RE: [xsl] XSL: For-Each Efficient or Not?

[Wendell Piez <wapiez at mulberrytech dot com>]

Bryan,

At 12:06 PM 7/1/2002, you wrote:
> Wendell wrote:
> >m:apply[factorof[not(preceding-sibling::*)]]
>
> >will be better than
>
> >m:apply[child::*[position()=1 and name()='factorof']]
>
> at first I didn't get your point on this one. I supposed on further
> consideration that it was right, although it seems like it would be
> mainly dependent on the order in which xpath is evaluated by the
> processor, I mean that a reasonably clever processor would evaluate
> [position()=1 and name()='factorof'] first and then from there look for
> any child::* which matched this, with the result that it would only
> check the first child to see if it had a name of factorof?
> Am I very wrong in this supposition?
Not very, in that it can be difficult to fathom the mysteries of optimization.

In this case, however, the processor does have to look at every node since 
each node has to provide the context for the evaluation of the 
expression(s) in the predicate. I.e. how does it evaluate name()='factorof' 
without picking up the node whose name it's testing? It would be a pretty 
smart processor that had already thrown away all the nodes in the wrong 
position before it did that.

I also confess that my preference for simple clean XPath is as much 
aesthetic as anything. Yet XSL in general is rife with examples where the 
elegant and expressive also turns out to be efficient.

Cheers,
Wendell


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Wendell Piez                            mailto:wapiez@mulberrytech.com
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