RE: [xml] XPath / LibXML question

["Buchcik, Kasimier" <k buchcik 4commerce de>]

Hi, 

> -----Original Message-----
> From: Alan_McCarthy scee net [mailto:Alan_McCarthy scee net] 
> Sent: Thursday, June 01, 2006 5:18 PM
> To: Buchcik, Kasimier
> Cc: Aron Stansvik; Buchcik, Kasimier; xml gnome org
> Subject: RE: [xml] XPath / LibXML question
>
> Well, that certainly returns just the one result correctly...
>
> I take it from your comments about it not being optimized that it will
> still find all results before returning the first, rather than just
> terminating at the first match?

Although it will return just one node, it will still try to apply the
predicate "[ att2='2' and parent::instances/parent::item[ att1='1']]"
on all "instance" elements it finds in the document. So actually it will
run even slower than your original expression. I'll try to optimize
this.

But the expression I proposed (even if optimized) has the drawback that
it won't be as efficient as a plain chain of node-tests in the child
axis,
if you *don't* know if the document will be of the expected structure.
Hmm, don't know if I expressed this understandable enough; I think this
will become clearer if we assume that the given document has a document
element with the name "foo"; here my expression will still search the
whole tree for "instance" elements, while your expression would stop
already at the document element. So it all depends on how much you can
assume about the queried tree.

Another possibility would be to use a specialized Libxml2 function,
which
internally will try to return only the first match. Maybe someone else
knows if we have such a function already. But such a function won't be
much efficient, since the XPath mechanism of Libxml2 will still gather
and test all nodes up to "/base/group/item[ att1=1]/instances", and then
it will have the chance to stop at the first match of
"instance[ att2=2]".

Regards,

Kasimier

> Thanks
>
> Alan
>
>
> "Buchcik, Kasimier" <k buchcik 4commerce de> wrote on 
> 01/06/2006 15:45:36:
>
> > Hi,
> >
> > > -----Original Message-----
> > > From: xml-bounces gnome org [mailto:xml-bounces gnome org] On
> > > Behalf Of Buchcik, Kasimier
> > > Hi,
> > >
> > > As Aron already pointed out, the position() should be of
> > > help. You need to select the descendant-or-self or descendant
> > > axis.
> > > Your expression could look like this:
> > > "//instance[ att2='2' and
> > parent::instances/parent::item[ att1='1']][1]"
> >
> > Whoops, sorry this should read:
> > "/descendant::instance[ att2='2' and
> > parent::instances/parent::item[ att1='1']][1]"
> >
> > Regards,
> >
> > Kasimier
>
>
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