Determinacy and Rewriting of Top-Down and MSO Tree Transformations

A query is determined by a view, if the result to the query can be reconstructed from the result of the view. We consider the problem of deciding for two given tree transformations, whether one is determined by the other. If the view transformation is induced by a tree transducer that may copy, then determinacy is undecidable, even for identity queries. For a large class of non-copying views, namely compositions of functional extended linear top-down tree transducers with regular look-ahead, we show that determinacy is decidable, where queries are given by deterministic top-down tree transducers with regular lookahead or by MSO tree transducers. We also show that if a query is determined, then it can be rewritten into a query that works directly over the view and is in the same class as the given query. The proof relies on the decidability of equivalence for the two considered classes of queries, and on their closure under composition.