Exchange-Repairs: Managing Inconsistency in Data Exchange

TL;DR

This paper introduces exchange-repairs, a new framework for managing inconsistencies in data exchange, enabling meaningful query answers even when no solutions exist for the source data.

Contribution

It formalizes exchange-repairs and shows how to compute XR-certain answers using existing repair-based query answering techniques for various schema mappings.

Findings

XR-certain answers can be computed via consistent query answering methods.

For GAV dependencies, XR-certain answers are equivalent to consistent answers over source repairs.

For GLAV constraints, XR-certain answers are obtainable through disjunctive logic programming.

Abstract

In a data exchange setting with target constraints, it is often the case that a given source instance has no solutions. In such cases, the semantics of target queries trivialize. The aim of this paper is to introduce and explore a new framework that gives meaningful semantics in such cases by using the notion of exchange-repairs. Informally, an exchange-repair of a source instance is another source instance that differs minimally from the first, but has a solution. Exchange-repairs give rise to a natural notion of exchange-repair certain answers (XR-certain answers) for target queries. We show that for schema mappings specified by source-to-target GAV dependencies and target equality-generating dependencies (egds), the XR-certain answers of a target conjunctive query can be rewritten as the consistent answers (in the sense of standard database repairs) of a union of conjunctive queries over the source schema with respect to a set of egds over the source schema, making it possible to use a consistent query-answering system to compute XR-certain answers in data exchange. We then examine the general case of schema mappings specified by source-to-target GLAV constraints, a weakly acyclic set of target tgds and a set of target egds. The main result asserts that, for such settings, the XR-certain answers of conjunctive queries can be rewritten as the certain answers of a union of conjunctive queries with respect to the stable models of a disjunctive logic program over a suitable expansion of the source schema.