Abstract: As the popularity of XML (eXtensible Markup Language) keeps growingrapidly, the management of XML compliant structured-document databaseshas become a very interesting and compelling research area. Queryoptimization for XML structured-documents stands out as one of the mostchallenging research issues in this area because of the much enlargedoptimization (search) space, which is a consequence of the intrinsiccomplexity of the underlying data model of XML data. We thereforepropose to apply deterministic transformations on query expressions tomost aggressively prune the search space and fast achieve a sufficientlyimproved alternative (if not the optimal) for each incoming queryexpression. This idea is not just exciting but practically attainable.This paper first provides an overview of our optimization strategy, andthen focuses on the key implementation issues of our rule-basedtransformation system for XML query optimization in a databaseenvironment. The performance results we obtained from experimentationshow that our approach is a valid and effective one.