RE: [xsl] Improving efficiency of an XPath expression?

["Michael Kay" <michael dot h dot kay at ntlworld dot com>]

Here's an XPath 2.0 solution (you didn't say it had to be XPath 1.0):

//l[@id=$targetid]/../(.|preceding-sibling::lg[1]|following-sibling::lg[
1])

I don't see any prizes for doing it in one XPath expression so with XSLT
1.0 I would do much the same:

<xsl:variable name="x" select="//l[@id=$targetid]/.."/>
<xsl...
select="$x|$x/preceding-sibling::lg[1]|$x/following-sibling::lg[1]">

And even that's doing it with one hand tied behind your back - it would
run much faster using keys.

Michael Kay
Software AG
home: Michael.H.Kay@ntlworld.com
work: Michael.Kay@softwareag.com 

> -----Original Message-----
> From: owner-xsl-list@lists.mulberrytech.com 
> [mailto:owner-xsl-list@lists.mulberrytech.com] On Behalf Of 
> Michael Beddow
> Sent: 07 June 2002 13:13
> To: XSL-list@lists.mulberrytech.com
> Subject: [xsl] Improving efficiency of an XPath expression?
> 
> 
> I have a document that consists of several thousand <lg> 
> elements each with varying numbers of <l> children. A 
> simplified extract looks like this:
> 
> <example>
> 
> <lg id="G0065">
> <l id="L0258">Putois li aynel afraie</l>
> <l id="L0259">Gopil cleye, thesson traie</l>
> <l id="L0260">Quant li venour li quer praie.</l>
> </lg>
> 
> <lg id="G0066">
> <l id="L0261">Ouwe jaungle, jars agroile</l>
> <l id="L0262" >Ane en mareis jaroile</l>
> <l id="L0263" >Mes il i ad jaroil e garoile</l>
> <l id="L0264">La difference dire vous voile:</l>
> </lg>
> 
> <lg id="G0067">
> <l id="L0265">Li ane jaroile en rivere</l>
> <l id="L0266">Si hom de falcoun la quere,</l>
> <l id="L0267">Mes devant un vile en guere</l>
> <l id="L0268">Afichom le garoil en tere</l>
> </lg>
> 
> </example>
> 
> Now the challenge: given the id attribute value of any one 
> <l>, find an XPath expression that will efficiently select a 
> node-set containing only: the <lg> that immediately precedes 
> the one where the <l> concerned is found, plus the <lg> that 
> is the parent of that <l> and the <lg> that immediately 
> follows the one that is the parent of the <l> specified. 
> (i.e. "L2063" as input id would return precisely the three 
> <lg> elements in the example.)
> 
> Its the "efficiently" bit that's got me stuck. (NB because 
> the number of <l>s per <lg> is variable, there's no algorithm 
> to derive the id's of the parent, preceding and following 
> <lg>s from a given <l>'s id)
> 
> I can do it OK with
> 
> select = '(//lg[following-sibling::lg/l/@id="$targetid"][last()] |
> //lg[l/@id="$targetid"]) | 
> //lg[preceding-sibling::lg/l/@id="$targetid"][1])'
> 
> (hope I pasted that all right) but understandably enough even 
> the zippy libsxlt/libxml-2 takes its time over that one.
> 
> In practice (since I'm doing this in a sequence of 
> server-side pipes) it much faster to select simply on 
> '//lg[l/@id="$targetid"]' to get the parent node, then parse 
> out the id of that node, derive the ids of its preceding and  
> following siblings, go back in again to pull them out 
> separately, then concatenate the results for further processing.
> 
> But can any resident wizard give me a better XPath expression 
> that will still grab my target set in one go, only faster 
> than my offering above?
> 
> Michael
> ---------------------------------------------------------
> Michael Beddow   http://www.mbeddow.net/
> XML and the Humanities page:  http://xml.lexilog.org.uk/
> The Anglo-Norman Dictionary http://anglo-norman.net/
> ---------------------------------------------------------
> 
> 
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> 
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