Projecting XML Documents

This chapter presents projection techniques that can be used to support main-memory XQuery evaluation over large XML documents. The chapter introduces a path analysis technique that infers the set of paths used for an arbitrary XQuery expression. The experiments show that this technique can be used to evaluate queries on files up to two Gigabyte even on machines with limited memory. The chapter reveals that XQuery is not only useful to query XML in databases, but also to applications that must process XML documents as files or streams. These applications suffer from the limitations of current main-memory XQuery processors, which break for rather small documents. This chapter proposes techniques, based on a notion of projection for XML, which can be used to drastically reduce memory requirements in XQuery processors. The chapter presents a loading algorithm that takes the resulting information to build a projected document, which is smaller than the original document, and on which the query yields the same result. This chapter implements projection in the Galax XQuery processor.