Constraint Objects Databases

Abstract

In many applications, our information about the world is incomplete. For example, we may know about a group of persons some information about their ages, such as that one person’s age is greater or less than forty or one person is older than another, without knowing their precise ages. Section 9.1 describes how our incomplete information about the whole world or universe can be represented as a single universal constraint object relation, or UCOR.

The UCOR requires different assumptions about the database than the assumptions made in earlier chapters. Section 9.2 describes the closed, open, and possible world assumptions. The UCOR satisfies the possible world assumption.

Refinement queries add (conjoin) new constraints to the UCOR. Each constraint added eliminates some of the possibilities or ambiguities from the UCOR. For example, as we add more constraints about the ages of the group of persons, we may get more precise knowledge about their ages. In the limit, this will approach the state where there is only one solution of the UCOR, and then we obtain the exact age of each person. Sections 9.3 and 9.4 describe the syntax and the semantics of refinement queries. Section 9.5 describes projection queries from the UCOR. Section 9.6 describes the evaluation of refinement queries. Finally, Section 9.7 considers the related notion of constraint XML objects.