Re: [xml] XPath / LibXML question

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

Hi,

I optimized the predicate evaluation for this case; but only
when using the short-hand form "[n]".
The following will now stop at the first "item" element which
satisfies [ attr1 = '1']:

"/base/group/descendant::instance[ att2='2' and
 parent::instances/parent::item[ att1='1']][1]"

Use xpath.c revision 1.136.

Regards,

Kasimier 

> -----Original Message-----
> From: xml-bounces gnome org [mailto:xml-bounces gnome org] On 
> Behalf Of Buchcik, Kasimier
> Sent: Thursday, June 01, 2006 7:57 PM
> To: Buchcik, Kasimier
> Cc: xml gnome org
> Subject: RE: [xml] XPath / LibXML question
>
> Hi, 
>
> > -----Original Message-----
> > From: xml-bounces gnome org [mailto:xml-bounces gnome org] On 
> > Behalf Of Buchcik, Kasimier
> > > -----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.
>
> [...] 
>
> Apparently I haven't thought the issue to the end. Of course there's
> also a way to restrict this to be an element inside /base/group:
>
> "/base/group/descendant::instance[ att2='2' and
>   parent::instances/parent::item[ att1='1']][1]"
>
> Regards,
>
> Kasimier
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