Data log and constraint satisfaction with infinite templates

On finite structures, there is a well-known connection between the expressive power of Data log, finite variable logics, the existential pebble game, and bounded hypertree duality. we study this connection for infinite structures. This has applications for constraint satisfaction with infinite templates. If the template Gamma is omega-categorical, we present various equivalent characterizations of those Gamma such that the constraint satisfaction problem (CSP) for Gamma can be solved by a Data log program. We also show that CSP(Gamma) can be solved in polynomial time for arbitrary omega-categorical structures Gamma if the input is restricted to instances of bounded treewidth. Finally, we characterize those omega-categorical templates whose CSP has Datalog width 1, and those whose CSP has strict Datalog width k. (C) 2012 Elsevier Inc. All rights reserved.