11. A system for processing a query, the system comprising: a processor-based database management system executed on a computer system and configured to store a structured document in its native format and to provide an inverted multi-path index configured to store a plurality of path-value pairs and a selectivity factor for each of the path-value pairs based on a number of the structured documents that include one of the path-value pairs, wherein each path-value pair references at least one structured document stored in the database system and comprises an index path expression of an indexed element in an indexed structured document and an indexed value associated with the indexed element; and a processor-based indexing engine executed on the computer system and configured to receive a clause including a path expression-value pair comprising a path expression associated with an element of a structured document, wherein the clause is included in a query for at least one structured document satisfying the clause, to determine that the clause can be processed by the inverted multi-path index, to process the clause to identify a path-value pair in the inverted multi-path index matching the path expression-value pair of the clause, and to identify the at least one structured document referenced by the matching path-value pair.
11. A system for processing a query, the system comprising: a processor-based database management system executed on a computer system and configured to store a structured document in its native format and to provide an inverted multi-path index configured to store a plurality of path-value pairs and a selectivity factor for each of the path-value pairs based on a number of the structured documents that include one of the path-value pairs, wherein each path-value pair references at least one structured document stored in the database system and comprises an index path expression of an indexed element in an indexed structured document and an indexed value associated with the indexed element; and a processor-based indexing engine executed on the computer system and configured to receive a clause including a path expression-value pair comprising a path expression associated with an element of a structured document, wherein the clause is included in a query for at least one structured document satisfying the clause, to determine that the clause can be processed by the inverted multi-path index, to process the clause to identify a path-value pair in the inverted multi-path index matching the path expression-value pair of the clause, and to identify the at least one structured document referenced by the matching path-value pair. 12. The system of claim 11 wherein each path-value pair in the inverted multi-path index is generated when the indexed element is indexed according to a sub-path definition covering the indexed element, wherein the sub-path definition is included in a multi-path index definition associated with the inverted multi-path index, and covers a plurality of elements descending from a root element and includes at least one index property.