Optimization of bound disjunctive queries with constraints

Abstract

This paper presents a technique for the optimization of bound queries over disjunctive deductive databases with constraints. The proposed approach is an extension of the wellknown Magic-Set technique and is well-suited for being integrated in current bottom-up (stable) model inference engines. More specifically, it is based on the exploitation of binding propagation techniques which reduce the size of the data relevant to answer the query and, consequently, reduces both the complexity of computing a single model and the number of models to be considered. The motivation of this work stems from the observation that traditional binding propagation optimization techniques for bottom-up model generator systems, simulating the goal driven evaluation of top-down engines, are only suitable for positive (disjunctive) queries, while hard problems are expressed using unstratified negation.The main contribution of the paper consists in the extension of a previous technique, defined for positive disjunctive queries, to queries containing both disjunctive heads and constraints (a simple and expressive form of unstratified negation). As the usual way of expressing declaratively hard problems is based on the guess-and-check technique, where the guess part is expressed by means of disjunctive rules and the check part is expressed by means of constraints, the technique proposed here is highly relevant for the optimization of queries expressing hard problems. The value of the technique has been proved by several experiments.

The Consistency Extractor System: Querying Inconsistent Databases Using Answer Set Programs

“…they compute all and only correct answers for the query. In [26] a sound but incomplete methodology is presented for disjunctive programs with program constraints of the form ← C(x), where C(x) is a conjunction of literals (i.e. positive or negated atoms).…”

“…More recently, the MS techniques have been extended to logic programs with stable models semantics [21,24,18,26]. In [12] it was shown how to adopt and adapt those techniques to our repair programs, resulting in a sound and complete MS methodology for the repair programs with program constraints.…”

The consistency extractor system: Answer set programs for consistent query answering in databases

“…Recently, the MS techniques have been extended to logic programs with stable models semantics [35,29,42,44].…”

“…the method computes all and only correct answers for the query. In [44] a sound but incomplete methodology is presented for disjunctive programs with program constraints of the form ← C (x), where C(x) is a conjunction of literals (i.e. positive or negated atoms).…”

“…This might happen if the query is related with a part of the program which is consistent with respect to the program constraint. The MS method focuses on that part of the program to answer the query [44]. So, the non-satisfaction of the program constraints will not be detected by the latter.…”