Thanks guys, using group by makes this query run beautifully. Just out of interest, if I was using a pre-XQuery 3.0 processor, would there have been any other approach than iterating over distinct-values?
Thanks also for the pointer about using 'null'. The Basex query editor seemed to approve it when I tried it once, and the habit stuck ;-) Cheers, Constantine -----Original Message----- From: Christian Grün [mailto:[email protected]] Sent: 09 April 2014 14:51 To: Dirk Kirsten Cc: Hondros, Constantine (ELS-AMS); [email protected] Subject: Re: [basex-talk] Efficient query for duplicates Thanks Dirk. A little addition: 'let' should probably be replaced with 'group': for $x in /dataset/item/pii group by $val := $x/string() where count($x) > 1 return <duplicate>{ $val }</duplicate> Christian On Wed, Apr 9, 2014 at 1:59 PM, Dirk Kirsten <[email protected]> wrote: > Hello Constantine, > > your query very much looks like you want to group by each <pii/> > element. Using XQuery 3.0 there even is a construct aimed at that and > I would guess it should also preserve memory. It is called group by > and you can find some more information at > https://docs.basex.org/wiki/XQuery_3.0#Group_By or in the spec. > It will look something like this: > > for $x in /dataset/item/pii > let $val := $x/string() > where count($x) > 1 > return <duplicate>{$val}</duplicate> > > By the way, it looks you are using null like a NULL statement in other > languages. Such a statement does not exist in XQuery. You might want > to return an empty set () instead. > > Cheers, > Dirk > > > On 09/04/14 13:37, Hondros, Constantine (ELS-AMS) wrote: >> I'm running out of memory (1.5 GB allocated) when querying for duplicate >> node values over a fairly flat XML database of approximately 450 MB. >> >> Can anyone suggest a more memory-efficient approach to framing this query >> than iterating over distinct-values as I do below? I'm hoping that there >> are some Basex tips and tricks to help out here. >> >> for $val in distinct-values(/dataset/item/pii) >> let $cnt := count(/dataset/item/pii[. = $val]) return >> if ($cnt > 1) then >> <duplicate>{$val}</duplicate> >> else >> null >> >> >> Thanks in advance, >> Constantine >> >> >> ________________________________ >> >> Elsevier B.V. Registered Office: Radarweg 29, 1043 NX Amsterdam, The >> Netherlands, Registration No. 33156677, Registered in The Netherlands. >> > > -- > Dirk Kirsten, BaseX GmbH, http://basex.org > |-- Firmensitz: Blarerstrasse 56, 78462 Konstanz > |-- Registergericht Freiburg, HRB: 708285, Geschäftsführer: > | Dr. Christian Grün, Dr. Alexander Holupirek, Michael Seiferle > `-- Phone: 0049 7531 28 28 676, Fax: 0049 7531 20 05 22 ________________________________ Elsevier B.V. Registered Office: Radarweg 29, 1043 NX Amsterdam, The Netherlands, Registration No. 33156677, Registered in The Netherlands.
