Developing a full-fledged cost-based XQuery optimizer is a fairly complex task. Nowadays, there is little knowledge concerning suitable cost formulae and optimization strategies for exploring and constraining the tremendously large search space. To allow for a fair assessment of different optimization strategies, physical algebra operators, and indexing approaches, we developed an extensible optimization framework. The framework is accompanied by a supportive visual explain tool that enables user interactions to refine the inspection and the comprehension of the query plans proposed. Using this tool, the optimizer can be dynamically reconfigured and the impact of different optimization strategies on the final query execution plan can be immediately visualized.

Even though an effective cost-based query optimizer is of utmost importance for the efficient evaluation of XQuery expressions in native XML database systems, such a component is currently out of sight, because former approaches do not pay attention to the latest advances in the area of physical operators (e. g., Holistic Twig Joins and advanced indexes) or just focus only on some of them.

XTCcmp, the XQuery Compiler of a native XML database system, extends Starburst's well-known Query Graph Model to serve as an internal representation and basis for query restructuring of XQuery expressions. Furthermore, XTCcmp is able to generate execution plans supporting a wide range of both well-known and newly developed variants of core XML processing algorithms and indexes. Our demo visualizes all rule-based transformation stages, i. e., simplification, algebraic rewriting, and plan generation. Furthermore, via an interface to the extensible rule configuration, it allows interaction with the query compiler to vary configuration parameters and to control the compilation outcome.