16. The machine-readable storage medium of claim 13 , wherein the instructions, when executed by the one or more processors, further cause the one or more processors to perform the steps of: determining how XML data, that is targeted by the first query, can be accessed; generating, based on the second query and how the XML data can be accessed, a third query that indicates one or more data structures; and executing the third query, wherein the one or more data structures are accessed to execute the third query.
16. The machine-readable storage medium of claim 13 , wherein the instructions, when executed by the one or more processors, further cause the one or more processors to perform the steps of: determining how XML data, that is targeted by the first query, can be accessed; generating, based on the second query and how the XML data can be accessed, a third query that indicates one or more data structures; and executing the third query, wherein the one or more data structures are accessed to execute the third query. 17. The machine-readable storage medium of claim 16 , wherein: determining how the XML data can be accessed includes determining that an XML index exists on the XML data; and the one or more data structures are one or more data structures of the XML index.