Re: [xsl] XPath equivalence question

[Michael Kay <mike@xxxxxxxxxxxx>]

On 30/11/2011 23:48, Graydon wrote:
> Does
>
> <xsl:template match="/A//B">
>
> match exactly the same thing as
>
> <xsl:template match="B[/A]">?
Yes, I think so.
> My presumption is that the second form is preferable on efficiency
> grounds -- on simple table lookup versus whatever tree-structure the
> descendant axis search produces -- but I don't actually know.
Saxon actually has different implementations for the two cases (I've 
often thought of normalizing patterns so they would end up the same 
internally, but have never got around to it). But the logic of the two 
implementations is pretty well identical, and I would be very suprised 
if there is any measurable performance difference.

You seem to be basing you reasoning on some kind of hypothetical 
implementation model which has no resemblance to the way it's 
implemented in Saxon and may have little resemblance to any other 
product. In both cases, in Saxon, the test whether a node matches the 
pattern will start by asking whether it is an element named B, and then 
proceed by asking whether it has an ancestor named A that is a child of 
the root document node. In both cases, if you do an apply-templates that 
selects a node named C, this rule won't even be considered and will have 
no impact on the performance.

Michael Kay
Saxonica