[bdbxml] Sort Query Performance

Hello friends ,
I am using dbxml 2.1.8 version .
I am executing a sort query on a container with more than 10000 
documents , but the execution
of the query is taking a lot of time(more than 15 mins).
The query is as follows --
 'for $i in collection("xyz.dbxml")/root/x order by $i/y[@a="abc"]/@b 
descending  return $i'

I have indexed the container as follows : -

Index: node-attribute-equality-string for node {}:id
Index: node-attribute-equality-string for node {}:a
Index: node-attribute-presence-none to node: {}:b

and the Xml Structure is as follows --


------XML structure ------

 <root id = "pqr" >
        <x>
            <y a="abc" b="300"> </y>
        <y a="def" b="123"> </y>
        <y a="pqr" b="432"> </y>
        ...
        ...
    </x>
    ...
    ...
    ...
    ...
 </root>


 Can you please suggest me what should I do make the query execution 
faster ?
 Thank You !
 Regards ,
 Ajinkya N


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