I have a general question about predicting performance in the general
case. I know that the best answer is "try it and see" but I'm wondering
if there's a general principle that can guide design in this particular
case.
In most of the work we do, which is processing technical documents to
generate various outputs, we have to do applicability checks on pretty
much every element to see if a particular element is applicable to the
current processing conditions (target output, national language,
computer platform, etc.). These applicability checks are fairly
expensive computationally because they may need to investigate any
number of properties of an element or its ancestry, neighbors, etc. It
may also require the use of external extension functions and so on.
My question is where, in general, is the best place to use these functions:
- In apply-templates specifications?
- In match specifications?
- As IF blocks within templates?
For example, I could do this:
<xsl:apply-templates select="*[util:is_applicable()]"/>
Or
<xsl:template match="foo[util:is_applicable()]">
or
<xsl:template match="foo">
<xsl:if test="util:is_applicable()">
</xsl:if>
</xsl:template>
I think that the IF approach ensures the fewest calls but also makes the
code more cluttered.
So I guess my question is: if not using the IF approach, would it be
better to put the check in the apply-templates select or the match or
does it matter or is it entirely a function of how a given XSLT
implementation does its optimization?
Another option of course is to do the applicability processing as a
separate step so that the base processing templates don't have to care
about applicability. That would ensure that each element is only
processed once for applicability but might introduce other performance
or scalability issues since one would have to generate either a new
serialized instance or a new result tree reflecting the input
document(s). It would be a cleaner engineering solution as it would mean
base template writers wouldn't have to know about the need to do
applicability checks.
Thanks,
Eliot
--
W. Eliot Kimber
Professional Services
Innodata Isogen
9030 Research Blvd, #410
Austin, TX 78758
(512) 372-8122
eliot@xxxxxxxxxxxxxxxxxxx
www.innodata-isogen.com