ON THE EXPRESSIVE POWER OF DATALOG - TOOLS AND A CASE-STUDY

We study here the language Datalog(not equal), which is the query language obtained from Datalog by allowing equalities and inequalities in the bodies of the rules. We view Datalog(not equal) as a fragment of an infinitary logic L(omega) and show that L(omega) can be characterized in terms of certain two-person pebble games. This characterization provides us with tools for investigating the expressive power of Datalog(not equal). As a case study, we classify the expressibility of fixed subgraph homeomorphism queries on directed graphs. S. Fortune, J. Hopcroft, and J. Wyllie (Theoret. Comput. Sci. 10 (1980), 111-121) classified the computational complexity of these queries by establishing two dichotomies, which are proper only if P not equal NP. Without using any complexity-theoretic assumptions, we show here that the two dichotomies are indeed proper in terms of expressibility in Datalog(not equal). (C) 1995 Academic Press, Inc.