Re: [xsl] Efficiency: predicate vs function

["Andrew Welch" <andrew.j.welch@xxxxxxxxx>]

On 7/22/07, Michael Kay <mike@xxxxxxxxxxxx> wrote:
> > For example, to return a sequence of all items but the last [sic: first],
> > which of the following is better?
> >
> > $x[ position() > 1 ]
> >
> > or
> >
> > remove( $x , 1 )
> >
>
> It's impossible to say in general, it's processor-dependent. Saxon compiles
> both of these into exactly the same code, effectively an expression
> tail($x). This construct is specially recognized at run-time to avoid
> copying the sequence unnecessarily.
>
> Use saxon:explain="yes" if you're interested in details of how Saxon has
> compiled the code.

For some anecdotal evidence, when I had this exact requirement for the
Sudoku Solver [1] and switched from $seq[position() > 1] to
subsequence($seq, 2) the performance increase was significant.

The change was made in some heavily used parts of the stylesheet and
although I never benchmarked the difference it was significant enough
to stick in my mind - around say 10 seconds on a transform that was
taking ~50 seconds at the time.  (which now takes around ~1.5secs in
the latest version :-)

[1] http://andrewjwelch.com/code/xslt/sudoku/sudoku-solver.html

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