Count of Distinct elements performance problem

[Kusunam, Srinivas]

I am trying to find count of distinct elements (Model Year). Here is my
XQuery. It takes 4 hrs to get the count from 8GB file. There are around
3000 distinct Model years in this file. 

let $mdoc := doc('input.xml')/Body
let $sourModelYEAR := $mdoc/Title/ModelYear
return
<Elements>
    <Element name="ModelYear"> 
        <Distribution>
        {
          for $dvalue in fn:distinct-values($sourModelYEAR)
          let $eachcount := count($mdoc/Title[ModelYear=$dvalue])
          return  
              <distribution>
                <value>{ $dvalue }</value>
                <count>{ $eachcount }</count>
              </distribution>
        }
        </Distribution>
    </Element>      
 </Elements>

This Query seems to loop through the document for each value i.e.
overall 3000 times. I know this should be easily achievable if we have
Group-by in XQuery. Do any XQuery engine supports (custom) Group-By now?
Or is there any other way to make this query efficient?

Where as if I add one more element to find the pattern of the data it
finishes the job within 40 minutes? Why is this odd behavior?

let $mdoc := doc('input.xml')/Body
let $sourModelYEAR := $mdoc/Title/ModelYear
return
<Elements>
    <Element name="ModelYear"> 
        <Distribution>
        {
          for $dvalue in fn:distinct-values($sourModelYEAR)
          let $eachcount := count($mdoc/Title[ModelYear=$dvalue])
          return  
              <distribution>
                <value>{ $dvalue }</value>
                <count>{ $eachcount }</count>
              </distribution>
        }
        </Distribution>
        <PatternDistribution>
        {
            for $phonenum in distinct-values($sourModelYEAR/translate(.,
'0123456789','9999999999'))
            return
                <pattern>
                    <type>{ $phonenum }</type>
                    <count>{count($sourModelYEAR[translate(.,
'0123456789', '9999999999') eq $phonenum])}</count>
                </pattern>
        }
        </PatternDistribution>
    </Element>      
</Elements>


Thanks,
Srini
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