Algorithms for Core Computation in Data Exchange

We describe the state of the art in the area of core computation for data exchange. Two main approaches are considered: post-processing core computation, applied to a canonical universal solution constructed by chasing a given schema mapping, and direct core computation, where the mapping is first rewritten in order to create core universal solutions by chasing it

Ontology for Representing Human Needs

Need satisfaction plays a fundamental role in human well-being. Hence understanding citizens' needs is crucial for developing a successful social and economic policy. This notwithstanding, the concept of need has not yet found its place in information systems and online tools. Furthermore, assessing needs itself remains a labor-intensive, mostly offline activity, where only a limited support by computational tools is available. In this paper, we make the first step towards employing need management in the design of information systems supporting participation and participatory innovation by proposing OpeNeeD, a family of ontologies for representing human needs data. As a proof of concept, OpeNeeD has been used to represent, enrich and query the results of a needs assessment study in a local citizen community in one of the Vienna districts. The proposed ontology will facilitate such studies and enable the representation of citizens' needs as Linked Data, fostering its co-creation and incentivizing the use of Open Data and services based on it

Geo-Semantic Labelling of Open Data

In the past years Open Data has become a trend among governments to increase transparency and public engagement by opening up national, regional, and local datasets. However, while many of these datasets come in semi-structured file formats, they use different schemata and lack geo-references or semantically meaningful links and descriptions of the corresponding geo-entities. We aim to address this by detecting and establishing links to geo-entities in the datasets found in Open Data catalogs and their respective metadata descriptions and link them to a knowledge graph of geo-entities. This knowledge graph does not yet readily exist, though, or at least, not a single one: so, we integrate and interlink several datasets to construct our (extensible) base geo-entities knowledge graph: (i) the openly available geospatial data repository GeoNames, (ii) the map service OpenStreetMap, (iii) country-specific sets of postal codes, and (iv) the European Union¿s classification system NUTS. As a second step, this base knowledge graph is used to add semantic labels to the open datasets, i.e., we heuristically disambiguate the geo-entities in CSV columns using the context of the labels and the hierarchical graph structure of our base knowledge graph. Finally, in order to interact with and retrieve the content, we index the datasets and provide a demo user interface. Currently we indexed resources from four Open Data portals, and allow search queries for geo-entities as well as full-text matches at http://data.wu.ac.at/odgraph/

Conversational Browsing

How can we better understand the mechanisms behind multi-turn information seeking dialogues? How can we use these insights to design a dialogue system that does not require explicit query formulation upfront as in question answering? To answer these questions, we collected observations of human participants performing a similar task to obtain inspiration for the system design. Then, we studied the structure of conversations that occurred in these settings and used the resulting insights to develop a grounded theory, design and evaluate a first system prototype. Evaluation results show that our approach is effective and can complement query-based information retrieval approaches. We contribute new insights about information-seeking behavior by analyzing and providing automated support for a type of information-seeking strategy that is effective when the clarity of the information need and familiarity with the collection content are low

Cendrarsiana

Schema mappings have been extensively studied in the context of data exchange and data integration, where they have turned out to be the right level of abstraction for formalizing data interoperability tasks. Up to now and for the most part, schema mappings have been studied as static objects, in the sense that each time the focus has been on a single schema mapping of interest or, in the case of composition, on a pair of schema mappings of interest. In this paper, we adopt a dynamic viewpoint and embark on a study of sequences of schema mappings and of the limiting behavior of such sequences. To this effect, we first introduce a natural notion of distance on sets of finite target instances that expresses how "Close" two sets of target instances are as regards the certain answers of conjunctive queries on these sets. Using this notion of distance, we investigate pointwise limits and uniform limits of sequences of schema mappings, as well as the companion notions of pointwise Cauchy and uniformly Cauchy sequences of schema mappings. We obtain a number of results about the limits of sequences of GAV schema mappings and the limits of sequences of LAV schema mappings that reveal striking differences between these two classes of schema mappings. We also consider the completion of the metric space of sets of target instances and obtain concrete representations of limits of sequences of schema mappings in terms of generalized schema mappings, that is, schema mappings with infinite target instances as solutions to (finite) source instances

