Answering Non-Monotonic Queries in Relational Data Exchange

TL;DR

This paper introduces the GCWA*-semantics for non-monotonic queries in relational data exchange, aligning with existing semantics for monotonic queries and analyzing the complexity of query evaluation.

Contribution

It proposes a new semantics for non-monotonic queries in data exchange, inspired by deductive database semantics, and analyzes its computational complexity.

Findings

GCWA*-semantics coincides with open world semantics on monotonic queries

Polynomial-time evaluation of universal queries on certain schema mappings

Complexity analysis of non-monotonic query evaluation in data exchange

Abstract

Relational data exchange is the problem of translating relational data from a source schema into a target schema, according to a specification of the relationship between the source data and the target data. One of the basic issues is how to answer queries that are posed against target data. While consensus has been reached on the definitive semantics for monotonic queries, this issue turned out to be considerably more difficult for non-monotonic queries. Several semantics for non-monotonic queries have been proposed in the past few years. This article proposes a new semantics for non-monotonic queries, called the GCWA*-semantics. It is inspired by semantics from the area of deductive databases. We show that the GCWA*-semantics coincides with the standard open world semantics on monotonic queries, and we further explore the (data) complexity of evaluating non-monotonic queries under the GCWA*-semantics. In particular, we introduce a class of schema mappings for which universal queries can be evaluated under the GCWA*-semantics in polynomial time (data complexity) on the core of the universal solutions.