[Monetdb-developers] problems with XQuery aggregation

Henning Rode h.rode at cs.utwente.nl
Thu Oct 25 12:39:07 CEST 2007

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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