A method and apparatus for resolving conjunctive queries, comprising relational predicates and external predicates, such as XML predicates. The method and apparatus obtain results from a relational engine or from an external engine, and then use the obtained results as input for the other engine, so as to optimize the activity of the other engine. The process may repeat further times, with the results obtained by the other engine being input to the first engine. The method and apparatus further disclose handling value, being isomorphic hierarchical subtrees.