About certainty-based queries against possibilistic databases

This paper is situated in the area of possibilistic relational databases, i.e., where some attribute values are imprecise and represented as possibility distributions. Any such database has a canonical interpretation as a set of regular relational databases, called worlds. This view provides the basic semantics of any query addressed to a possibilistic database. However, a query cannot be run this way for tractability reasons. This situation has led us to consider specific families of queries that can be processed in a compact way, i.e., directly on possibilistic relations. The queries dealt with in this paper, called necessity-based queries, are of the form: "to what extent is it certain that tuple t belongs to the result of query Q", where Q denotes a regular relational query. The major contribution of this paper is to identify the constraints over Q (in terms of algebraic operations) which must be imposed so that these queries are tractable.