its own conceptual schema The multidatabase system must integrate these separate schemas into one common schema Schema integration is a complicated task, mainly because of the semantic heterogeneity Schema integration is not simply straightforward translation between data-de nition languages The same attribute names may appear in different local databases but with different meanings The data types used in one system may not be supported by other systems, and translation between types may not be simple Even for identical data types, problems may arise from the physical representation of data: One system may use ASCII, another EBCDIC; oating-point representations may differ; integers may be represented in big-endian or little-endian form At the semantic level, an integer value for length may be inches in one system and millimeters in another, thus creating an awkward situation in which equality of integers is only an approximate notion (as is always the case for oating-point numbers) The same name may appear in different languages in different systems For example, a system based in the United States may refer to the city Cologne, whereas one in Germany refers to it as Koln
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