[xquery-talk] Count of Distinct elements performance problem

[Michael Kay]

A performance question like this can only be answered with respect to a
specific product.

There aren't many XQuery engines that will handle an 8Gb file, so you're
doing quite well.

You might find that a multi-pass approach works faster:

(a) extract the values

(b) sort them

(c) use a recursive scan to do positional grouping - depends on your product
supporting tail call optimization


That's likely to have O(n*log(n)) performance rather than O(n^2). 

Michael Kay
http://www.saxonica.com/



> -----Original Message-----
> From: talk-bounces at xquery.com 
> [mailto:talk-bounces at xquery.com] On Behalf Of Kusunam, Srinivas
> Sent: 22 August 2006 18:36
> To: talk at xquery.com
> Subject: [xquery-talk] Count of Distinct elements performance problem
> 
> 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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