[Monetdb-developers] problems with XQuery aggregation

Keulen, M. van (Maurice) m.vankeulen at utwente.nl
Thu Oct 25 16:11:49 CEST 2007


I'm sure it did recognize the join in this query some time/versions ago ...
Peter, although we decided to not spend a lot of time into maintaining 
the mps-backend, it would be a pity if the reason for it is related to 
the value index recently introduced. Perhaps you may want to check if it 
is simply a matter of the join pattern not being recognized anymore due 
to the XQuery-Core rewritings for the value index.
Maurice.

Stefan Manegold wrote:
> On Thu, Oct 25, 2007 at 02:44:44PM +0200, Henning Rode wrote:
>   
>> oh, amazing...
>> with the algebra backend it works in seconds.
>>
>> thanks a lot. is MPS not able to create an efficient query plan here?
>>     
>
> apparently.
>
> Stefan
>
>   
>> -henning
>>
>> Stefan Manegold wrote:
>>     
>>> Henning,
>>>
>>> I suppose you use the default MPS ("milprint_summer") backend, right?
>>> (i.e., a simple `mclient -lxquery q.xq` or `pf [-M] q.xq | mclient -lmil`.)
>>>
>>> Could you please also try the algebra backend via
>>> 	`pf -A q.xq | mclient -lmil`
>>> and report the results/behaviour?
>>>
>>> Stefan
>>>
>>> On Thu, Oct 25, 2007 at 12:39:07PM +0200, Henning Rode wrote:
>>>       
>>>> hej monetdb developers,
>>>>
>>>> i tried to write a simple aggregation query:
>>>>
>>>> let $cands := doc("x.xml")//c
>>>> for $eid in distinct-values($cands/@id)
>>>> return <candidate id="{$eid}" count="{count($cands[@id eq $eid])}"/>
>>>>
>>>> however, executing the above query turns out to be rather difficult. i
>>>> always killed the process after waiting for more than half an hour
>>>> without getting any result.
>>>>
>>>> x.xml is 17MB large and contains 300000 candidates nodes c.
>>>> i run it on Monet Database Server V4.19.0 where the query crashed
>>>>
>>>> ERROR = !ERROR: GDKload: cannot mmap(): name=10/1030, ext=buns.priv
>>>>         !OS: Cannot allocate memory
>>>>         !ERROR: GDKload failed: name=10/1030, ext=buns.priv
>>>>         !ERROR: CMDleftjoin: operation failed.
>>>>
>>>> and on the newer stable release MonetDB Server v4.20.0 on a machine with
>>>> 16GB memory where the query just runs endless.
>>>>
>>>> apparently the query compiler does not recognise the necessary join and
>>>> aggregation here. is that a known problem?
>>>>
>>>> btw. sorting the candidates by @id value is done in seconds:
>>>>
>>>> let $cands := doc("x.xml")//c
>>>> for $c in $cands
>>>> order by $c/@id
>>>> return $c
>>>>
>>>> best -henning
>>>>
>>>>
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>
>   

-- 
----------------------------------------------------------------------
Dr.Ir. M. van Keulen - Assistant Professor, Data Management Technology
Univ. of Twente, Dept of EEMCS, POBox 217, 7500 AE Enschede, Netherlands
Email: m.vankeulen at utwente.nl, Phone: +31 534893688, Fax: +31 534892927
Room: ZI 3039, WWW: http://www.cs.utwente.nl/~keulen





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