A Semideterministic Approach to Object Creation and Nondeterminism in Database Queries

We introduce and study the concept of semi determinism A non deterministic generic query is called semi deterministic if any two possible results of the query to a database are isomorphic Semi determinism is a generalization of determinacy proposed by Abiteboul and Kanellakis in the context of object creating query languages The framework of semi deterministic queries is less restrictive than that of the determinate queries and avoids the problem of copy elimination connected with determinacy By o ering a less restrictive framework it avoids the problem of copy elimination connected with determinacy We argue that semi determinism is also interesting in its own right and show that it is natural and desirable though hard to achieve in general Nevertheless we exhibit two major applications where semi deterministic computations are possible First we show that there is a universal procedure to compute any semi deterministic query in a semi deterministic manner Second we show that the polynomial time counting queries can be e ciently expressed semi deterministically An extended abstract of a preliminary version of this paper was presented at the th ACM Symposium on Principles of Database Systems Research Assistant of the NFWO Address Dept Math Computer Sci Uni versity of Antwerp UIA Universiteitsplein B Antwerp Belgium E mail vdbuss uia ac be Address Computer Sci Dept Indiana University Bloomington IN USA E mail vgucht cs indiana edu