1. A computer-implemented method comprising: storing an XML document in a persistent database of a database system; storing persistently in said database an XML tree index on said XML document, wherein the XML tree index comprises a set of fixed-sized index entries, each index entry of the set of fixed-sized index entries having a one-to-one correspondence with a node of said XML document; wherein each index entry of at least a portion of said set of fixed-sized index entries contains a link referring to another respective index entry that corresponds to either a parent node, child node, or sibling node of the respective corresponding node of said each index entry; evaluating a path expression, wherein evaluating a path expression includes traversing one or more of the links contained in the index entries of said portion of said set of fixed-sized index entries; wherein the method is performed by one or more computing devices.