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Approximating block accesses in database organizations

By S. B. Yao

Communications of the ACM, Vol. 20 No. 4, Pages 260-261

When data records are grouped into blocks in secondary storage, it is frequently necessary to estimate the number of blocks XD accessed for a given query. In a recent paper [1], Cardenas gave the expression XD = m(1 - (1 - 1/m)k), (1) assuming that there are n records divided into m blocks and that the k records satisfying the query are distributed uniformly among the m blocks. The derivation of the expression was left to the reader as an exercise.

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