Database Research Group

WSI – Database Systems Research Group

Let SQL Drive the XQuery Workhorse.

Torsten Grust, Manuel Mayr, Jan Rittinger

Proceedings of the 13th Int'l Conference on Extending Database Technology (EDBT 2010), Lausanne, Switzerland, March 2010.

Online version

A truly relational account of the true XQuery semantics can turn any relational database system into an XQuery processor. Compiling nested expressions of the fully compositional XQuery language, however, yields odd algebraic plan shapes featuring scattered distributions of join operators that currently overwhelm commercial SQL query optimizers.

This work rewrites such plans before submission to the relational database back-end. Once cast into the shape of join graphs, we have found off-the-shelf relational query optimizers - the B-tree indexing subsystem and join tree planner, in particular - to cope and even be autonomously capable of "reinventing" advanced processing strategies that have originally been devised specifically for the XQuery domain, e.g., XPath step reordering, axis reversal, and path stitching. Performance asssessments provide evidence that relational query engines are among the most versatile and efficient XQuery processors readily available today.