27,359 research outputs found
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
Answer Sets for Consistent Query Answering in Inconsistent Databases
A relational database is inconsistent if it does not satisfy a given set of
integrity constraints. Nevertheless, it is likely that most of the data in it
is consistent with the constraints. In this paper we apply logic programming
based on answer sets to the problem of retrieving consistent information from a
possibly inconsistent database. Since consistent information persists from the
original database to every of its minimal repairs, the approach is based on a
specification of database repairs using disjunctive logic programs with
exceptions, whose answer set semantics can be represented and computed by
systems that implement stable model semantics. These programs allow us to
declare persistence by defaults and repairing changes by exceptions. We
concentrate mainly on logic programs for binary integrity constraints, among
which we find most of the integrity constraints found in practice.Comment: 34 page
A SAT-based System for Consistent Query Answering
An inconsistent database is a database that violates one or more integrity
constraints, such as functional dependencies. Consistent Query Answering is a
rigorous and principled approach to the semantics of queries posed against
inconsistent databases. The consistent answers to a query on an inconsistent
database is the intersection of the answers to the query on every repair, i.e.,
on every consistent database that differs from the given inconsistent one in a
minimal way. Computing the consistent answers of a fixed conjunctive query on a
given inconsistent database can be a coNP-hard problem, even though every fixed
conjunctive query is efficiently computable on a given consistent database.
We designed, implemented, and evaluated CAvSAT, a SAT-based system for
consistent query answering. CAvSAT leverages a set of natural reductions from
the complement of consistent query answering to SAT and to Weighted MaxSAT. The
system is capable of handling unions of conjunctive queries and arbitrary
denial constraints, which include functional dependencies as a special case. We
report results from experiments evaluating CAvSAT on both synthetic and
real-world databases. These results provide evidence that a SAT-based approach
can give rise to a comprehensive and scalable system for consistent query
answering.Comment: 25 pages including appendix, to appear in the 22nd International
Conference on Theory and Applications of Satisfiability Testin
Inconsistency-tolerant Query Answering in Ontology-based Data Access
Ontology-based data access (OBDA) is receiving great attention as a new paradigm for managing information systems through semantic technologies. According to this paradigm, a Description Logic ontology provides an abstract and formal representation of the domain of interest to the information system, and is used as a sophisticated schema for accessing the data and formulating queries over them. In this paper, we address the problem of dealing with inconsistencies in OBDA. Our general goal is both to study DL semantical frameworks that are inconsistency-tolerant, and to devise techniques for answering unions of conjunctive queries under such inconsistency-tolerant semantics. Our work is inspired by the approaches to consistent query answering in databases, which are based on the idea of living with inconsistencies in the database, but trying to obtain only consistent information during query answering, by relying on the notion of database repair. We first adapt the notion of database repair to our context, and show that, according to such a notion, inconsistency-tolerant query answering is intractable, even for very simple DLs. Therefore, we propose a different repair-based semantics, with the goal of reaching a good compromise between the expressive power of the semantics and the computational complexity of inconsistency-tolerant query answering. Indeed, we show that query answering under the new semantics is first-order rewritable in OBDA, even if the ontology is expressed in one of the most expressive members of the DL-Lite family
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
On the Data Complexity of Consistent Query Answering over Graph Databases
Areas in which graph databases are applied - such as the semantic web, social networks and scientific databases - are prone to inconsistency, mainly due to interoperability issues. This raises the need for understanding query answering over inconsistent graph databases in a framework that is simple yet general enough to accommodate many of its applications. We follow the well-known approach of consistent query answering (CQA), and study the data complexity of CQA over graph databases for regular path queries (RPQs) and regular path constraints (RPCs), which are frequently used. We concentrate on subset, superset and symmetric difference repairs. Without further restrictions, CQA is undecidable for the semantics based on superset and symmetric difference repairs, and Pi_2^P-complete for subset repairs. However, we provide several tractable restrictions on both RPCs and the structure of graph databases that lead to decidability, and even tractability of CQA. We also compare our results with those obtained for CQA in the context of relational databases
- …