InterJoin: Exploiting Indexes and Materialized Views in XPath Evaluation

XML has become the standard for data exchange for a wide variety of applications, particularly in the scientific community. In order to efficiently process queries on XML representations of scientific data, we require specialized techniques for evaluating XPath expressions. Exploiting materialized views in query processing significantly enhances query processing performance. We propose a novel view definition that allows for intermediate (structural) join results to be stored and reused in XML query evaluation. Unlike current XML view proposals, our views do not require navigation in the original document or path-based pattern matching. Hence, they are evaluated significantly faster and are easily costed as part of a query plan. In general, current structural joins cannot exploit views efficiently when the view definition is not a prefix (or a suffix) of the XPath query. To increase the applicability of our proposed view definition, we propose a novel physical structural join operator called InterJoin. The InterJoin operator allows for joining interleaving XPath expressions, e.g., joining //A//C with //B to evaluate //A//B//C. InterJoin allows for more join alternatives in XML query plans. We propose several physical implementations for InterJoin, including a technique to exploit spatial indexes on the inputs. We give analytic cost models for the implementations so they can be costed in an existing XML query optimizer. Experiments on real and synthetic XML data show significant speed-ups of up to 200% using InterJoin, and speed-ups of up to 400% using our materialized views