Exchange-Repairs: Managing Inconsistency in Data Exchange

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.Comment: 29 pages, 13 figures, submitted to the Journal on Data Semantic

Query Rewriting with Disjunctive Existential Rules and Mappings

We consider the issue of answering unions of conjunctive queries (UCQs) with disjunctive existential rules and mappings. While this issue has already been well studied from a chase perspective, query rewriting within UCQs has hardly been addressed yet. We first propose a sound and complete query rewriting operator, which has the advantage of establishing a tight relationship between a chase step and a rewriting step. The associated breadth-first query rewriting algorithm outputs a minimal UCQ-rewriting when one exists. Second, we show that for any ``truly disjunctive'' nonrecursive rule, there exists a conjunctive query that has no UCQ-rewriting. It follows that the notion of finite unification sets (fus), which denotes sets of existential rules such that any UCQ admits a UCQ-rewriting, seems to have little relevance in this setting. Finally, turning our attention to mappings, we show that the problem of determining whether a UCQ admits a UCQ-rewriting through a disjunctive mapping is undecidable. We conclude with a number of open problems.Comment: This report contains the paper accepted at KR 2023 and an appendix with full proofs. 24 page

Abstraction in ontology-based data management

In many aspects of our society there is growing awareness and consent on the need for data-driven approaches that are resilient, transparent, and fully accountable. But in order to fulfil the promises and benefits of a data-driven society, it is necessary that the data services exposed by the organisations' information systems are well-documented, and their semantics is clearly specified. Effectively documenting data services is indeed a crucial issue for organisations, not only for governing their own data, but also for interoperation purposes. In this thesis, we propose a new approach to automatically associate formal semantic descriptions to data services, thus bringing them into compliance with the FAIR guiding principles, i.e., make data services automatically Findable, Accessible, Interoperable, and Reusable (FAIR). We base our proposal on the Ontology-based Data Management (OBDM) paradigm, where a domain ontology is used to provide a semantic layer mapped to the data sources of an organisation, thus abstracting from the technical details of the data layer implementation. The basic idea is to characterise or explain the semantics of a given data service expressed as query over the source schema in terms of a query over the ontology. Thus, the query over the ontology represents an abstraction of the given data service in terms of the domain ontology through the mapping, and, together with the elements in the vocabulary of the ontology, such abstraction forms a basis for annotating the given data service with suitable metadata expressing its semantics. We illustrate a formal framework for the task of automatically produce a semantic characterisation of a given data service expressed as a query over the source schema. The framework is based on three semantically well-founded notions, namely perfect, sound, and complete source-to-ontology rewriting, and on two associated basic computational problems, namely verification and computation. The former verifies whether a given query over the ontology is a perfect (respectively, sound, complete) source-to-ontology rewriting of a given data service expressed as a query over the source schema, whereas the latter computes one such rewriting, provided it exists. We provide an in-depth complexity analysis of these two computational problems in a very general scenario which uses languages amongst the most popular considered in the literature of managing data through an ontology. Furthermore, since we study also cases where the target query language for expressing source-to-ontology rewritings allows inequality atoms, we also investigate the problem of answering queries with inequalities over lightweight ontologies, a problem that has been rarely addressed. In another direction, we study and advocate the use of a non-monotonic target query language for expressing source-to-ontology rewritings. Last but not least, we outline a detailed related work, which illustrates how the results achieved in this thesis notably contributes to new results in the Semantic Web context, in the relational database theory, and in view-based query processing

Integrating Ontologies and Relational Data

In recent years, an increasing number of scientific and other domains have attempted to standardize their terminology and provide reasoning capabilities through ontologies, in order to facilitate data exchange. This has spurred research into Web-based languages, formalisms, and especially query systems based on ontologies. Yet we argue that DBMS techniques can be extended to provide many of the same capabilities, with benefits in scalability and performance. We present OWLDB, a lightweight and extensible approach for the integration of relational databases and description logic based ontologies. One of the key differences between relational databases and ontologies is the high degree of implicit information contained in ontologies. OWLDB integrates the two schemes by codifying ontologies\u27 implicit information using a set of sound and complete inference rules for SHOIN (the description logic behind OWL ontologies. These inference rules can be translated into queries on a relational DBMS instance, and the query results (representing inferences) can be added back to this database. Subsequently, database applications can make direct use of this inferred, previously implicit knowledge, e.g., in the annotation of biomedical databases. As our experimental comparison to a native description logic reasoner and a triple store shows, OWLDB provides significantly greater scalability and query capabilities, without sacrifcing performance with respect to inference

Modified Query-Roles Based Access Control Model (Q-RBAC) for Interactive Access of Ontology Data

The data access model plays an important role during accessing and querying the stored data from the database. It provides an access right and authorization of accessing data into a database. It can distinguish the access boundaries between the administrators and the users where the database administrators can create certain policies either from the client application side or directly from the database side, depending upon the nature of running application. However, the emerging technology on the ontology repository has forced some database developers to adapt most of the access policies from the traditional database system and many of the policies were inherited from the relational database. This method of adopting or borrowing access policies from other storage system has created an unnecessary layer between the ontology repository and database. Most of the emerging ontology repositories lack an independent access model that provides or distinguishes access right between the administrators and users or between the ontology data. This paper proposed the improved access layer from the ontology repository with an additional users’ policy creation layer that will lead to increase data security and also increase the performance of querying data. Our effort relies on re-modifying the role based access control model from the traditional one to the new proposed model that organized by the rich users’ policies and perfect query rewriting layer. Although it is associated with query module, the proposed model has an additional security layer to restrict unauthorized users from accessing stored data in order to improve querying and data access performance Keywords: Access methods, Access control, Rule based access control model. Oracle NoSQL database, Virtual data layer, Ontology Query

Pseudo-contractions as Gentle Repairs

Updating a knowledge base to remove an unwanted consequence is a challenging task. Some of the original sentences must be either deleted or weakened in such a way that the sentence to be removed is no longer entailed by the resulting set. On the other hand, it is desirable that the existing knowledge be preserved as much as possible, minimising the loss of information. Several approaches to this problem can be found in the literature. In particular, when the knowledge is represented by an ontology, two different families of frameworks have been developed in the literature in the past decades with numerous ideas in common but with little interaction between the communities: applications of AGM-like Belief Change and justification-based Ontology Repair. In this paper, we investigate the relationship between pseudo-contraction operations and gentle repairs. Both aim to avoid the complete deletion of sentences when replacing them with weaker versions is enough to prevent the entailment of the unwanted formula. We show the correspondence between concepts on both sides and investigate under which conditions they are equivalent. Furthermore, we propose a unified notation for the two approaches, which might contribute to the integration of the two areas

State-of-the-art on evolution and reactivity

This report starts by, in Chapter 1, outlining aspects of querying and updating resources on the Web and on the Semantic Web, including the development of query and update languages to be carried out within the Rewerse project. From this outline, it becomes clear that several existing research areas and topics are of interest for this work in Rewerse. In the remainder of this report we further present state of the art surveys in a selection of such areas and topics. More precisely: in Chapter 2 we give an overview of logics for reasoning about state change and updates; Chapter 3 is devoted to briefly describing existing update languages for the Web, and also for updating logic programs; in Chapter 4 event-condition-action rules, both in the context of active database systems and in the context of semistructured data, are surveyed; in Chapter 5 we give an overview of some relevant rule-based agents frameworks