An effective approach to support XML updates is to use XQuery extended with update operations. This approach results in very expressive languages which are convenient for users but are difficult to optimize or reason about. A crucial question underlying many static analysis problems for such languages, from optimization to view maintenance, is whether two expressions commute. Unfortunately, commutativity is undecidable for most existing XML update languages. In this paper, we propose a conservative analysis for an expressive XML update language that can be used to determine commutativity. The approach relies on a form of path analysis that computes upper bounds for the nodes that are accessed or modified in a given expression. Our main result is a commutativity theorem that can be used to identify commuting expressions. We illustrate how the technique applies to concrete examples of query optimization in the presence of updates
Abstract. A common approach to XML updates is to extend XQuery with update operations. This approach results in very expressive languages which are convenient for users but are difficult to reason about. Deciding whether two expressions can commute has numerous applications from view maintenance to rewriting-based optimizations. Unfortunately, commutativity is undecidable in most recent XML update languages. In this paper, we propose a conservative analysis for an expressive XML update language that can be used to determine whether two expressions commute. The approach relies on a form of path analysis that computes upper bounds for the nodes that are accessed or modified in a given update expression. Our main result is a commutativity theorem that can be used to identify commuting expressions.
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