33,522 research outputs found
Consistent Query Answers in the Presence of Universal Constraints
The framework of consistent query answers and repairs has been introduced to
alleviate the impact of inconsistent data on the answers to a query. A repair
is a minimally different consistent instance and an answer is consistent if it
is present in every repair. In this article we study the complexity of
consistent query answers and repair checking in the presence of universal
constraints.
We propose an extended version of the conflict hypergraph which allows to
capture all repairs w.r.t. a set of universal constraints. We show that repair
checking is in PTIME for the class of full tuple-generating dependencies and
denial constraints, and we present a polynomial repair algorithm. This
algorithm is sound, i.e. always produces a repair, but also complete, i.e.
every repair can be constructed. Next, we present a polynomial-time algorithm
computing consistent answers to ground quantifier-free queries in the presence
of denial constraints, join dependencies, and acyclic full-tuple generating
dependencies. Finally, we show that extending the class of constraints leads to
intractability. For arbitrary full tuple-generating dependencies consistent
query answering becomes coNP-complete. For arbitrary universal constraints
consistent query answering is \Pi_2^p-complete and repair checking
coNP-complete.Comment: Submitted to Information System
Prioritized Repairing and Consistent Query Answering in Relational Databases
A consistent query answer in an inconsistent database is an answer obtained
in every (minimal) repair. The repairs are obtained by resolving all conflicts
in all possible ways. Often, however, the user is able to provide a preference
on how conflicts should be resolved. We investigate here the framework of
preferred consistent query answers, in which user preferences are used to
narrow down the set of repairs to a set of preferred repairs. We axiomatize
desirable properties of preferred repairs. We present three different families
of preferred repairs and study their mutual relationships. Finally, we
investigate the complexity of preferred repairing and computing preferred
consistent query answers.Comment: Accepted to the special SUM'08 issue of AMA
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
From Causes for Database Queries to Repairs and Model-Based Diagnosis and Back
In this work we establish and investigate connections between causes for
query answers in databases, database repairs wrt. denial constraints, and
consistency-based diagnosis. The first two are relatively new research areas in
databases, and the third one is an established subject in knowledge
representation. We show how to obtain database repairs from causes, and the
other way around. Causality problems are formulated as diagnosis problems, and
the diagnoses provide causes and their responsibilities. The vast body of
research on database repairs can be applied to the newer problems of computing
actual causes for query answers and their responsibilities. These connections,
which are interesting per se, allow us, after a transition -inspired by
consistency-based diagnosis- to computational problems on hitting sets and
vertex covers in hypergraphs, to obtain several new algorithmic and complexity
results for database causality.Comment: To appear in Theory of Computing Systems. By invitation to special
issue with extended papers from ICDT 2015 (paper arXiv:1412.4311
Priority-Based Conflict Resolution in Inconsistent Relational Databases
We study here the impact of priorities on conflict resolution in inconsistent
relational databases. We extend the framework of repairs and consistent query
answers. We propose a set of postulates that an extended framework should
satisfy and consider two instantiations of the framework: (locally preferred)
l-repairs and (globally preferred) g-repairs. We study the relationships
between them and the impact each notion of repair has on the computational
complexity of repair checking and consistent query answers
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