Rewriting-based optimization for XQuery transformational queries

Abstract

The modern XML query language called XQuery includes advanced facilities both to query and to transform XML data. An XQuery query optimizer should be able to optimize any query. For "querying" queries almost all techniques inherited from SQL-oriented DBMS may be applied. The XQuery transformation facilities are XML-specific and have no counterparts in other query languages. That is why XQuery transformational queries need to be optimized with novel techniques. In this paper two kinds of such techniques (namely push predicates down XML element constructors and projection of transformation) are considered. A subset of XQuery for which these techniques can be fully implemented is identified. This subset seems to be the most interesting from the practical viewpoint. Rewriting rules for this subset are proposed and the correctness of these rules is formally justified. For the rest of the language we propose solutions that work for the most of common cases or consider the problems we have encountered.