Bitmap indexes for relational XML twig query processing

Due to an increasing volume of XML data, it is considered prudent to store XML data on an industry-strength database system instead of relying on a domain specific application or a file system. For shredded XML data stored in the relational tables, however, it may not be straightforward to apply existing algorithms for twig query processing, because most of the algorithms require XML data to be accessed in a form of streams of elements grouped by their tags and sorted in a particular order. In order to support XML query processing within the common framework of relational database systems, we first propose several bitmap indexes for supporting holistic twig joins on XML data stored in the relational tables. Since bitmap indexes are well supported in most of the commercial and open-source database systems, the proposed bitmap indexes and twig query processing algorithms can be incorporated into the relational query processing framework with more ease. The proposed query processing algorithms are efficient in terms of both time and space, since the compressed bitmap indexes stay compressed during query processing. In addition, we propose a hybrid index which computes twig query solutions with only bit-vectors, without accessing labeled XML elements stored in the relational tables.