## The Closed World Assumption of Databases

“Deductive question-answering systems generally evaluate queries under one of two possible assumptions which we in this paper refer to as the open and closed world assumptions. The open world assumption corresponds to the usual first order approach to query evaluation: Given a data base DB and a query Q, the only answers to Q are those which obtain from proofs of Q given DB as hypotheses. Under the closed world assumption, certain answers are admitted as a result of failure to find a proof. More specifically, if no proof of a positive ground literal exists, then the negation of that literal is assumed true. This can be viewed as equivalent to implicitly augmenting the given data base with all such negated literals.

“For many domains of application, closed world query evaluation is appropriate since, in such domains, it is natural to explicitly represent only positive knowledge and to assume the truth of negative facts by default. For example, in an airline data base, all flights and the cities which they connect will be explicitly represented. Failure to find an entry indicating that Air Canada flight 103 connects Vancouver with Toulouse permits one to conclude that it does not…

“Unfortunately, the number of negative facts about a given domain will, in general, far exceed the number of positive ones so that the requirement that all facts, both positive and negative, be explicitly represented may well be unfeasible. In the case of purely extensional data bases there is a ready solution to this problem. Merely explicitly represent positive facts. A negative fact is implicitly present provided its positive counterpart is not explicitly present. Notice, however, that by adopting this convention, we are making an assumption about our knowledge about the domain, namely, that we know evertyhing about each predicate of the domain. There are no gaps in our knowledge…This is an important point. The implicit representation of negative facts presumes total knowledge about the domain being represented. Fortunately, in most applications, such an assumption is warranted. We shall refer to this as the closed world assumption (CWA). Its opposite, the open world assumption (OWA), assumes only the information given in the data base and hence requires all facts, both positive and negative, to be explicitly represented. Under the OWA, ‘gaps’ in one’s knowledge about the domain are permitted…

“We have introduced the notion of the closed world assumption for deductive question-answering. This says, in effect, ‘Every positive statement that you don’t know to be true may be assumed false.’ We have then shown how query evaluation under the closed world assumption reduces to the usual first order proof theoretic approach to query evaluation as applied to atomic queries. Finally, we have shown that consistent Horn data bases remain consistent under the closed world assumption and that definite data bases are consistent with the closed world assumption.”

Reference: Reiter, R (1978) On closed world data bases. In Logic and Data Bases, Gallaire H and Minker J. (ed) pp 55 – 76. Plenum Press: New York.