Observational Distinguishability of Databases with Object Identity

We will examine the problem of distinguishing between database instances and values in models which incorporate object-identities and recursive data-structures. We will show that the notion of observational distinguishability is intricately linked to the languages available for querying a database. In particular we will show that, given a simple query language incorporating a test for equality of object-identities, database instances are indistinguishable if they are isomorphic, and that, in a language without any operators on objectidentities, database instances are indistinguishable if a bisimilarity relation holds between them. Further, such a bisimulation relation may be computed on values, but doing so requires the ability to recurse over all the object-identities in an instance. We will then show that systems of keys give rise to observational distinguishability relations which lie between these two extremes. We show that a system of keys satisfying certain restrictions provides us with an efficient means of comparing values, while avoiding the need to compare object identities directly. Comments University of Pennsylvania Department of Computer and Information Science Technical Report No. MSCIS-95-20. This technical report is available at ScholarlyCommons: http://repository.upenn.edu/cis_reports/227 Observational Distinguishability of Databases with Object Identity ' MS-CIS-95-20 Logic and Computation 91