Data Integration under Integrity Constraints

Data integration systems provide access to a set of heteroge ne us, autonomous data sources through a so-called global schema. Th ere are basically two approaches for designing a data integration system. In t he global-centric approach, one defines the elements of the global schema as views ov r the sources, whereas in the local-centric approach, one characterizes t he ources as views over the global schema. It is well known that processing queries i n the latter approach is similar to query answering with incomplete information, and, therefore, is a complex task. On the other hand, it is a common opinion that qu ery processing is much easier in the former approach. In this paper we sho w t e surprising result that, when the global schema is expressed in the relat ional model with integrity constraints, even of simple types, the problem of in complete information implicitly arises, making query processing difficult in the global-centric approach as well. We then focus on global schemas with key and foreign k ey constraints, which represents a situation which is very common in practic e, and we illustrate techniques for effectively answering queries posed to the d ata integration system