XJoin index: indexing XML data for efficient handling of branching path expressions

We consider the problem of indexing XML data for solving branching path expressions with the aim of reducing the number of joins to be executed and we propose a simple yet efficient join indexing approach to shrink the twig before applying any structural join algorithm. The indexing technique we propose, that we call XJoin Index, precomputes some structural (semi-)join results thus reducing the number of joins to be computed. Precomputed (semi-)joins support the following operations: (i) attribute selections, possibly involving several attributes; (ii) detection of parent-child relationships; (ii) counting selections, like Find all books with at least 3 authors. Unlike other approaches, based on specialized data structures XJoin Index is entirely based on B⁺-trees and can be coupled with any structural join algorithm proposed so far.