[bdbxml] Indexing attributes

I have a simple database and most of the queries have the following 
structure, locate some element by it's id attribute and sort the child 
elements by the their number attribute.

query '
<div class="paragraphlist">
        {
                for $paragraph in 
collection("books.dbxml")/collection/book/chapter[@id = 
"b4.2.4"]/paragraph order by $paragraph/@number cast as xs:decimal
                return $paragraph
        }
</div>
'

I have tried adding indexes to speed things up. An unindexed query took 
around 5.4 seconds, whatever indexes I add only make things worse, 
adding on average another second to the query.

In this situation is it possible to speed things up with an index?



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