Andrea Calì

On the decidability and complexity of query answering over inconsistent and incomplete databases

In databases with integrity constraints, data may not satisfy the constraints. In this paper, we address the problem of obtaining consistent answers in such a setting, when key and inclusion dependencies are expressed on the database schema. We establish decidability and complexity results for query answering under different assumptions on data (soundness and/or completeness). In particular, after showing that the problem is in general undecidable, we identify the maximal class of inclusion dependencies under which query answering is decidable in the presence of key dependencies. Although obtained in a single database context, such results are directly applicable to data integration, where multiple information sources may provide data that are inconsistent with respect to the global view of the sources.

Data integration under integrity constraints

Data integration systems provide access to a set of heterogeneous, autonomous data sources through a so-called global schema. There are basically two approaches for designing a data integration system. In the global-as-view approach, one defines the elements of the global schema as views over the sources, whereas in the local-as-view approach, one characterizes the sources as views over the global schema. It is well known that processing queries in the latter approach is similar to query answering with incomplete information, and, therefore, is a complex task. On the other hand, it is a common opinion that query processing is much easier in the former approach. In this paper we show the surprising result that, when the global schema is expressed in the relational model with integrity constraints, even of simple types, the problem of incomplete information implicitly arises, making query processing difficult in the global-as-view approach as well. We then focus on global schemas with key and foreign key constraints, which represents a situation which is very common in practice, and we illustrate techniques for effectively answering queries posed to the data integration system in this case.

Accessing Data Integration Systems through Conceptual Schemas

Data integration systems provide access to a set of heterogeneous, autonomous data sources through a so-called global, or mediated view. There is a general consensus that the best way to describe the global view is through a conceptual data model, and that there are basically two approaches for designing a data integration system. In the global-as-view approach, one defines the concepts in the global schema as views over the sources, whereas in the local-as-view approach, one characterizes the sources as views over the global schema. It is well known that processing queries in the latter approach is similar to query answering with incomplete information, and, therefore, is a complex task. On the other hand, it is a common opinion that query processing is much easier in the former approach. In this paper we show the surprising result that, when the global schema is expressed in terms of a conceptual data model, even a very simple one, query processing becomes difficult in the global-as-view approach also. We demonstrate that the problem of incomplete information arises in this case too, and we illustrate some basic techniques for effectively answering queries posed to the global schema of the data integration system.

A Formal Framework for Reasoning on UML Class Diagrams

In this paper we formalize UML class diagrams in terms of a logic belonging to Description Logics, which are subsets of First-Order Logic that have been thoroughly investigated in Knowledge Representation. The logic we have devised is specifically tailored towards the high expressiveness of UML information structuring mechanisms, and allows one to formally model important properties which typically can only be specified by means of qualifiers. The logic is equipped with decidable reasoning procedures which can be profitably exploited in reasoning on UML class diagrams. This makes it possible to provide computer aided support during the application design phase in order to automatically detect relevant properties, such as inconsistencies and redundancies.

A Logic-Based Approach for Matching User Profiles

Several applications require the matching of user profiles, e.g., job recruitment or dating systems. In this paper we present a logical framework for specifying user profiles that allows profile description to be incomplete in the parts that are unavailable or are considered irrelevant by the user. We present an algorithm for matching demands and supplies of profiles, taking into account incompleteness of profiles and incompatibility between demand and supply. We specialize our framework to dating services; however, the same techniques can be directly applied to several other contexts.

Reasoning in Data Integration Systems: Why LAV and GAV Are Siblings

Data integration consists in providing a uniform access to a set of data sources, through a unified representation of the data called global schema; a mapping specifies the relationship between the global schema and the sources. Integrity constraints (ICs) are expressed on the global schema to better represent the domain of interest; in general, ICs are not satisfied by the data at the sources. In this paper we address the problem of query answering in GLAV data integration systems, where tuple-generating dependencies are expressed on the global schema. We solve the problem in an intensional fashion, by presenting a rewriting technique that, taking into account both the ICs and the mapping, allows us to compute the answers to a query, expressed over the global schema, by evaluating the rewritten query directly over the sources. Since the GLAV approach is a generalisation of the basic approaches LAV and GAV, we show that query answering under ICs can be done in the same way in LAV and GAV systems, thus proving that LAV and GAV are siblings, and not opposites.

IBIS: semantic data integration at work

In this paper we present IBIS (Internet-Based Information System), a system for the semantic integration of heterogeneous data sources, which adopts innovative and state-of-the-art solutions to deal with all aspects of a complex data-integration environment, including query answering under integrity constraints and limitations on source access. IBIS is based on the global-as-view approach, using a relational mediated schema to query the data at the sources. Sources are wrapped so as to provide a relational view on them. A key issue is that the system allows the specification of integrity constraints (modeling constraints in the domain of interest) in the global schema. Since sources are autonomous, the extracted data in general do not satisfy the constraints. IBIS adapts and integrates the data extracted from the sources making use of the constraints in the global schema, so as to answer queries at best with the information available. IBIS deals with limitations in accessing data sources, and exploits techniques developed for querying sources with access limitations in order to retrieve the maximum set of answers. In particular, it may use integrity constraints available on the sources to improve the efficiency of the extraction process.

Optimized Querying of Integrated Data over the Web

Information Integration is the problem of providing a uniform access to multiple and heterogeneous data sources. The most common approach to this task, called global-as-view, consists in providing a global schema of the data, in which each relation is defined as a view over a set of data sources. Recent works deal with this problem in the case of limited source capabilities, where, in general, sources can only be accessed respecting certain binding patterns for their attributes. In this case, computing the answer to a user query over the global schema cannot be done by simply substituting the concepts appearing in the query with their definitions. Instead, it may require the evaluation of a suitable recursive Datalog program.

Interaction-Based adaptation for small screen devices

This paper explores an original approach to overcome current issues in the use of mobile devices, such as limited screen space and interaction modalities, based on exploiting interface adaptation and adaptive techniques. Specifically, the paper describes the application of this approach to a web searching prototype, which collects usage data to model interaction and provide a personalized version of the web facility visited by the user.

Experimenting data integration with DIS@DIS

Data integration consists in providing a uniform access to a set of heterogeneous sources through a common representation called global schema. In this paper we present DIS@DIS, a data integration system that adopts innovative techniques for query answering in a complex integration environment. In particular, DIS@DIS is able to deal with integrity constraints, which are used to enhance the expressiveness of the global schema. Since data at the sources may not satisfy the constraints, DIS@DIS is capable of reasoning in the presence of incomplete and inconsistent information, so as to provide consistent answers to user queries. Moreover, DIS@DIS is able to deal with both local-as-view and global-as-view approaches for the specification of the mapping between the global schema and the sources. DIS@DIS incorporates novel optimization techniques for query processing, which speed up query answering time even in the presence of complex global schemata and large amounts of data. Indeed, we show experimental results that prove the feasibility of our approach.