[Monetdb-developers] problems with XQuery aggregation

Stefan Manegold Stefan.Manegold at cwi.nl
Thu Oct 25 14:53:45 CEST 2007


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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> 
> 
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| Dr. Stefan Manegold | mailto:Stefan.Manegold at cwi.nl |
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