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Algorithms that make use of an existing index (e.g., B⁺-tree)

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• Operators that benefit from an index
  • The following algorithm can be very efficient if an appropriate index is available:

    σAttr = constant(R)   --   and there is an index on the select attribute(s) σAttr ≥ constant(R)   --   and there is an index on the select attribute(s) R ⋈ S   --   and there is an index on the join attribute(s)

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• Terminology: clustering index
  • Recall: clustered and non-clustered file (see: click here

    Clustered file: A file that stores records of one relation Unclustered file: (less common) A file that stores records of multiple relation

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  • Definition: clustering index

    Clustering index: An index on attribute(s) A on a file is a clustering index when: All tuples with attribute value A = a are stored sequentially (= consecutively) in the data file Graphically:

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  • Common example of a clustering index:

    When the relation R is sorted on the attribute(s) A: