5 research outputs found
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
Consistent Query Answering for Expressive Constraints under Tuple-Deletion Semantics
We study consistent query answering in relational databases. We consider an
expressive class of schema constraints that generalizes both tuple-generating
dependencies and equality-generating dependencies. We establish the complexity
of consistent query answering and repair checking under tuple-deletion
semantics for different fragments of the above constraint language. In
particular, we identify new subclasses of constraints in which the above
problems are tractable or even first-order rewritable
Management of Inconsistencies in Data Integration
Data integration aims at providing a unified view over data coming from various sources. One of the most challenging tasks for data integration is handling the inconsistencies that appear in the integrated data in an efficient and effective manner. In this chapter, we provide a survey on techniques introduced for handling inconsistencies in data integration, focusing on two groups. The first group contains techniques for computing consistent query answers, and includes mechanisms for the compact representation of repairs, query rewriting, and logic programs. The second group contains techniques focusing on the resolution of inconsistencies. This includes methodologies for computing similarity between atomic values as well as similarity between groups of data, collective techniques, scaling to large datasets, and dealing with uncertainty that is related to inconsistencies
The chase procedure and its applications
The goal of this thesis is not only to introduce and present new chase-based algorithms, but also to investigate the differences between the main existing chase procedures. In order to achieve this, first we will investigate and do a clear delimitation between the existing chase algorithms based on their termination criteria. This will give a better picture of which chase algorithm can be used for different dependency classes. Next, we will investigate the data exchange, data repair and data correspondence problems and show how the chase algorithm can be used to characterize different types of solutions. For the later two problems, we will also investigate the data complexity of solution-existence and solution-check problems. Further, we will introduce a new chase based algorithm which computes representative solutions under constructible models, a new closed world semantics. This new semantics is, in our view, appropriate to be used as a closed world semantics in data exchange. We will also show that the conditional table computed by this chase algorithm can help to get both possible and certain answers for general queries. And finally, we will investigate strong representation systems and strong data exchange representation system. We will prove, by introducing a new chase based algorithm, that mappings specified by source-to-target second order dependencies and target richly acyclic TGD’s are strong data exchange representation systems for the class of first order queries