Data Exchange: Algorithm for Computing Maybe Answers for Relational Algebra Queries

The concept of certain answers is usually used in the definition of the semantics of answering queries in data exchange. But as certain answers are defined as a relation without null values, the approaches for answering queries over databases do not always lead to semantically correct answer. In order to obtain all possible semantically correct answers, maybe answers have been considered along with certain answers. In this paper, an algorithm is presented that produces semantically correct maybe answers to simple relational algebra queries over target schemas of data exchange under the closed world assumption. As there may be infinitely many maybe answers, sophisticated and restricted representation techniques are used to represent them compactly. Then using the compact representation, an algorithm to compute possible (i.e., maybe) answers incrementally is designed. Finally the algorithm is implemented for a fragment of relational algebra.