Why off-the-shelf RDBMSs are better at XPath than you might expect
Torsten Grust, Jan Rittinger, Jens Teubner · 2007
To compensate for the inherent impedance mismatch between the relational data model (tables of tuples) and XML (ordered, unranked trees), tree join algorithms have become the prevalent means to process XML data in relational databases, most notably the TwigStack[6], structural join[1], and staircase join[13] algorithms. However, the addition of these algorithms to existing systems depends on a significant invasion of the underlying database kernel, an option intolerable for most database vendors.