[Monetdb-developers] problems with XQuery aggregation

Henning Rode h.rode at cs.utwente.nl
Thu Oct 25 14:44:44 CEST 2007

oh, amazing...
with the algebra backend it works in seconds.

thanks a lot. is MPS not able to create an efficient query plan here?

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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