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Block Access Estimation for Clustered Data
August 1993 (vol. 5 no. 4)
pp. 712-718

A method is proposed for dealing with nonuniform data distributions in database organizations in order to estimate the expected number of blocks containing the tuples requested by a query. When tuples with equal attribute value are not uniformly distributed over the blocks of secondary memory that store the relation, a clustering effect is observed. This can be detected by means of a single parameter, the clustering factor, which can be stored in the system catalog. The method can be applied to uniform data distributions as well, since it is shown that a uniform distribution can be viewed as a particular instance of a class of clustered distributions. In this case the proposed method allows considerable reduction of the number of computational steps needed to compute the estimated result.

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Index Terms:
relational databases; clustered data; nonuniform data distributions; database organizations; tuples; query; clustering effect; clustering factor; system catalog; relational databases
P. Ciaccia, "Block Access Estimation for Clustered Data," IEEE Transactions on Knowledge and Data Engineering, vol. 5, no. 4, pp. 712-718, Aug. 1993, doi:10.1109/69.234782
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