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Semijoin Program

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Encyclopedia of Database Systems

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Synonyms

Semijoin reducer

Definition

A semijoin program is a query execution plan for queries to distributed database systems that uses semijoins to reduce the size of relation instances before they are transmitted and further joined. Yet the reduction itself requires that a projection of the relation instances involved in the join onto the join attributes be transmitted. The maximum amount of reduction can be achieved by a semijoin program called a full reducer. Full reducers that do not require the computation of a fixpoint exist for acyclic queries. Fully reducing relation instances is rarely beneficial. However semijoin programs partially reducing selected relation instances may be an effective optimization when the dominant cost of query execution is communication. Considering semijoin programs considerably increases the distributed query optimization search space.

Historical Background

Semijoin programs were first introduced to improve the performance, input/output operations and...

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Recommended Reading

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Author information

Authors and Affiliations

  1. National University of Singapore, Singapore, Singapore

    Stéphane Bressan

Authors
  1. Stéphane Bressan
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Editor information

Editors and Affiliations

  1. College of Computing, Georgia Institute of Technology, 266 Ferst Drive, 30332-0765, Atlanta, GA, USA

    LING LIU (Professor) (Professor)

  2. Database Research Group David R. Cheriton School of Computer Science, University of Waterloo, 200 University Avenue West, N2L 3G1, Waterloo, ON, Canada

    M. TAMER ÖZSU (Professor and Director, University Research Chair) (Professor and Director, University Research Chair)

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© 2009 Springer Science+Business Media, LLC

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Bressan, S. (2009). Semijoin Program. In: LIU, L., ÖZSU, M.T. (eds) Encyclopedia of Database Systems. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-39940-9_709

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