This chapter describes the NEXT generalization of tree patterns that enables logical optimization of XQuery and demonstrates its value by developing an effective technique for minimization of nested XQueries, which removes redundancy across and within subqueries. A key ingredient of NEXT is the groupby operation that reduces mixed semantics to pure set semantics that provides the typical framework for logical optimization such as minimization. Furthermore, it enables consolidation of all navigation in the XTableaux. The rewriting rules reduce any query from the OptXQuery subset of XQuery into NEXT. The minimization algorithm also capitalizes on the groupby of NEXT that allows the navigation performed on a nesting level to reuse the navigation performed on higher levels. Additionally, the minimization algorithm go fundamentally beyond prior minimization algorithms for tree patterns and conjunctive queries by introducing a new type of minimization step called collapsing.
