 In the area of artificial intelligence, he is best known for his influential early work on the complexity of nonmonotonic logics and on (generalised) hypertree decompositions, a framework for obtaining tractable structural classes of constraint satisfaction problems, and a generalisation of the notion of tree decomposition from graph theory.This work has also had substantial impact in database theory, since it is known that the problem of evaluating conjunctive queries on relational databases is equivalent to the constraint satisfaction problem.His recent work on XML query languages (notably XPath) has helped create the complexity-theoretical foundations of this area.