The last few years have seen the rapid emerging of the eXtensible Markup Language (XML). XML was designed as a simplification of SGML, and it has become the most widely used universal data representation format. In particular, the ability of XML to easily represent data with irregular structure has imposed XML as the standard incarnation for semistrucured data, i.e., data with irregular, unstable, or even unknown structure. In the context of XML data management systems, the estimation of query cardinality is becoming more and more important: the information provided by a query result estimator can be used as input to the query optimizer, as an early feedback to user queries, as well as input for determining an optimal storage schema. This Thesis describes the result size estimation model of Xtasy, a prototype XML database management system. Unlike other existing models, which focus on very limited subsets of XQuery, the proposed model covers the FLWR core of XQuery, and estimate not only the raw cardinality of query results, but also their distribution
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