Andrei Lopatenko

A Robust Logical and Computational Characterisation of Peer-to-Peer Database Systems

In this paper we give a robust logical and computational characterisation of peer-to-peer (p2p) database systems. We first define a precise model-theoretic semantics of a p2p system, which allows for local inconsistency handling. We then characterise the general computational properties for the problem of answering queries to such a p2p system. Finally, we devise tight complexity bounds and distributed procedures for the problem of answering queries in few relevant special cases.

Complexity of consistent query answering in databases under cardinality-based and incremental repair semantics

A database D may be inconsistent wrt a given set IC of integrity constraints. Consistent Query Answering (CQA) is the problem of computing from D the answers to a query that are consistent wrt IC. Consistent answers are invariant under all the repairs of D, i.e. the consistent instances that minimally depart from D. Three classes of repair have been considered in the literature: those that minimize set-theoretically the set of tuples in the symmetric difference; those that minimize the changes of attribute values, and those that minimize the cardinality of the set of tuples in the symmetric difference. The latter class has not been systematically investigated. In this paper we obtain algorithmic and complexity theoretic results for CQA under this cardinality-based repair semantics. We do this in the usual, static setting, but also in a dynamic framework where a consistent database is affected by a sequence of updates, which may make it inconsistent. We also establish comparative results with the other two kinds of repairs in the dynamic case.

Queries and updates in the coDB peer to peer database system

In this short paper we present the coDB P2P DB system. A network of databases, possibly with different schemas, are interconnected by means of GLAV coordination rules, which are inclusions of conjunctive queries, with possibly existential variables in the head; coordination rules may be cyclic. Each node can be queried in its schema for data, which the node can fetch from its neighbours, if a coordination rule is involved.

Efficient Approximation Algorithms for Repairing Inconsistent Databases

We consider the problem of repairing a database that is inconsistent wrt a set of integrity constraints by updating numerical values. In particular, we concentrate on denial integrity constraints with numerical built-in predicates. So far research in this context has concentrated in computational complexity analysis. In this paper we focus on efficient approximation algorithms to obtain a database repair and we present an algorithm that runs in O(n log n) wrt the size of the database. Our experimental evaluations show that even for large databases an approximate repair of the database can be computed efficiently despite the fact that the exact problem is computationally intractable. Finally, we show that our results can also be applied to database repairs obtained by a minimal number of tuple deletions.

Complexity and approximation of fixing numerical attributes in databases under integrity constraints

Consistent query answering is the problem of computing the answers from a database that are consistent with respect to certain integrity constraints that the database as a whole may fail to satisfy. Those answers are characterized as those that are invariant under minimal forms of restoring the consistency of the database. In this context, we study the problem of repairing databases by fixing integer numerical values at the attribute level with respect to denial and aggregation constraints. We introduce a quantitative definition of database fix, and investigate the complexity of several decision and optimization problems, including DFP, i.e. the existence of fixes within a given distance from the original instance, and CQA, i.e. deciding consistency of answers to aggregate conjunctive queries under different semantics. We provide sharp complexity bounds, identify relevant tractable cases; and introduce approximation algorithms for some of those that are intractable. More specifically, we obtain results like undecidability of existence of fixes for aggregation constraints; MAXSNP-hardness of DFP, but a good approximation algorithm for a relevant special case; and intractability but good approximation for CQA for aggregate queries for one database atom denials (plus built-ins).

A distributed algorithm for robust data sharing and updates in p2p database networks

In this paper we thoroughly analyze a distributed procedure for the problem of local database update in a network of database peers, useful for data exchange scenarios The algorithm supports dynamic networks: even if nodes and coordination rules appear or disappear during the computation, the proposed algorithm will eventually terminate with a sound and complete result.

Consistent Query Answering By Minimal-Size Repairs

This paper briefly reviews the recent literature on consistent query answering, an approach to handle database inconsistency in a systematic and logical manner based on the notion of repair. It discusses some computational and semantic limitations of consistent query answering, and summarizes selected research directions in this area

Fixing inconsistent databases by updating numerical attributes

For several reasons a database may not satisfy certain integrity constraints (ICs). However most likely most of the information in it is still consistent with the ICs; and could be retrieved when queries are answered. Consistent answers with respect to a set of ICs have been characterized as answers that can be obtained from every possible minimal repair of the database, where the repairs are obtained by deletions/insertion of whole tuples. In this paper we study the problem of fixing (repairing) databases by updating numerical attributes of tuples with respect to denial constraints. We introduce a quantitative definition of database fix, identify relevant tractable cases and introduce approximation algorithms for some of those that are intractable.