[MarkLogic Dev General] sorting efficiency question

[Charles Blair]

hello all. i need to do a 3-level sort, somewhat as follows:

let $docs := for $i in fn:distinct-values($results//titleName),
                  $j in fn:distinct-values($results//titleTypeName),
                   $k in fn:distinct-values($results//titlePhotoSequenceNumber)
             let $p := $results[//titleName = $i and
                                //titleTypeName = $j and
                                //titlePhotoSequenceNumber = $k]
             order by $i, $j, $k
             return $p

the above works, and is easy enough to write, but it's not terribly
speedy at returning results. does someone have, or can someone
suggest, a better approach? 

thanks.