Foundational aspects of schema mapping optimization and normalization

Zsfassung in dt. SpracheSchemaabbildungen sind ein grundlegendes Konzept in Datenintegration und Datenaustausch, welches die Beziehung zwischen zwei Datenbankschemas beschreibt. Schemaabbildungen können als deklarative Programme verstanden werden, die entweder Daten von einem Schema in ein anderes transferieren, oder eine Abfrage über einem Schema in eine Abfrage über einem anderem Schema übersetzen. Der erste Teil dieser Arbeit beschäftigt sich mit der Frage der Optimierung und Normalisierung von Schemaabbildungen bezüglich logischer Äquivalenz. Für Schemaabbildungen, definiert durch sogenannte "embedded dependencies", formulieren wir konkrete Optimalitätskriterien und definieren ein System von Transformationsregeln, das eine als sogenannte Quelle-zu-Ziel Beziehungen gegebene Abbildung in eine optimale Darstellung überführt. Wir beweisen, dass das Ergebnis der Anwendung dieser Transformationsregeln eindeutig ist. Dieses Resultat ist dann für Schemaabbildungen erweitert, welche zusätzlich Gleichheit erzeugende Abhängigkeiten auf dem Ziel-Schema erlauben. Der zweite Teil der Arbeit betrachtet weniger strenge Äquivalenzbegriffe, wie sie von Fagin et al. definiert wurden, nämlich Datenaustauschäquivalenz (DE-Äquivalenz) und Äquivalenz bezüglich konjunktiver Abfragen (CQ-Äquivalenz). Es ist bekannt, dass die gelockerten Äquivalenzbegriffe im Allgemeinen unentscheidbar sind, wenn diese Ziel-Bedingungen nicht fixiert sind. Für CQ-Äquivalenz gilt dies sogar für den Fall dass diese Bedingungen nur Schlüssel sind. Wir identifizieren eine praktisch relevante Klasse von Ziel-Bedingungen, die sowohl die Funktionalen- als auch Inklusionsabhängigkeiten beinhaltet.Für diese ist DE-Äquivalenz entscheidbar, und darüber hinaus bietet sie zusätzliches Optimierungspotential gegenüber logischer Äquivalenz.Abschließend betrachten wir das Problem SO-Abbildungen ("SO-tgds") auf CQ-Äquivalenz zu überprüfen. Wir zeigen, dass dieses Problem unter der realistischen Annahme, dass das Quell-Schema Schlüssel besitzt, unentscheidbar ist.Schema mappings represent a basic concept of data integration and data exchange, expressing the relationship between database schemas.They can be seen as declarative programs, transforming the data from one schema to another, or rewriting the query against one schema as a query against another schema. The first part of this dissertation deals with the question of optimization and normalization of schema mappings with respect to logical equivalence. We prove that a unique normal form exists for the set of source-to-target tuple generating dependencies and that this form is unique up to isomorphism. Moreover, we extend this result by defining a normal form in presence of target equality generating dependencies and show the trade-off between uniqueness and optimality in this setting.In the second part of the dissertation, we move on to relaxed notions of equivalence, proposed by Fagin et al. in 2008: namely, data exchange equivalence and conjunctive query equivalence. If no integrity constraints are defined over the target schema, these notions are known to coincide with logical equivalence. In contrast, conjunctive query equivalence becomes undecidable even in presence of target key dependencies. We identify a practically relevant class of target dependencies, including functional and inclusion dependencies, for which data exchange equivalence is decidable, and thus offers more optimization potential than logical equivalence.Finally, we consider testing conjunctive query equivalence for mappings based on Second Order tuple generating dependencies and show undecidability of this task, under a realistic assumption that key dependencies are defined over the source schema.11