Due to the complex nature of XML, finding classes of integrity constraints for XML data that are both expressive and practical is an important but challenging task. In this paper, we study a class of XML functional dependencies (called pXFDs) defined on the basis of tree homomorphism, We establish a semantic equivalence between the implications problems for pXFDs and for propositional Horn clauses, which guarantees linear time decidability of pXFD implication. Hence, pXFDs cannot only be used to capture relevant data semantics, but also be reasoned about efficiently.
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