Mapping-equivalence and oid-equivalence of single-function object-creating conjunctive queries

TL;DR

This paper studies single-function object-creating conjunctive queries, providing a simpler characterization for oid-equivalence and solving the logical entailment problem, both within NP complexity, advancing understanding in data exchange and integration.

Contribution

It introduces a new NP-based characterization for oid-equivalence of sifo CQs and resolves the logical entailment problem for this class.

Findings

New NP characterization for oid-equivalence

Logical entailment for sifo CQs is in NP

Simplifies previous complexity results

Abstract

Conjunctive database queries have been extended with a mechanism for object creation to capture important applications such as data exchange, data integration, and ontology-based data access. Object creation generates new object identifiers in the result, that do not belong to the set of constants in the source database. The new object identifiers can be also seen as Skolem terms. Hence, object-creating conjunctive queries can also be regarded as restricted second-order tuple-generating dependencies (SO tgds), considered in the data exchange literature. In this paper, we focus on the class of single-function object-creating conjunctive queries, or sifo CQs for short. We give a new characterization for oid-equivalence of sifo CQs that is simpler than the one given by Hull and Yoshikawa and places the problem in the complexity class NP. Our characterization is based on Cohen's equivalence notions for conjunctive queries with multiplicities. We also solve the logical entailment problem for sifo CQs, showing that also this problem belongs to NP. Results by Pichler et al. have shown that logical equivalence for more general classes of SO tgds is either undecidable or decidable with as yet unknown complexity upper bounds.