Optimizing Tree Pattern Queries over Secure XML Databases

The rapid emergence of XML as a standard for data representation and exchange over the Web has sparked considerable interest in models and efficient mechanisms for controlled access, especially using queries, to information represented in XML (see, e.g., [3,5-7,ll-13,311). A naive approach to secure XML query evaluation is: (i) compute the query result without consideration of any security policy, and (ii) filter the query result using the access control policies in a post-processing step. While this approach may appear attractive, it's not secure. For example, consider the XML database with its DTD illustrated in Figure 1, which has information about items and customer accounts. Assume that a specific user is allowed access to the item (book and s o f t w a r e ) information but not to any account information. If only query results are filtered for accessibility, then the following XQuery path expression: