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Though inevitable for effective cost-based query rewriting, the derivation of meaningful cardinality estimates has remained a notoriously hard problem in the context of XQuery. By basing the estimation on a relational representation of the XQuery syntax, the authors' show how existing cardinality estimation techniques for XPath and proven relational estimation machinery can play together to yield dependable forecasts for arbitrary XQuery (sub)expressions. Their approach benefits from a light-weight form of data flow analysis. Abstract domain identifiers guide the query analyzer through the estimation process and allow for informed decisions even in case of deeply nested XQuery expressions.
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