ALLOCATING RELATIONS IN A DISTRIBUTED DATABASE SYSTEM

A model is proposed that allocates tables of a relational database to the sites of a distributed system in order that the total cost of exec uting a given collection of join queries is minimized. This model is p resented in the convenient form of an integer linear program. Each ind ividual query specifies that several logically distinct data sets, or relations, are to be amalgamated and presented to the particular user that issued the request. Performing this task requires the utilization of limited system resources; both processors, and the communications facilities that interconnect them, may be used. An optimal strategy fo r executing a single query is, therefore, defined to be one that minim izes a weighted sum of the costs of computation, and those of informat ion interchange, incurred during the computation. One particular model , appearing in [1], conforms to this philosophy, and so forms the basi s for further investigations. The total cost of executing an entire gr oup of such queries depends upon the way in which the relevant informa tion is allocated to the sites of the network. Several copies of any p articular relation may be dispersed across the network; the replicatio n of data increases its availability, and potentially decreases the co sts of answering the given requests. However, only limited storage cap acities are available, and increased replication commands greater over heads in maintaining consistency. An optimization program is developed to design a data allocation plan that achieves a minimal total cost f or the execution of a given group of requests, while maintaining restr aints on the levels of data replication considered permissible.