Characterizing schema mappings via data examples

Abstract

Schema mappings are high-level specifications that describe the relationship between two database schemas; they are considered to be the essential building blocks in data exchange and data integration, and have been the object of extensive research investigations. Since in real-life applications schema mappings can be quite complex, it is important to develop methods and tools for understanding, explaining, and refining schema mappings. A promising approach to this effect is to use "good" data examples that illustrate the schema mapping at hand. We develop a foundation for the systematic investigation of data examples and obtain a number of results on both the capabilities and the limitations of data examples in explaining and understanding schema mappings. We focus on schema mappings specified by source-to-target tuple generating dependencies (s-t tgds) and investigate the following problem: which classes of s-t tgds can be "uniquely characterized" by a finite set of data examples? Our investigation begins by considering finite sets of positive and negative examples, which are arguably the most natural choice of data examples. However, we show that they are not powerful enough to yield interesting unique characterizations. We then consider finite sets of universal examples, where a universal example is a pair consisting of a source instance and a universal solution for that source instance. We unveil a tight connection between unique characterizations via universal examples and the existence of Armstrong bases (a relaxation of the classical notion of Armstrong databases). On the positive side, we show that every schema mapping specified by LAV s-t tgds is uniquely characterized by a finite set of universal examples with respect to the class of LAV s-t tgds. Moreover, this positive result extends to the much broader classes of n-modular schema mappings, n a positive integer. Finally, we show that, on the negative side, there are schema mappings specified by GAV s-t tgds that are not uniquely characterized by any finite set of universal examples and negative examples with respect to the class of GAV s-t tgds (hence also with respect to the class of all s-t tgds).