Hi Fabrice,
In a /a/b/c example schema, Is there any index that could speed up //c request ?
if there are no other descendant paths to <c>...</c> elements in your database, your query will automaticlly be rewritten to /a/b/c. The InfoView in the GUI or the QUERYINFO option [1] will give you more information on how the query is rewritten and optimized.
Hope this helps, Christian