Flexible infrastructure for gathering XML statistics and estimating query cardinality

A key component of XML data management systems is the result size estimator, which estimates the cardinalities of user queries. Estimated cardinalities are needed in a variety of tasks, including query optimization and cost-based storage design; and they can also be used to give users early feedback about the expected outcome of their queries. In [2], we proposed StatiX. In contrast to previously proposed result estimators, which use specialized data structures and estimation algorithms, StatiX uses histograms to uniformly capture both the structural and value skew present in documents. It also leverages schema information to produce high-quality and concise statistical summaries. In particular, it exploits XML Schema transformations [1] to obtain statistics at different granularities.