Implementation of Logical Query Languages for Databases

Abstract

We examine methods of implementing queries about relational databases in the case where these queries are expressed in first-order logic as a collection of Horn clauses. Because queries may be defined recursively, straightforward methods of query evaluation do not always work, and a variety of strategies have been proposed to handle subsets of recursive queries. We express such query evaluation techniques as “capture rules” on a graph representing clauses and predicates. One essential property of capture rules is that they can be applied independently, thus providing a clean interface for query-evaluation systems that use several different strategies in different situations. Another is that there be an efficient test for the applicability of a given rule. We define basic capture rules corresponding to application of operators from relational algebra, a top-down capture rule corresponding to “backward chaining,” that is, repeated resolution of goals, a bottom-up rule, corresponding to “forward chaining,” where we attempt to deduce all true facts in a given class, and a “sideways” rule that allows us to pass results from one goal to another.

DOI: 10.1145/3979.3980

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@article{Ullman1985ImplementationOL, title={Implementation of Logical Query Languages for Databases}, author={Jeffrey D. Ullman}, journal={ACM Trans. Database Syst.}, year={1985}, volume={10}, pages={289-321} }