Research and Advances

Analysis of design alternatives for virtual memory indexes

A class of index structures for use in a virtual memory environment is described. Design alternatives within this class of index structures are analyzed. These alternatives include a choice of search strategy, whether or not pages in the index are stuctured, and whether or not keys are compressed. The average cost of retrieving entries from these indexes is expressed as a weighted sum of the cost of a basis key comparison and the cost of crossing a page boundary in the index structure. Formulas for the retrieval costs possible combinations of design alternatives are given. These are used in numerical case studies which compare the retrieval costs of the alternatives. Qualitative comparisons of the maintenance costs (insertion, deletion, reorganization) of the design alternatives are also included.

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Research and Advances

Optimal reorganization of distributed space disk files

In most database organizations, the cost of accessing the database will increase due to structural changes caused by updates and insertions. By reorganizing the database, the access costs can be reduced. A basic problem is to establish the proper tradeoff between performance, storage costs, and reorganization costs. This paper considers the optimum points at which to reorganize a database. A disk file organization which allows for distributed free space is described. A cost function describing the excess costs due to physical disorganization is defined, and this function is minimized to obtain the optimum reorganization points. Numerical examples based on the characteristics of existing disk storage devices are given.

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