An index data structure (“MTree”) useful in creating indices for structured data is provided. The MTree index data structure is designed to meet the needs of the hierarchical XPath query language. The primary feature of MTree is the next subtree root node in document order for all axes are available to each context node in O(1). The MTree index data structure supports modification operations such as insert and delete. The MTree index structure is implemented in memory or in a digital storage medium. Improved performance in the MTree index structure utilizing a threading scheme is also provided.