XML indexing and storage: fulfilling the wish list

XML Indexing and Storage (XMIS) techniques are crucial for the functionality and the overall performance of an XML database management system (XDBMS). Because of the complexity of XQuery and performance demands of XML query processing, efficient path processing operators—including those for tree-pattern queries (so-called twigs)—are urgently needed for which tailor-made indexes and their flexible use are indispensable. Although XML indexing and storage are standard problems and, of course, manifold approaches have been proposed in the last decade, adaptive and broad-enough solutions for satisfactory query evaluation support of all path processing operators are missing in the XDBMS context. Therefore, we think that it is worthwhile to take a step back and look at the complete picture to derive a salient and holistic solution. To do so, we first compile an XMIS wish list containing what—in our opinion—are essential functional storage and indexing requirements in a modern XDBMS. With these desiderata in mind, we then develop a new XMIS scheme, which—by reconsidering previous work—can be seen as a practical and general approach to XML storage and indexing. Interestingly, by working on both problems at the same time, we can make the storage and index managers live in a kind of symbiotic partnership, because the document store re-uses ideas originally proposed by the indexing community and vice versa. The XMIS scheme is implemented in XTC, an XDBMS used for empirical tests.