As XQuery nears standardization, more sophisticated XQuery applications are emerging, which often exploit the entire language and are applied to non-trivial XML sources. We propose an algebra and optimization techniques that are suitable for building an XQuery compiler that is complete, correct, and efficient. We describe the compilation rules for the complete language into that algebra and present novel optimization techniques that address the needs of complex queries. These techniques include new query unnesting rewritings and specialized join algorithms that account for XQuery?s complex predicate semantics. The algebra and optimizations are implemented in the Galax XQuery engine, and yield execution plans that are up to three orders of magnitude faster than earlier versions of Galax.