This paper presents an efficient evaluation of XQuery in a heterogeneous distributed system. XQuery(W3C, 2005) is a rich and so a complex language. Its syntax allows us to express a large scale of queries over XML documents. We have extended (Chen et al., 2003) proposal to rewrite XQuery expressions in "canonical XQuery" in order to support the full XQuery specification. The XQuery expressiveness makes difficulties to obtain an exclusive internal representation within a system. Models based on Tree Patterns have been proposed, and we have extended the tree pattern model to a model called TGV that (a) integrates the whole functionalities of XQuery (b) uses an intuitive representation that provides a global visualization of the request in a mediation context and (c) provides a support for optimization and for cost information. Our paper is based on the XLive mediation system. XLive integrates sources in a uniform view. It is a running research vehicle designed at PRiSM Laboratory for assessing the integration system at every stage of the process starting from sources extraction to the user interface and is already used in several projects.