Re: [xsl] Sibling axis: All of kind A up to first of kind B

[Michael Ludwig <mlu@xxxxxxxxxxxxx>]

Michael Kay schrieb:
> > > >        <xsl:apply-templates mode="toc"
> > > >        select="(following-sibling::h1 | following-sibling::h2)
> > > >        [ 1 ]"/>
> > > Might be safer to do following-sibling::*[self::h1 or self::h2][1] -
> > > but depends on the processor.
> > Why might this be safer? Aren't the resulting node-sets the
> > same and the expressions therefore equivalent? Doesn't the
> > union operator impose document order?
>
> Well, I've no way of knowing, but it occurred to me that there might
> be processors that fail to recognize that the two operands of the
> union operator are both sorted node-sets and that the operation can
> therefore be done by a fully-streamed merge, stopping as soon as
> either node-set delivers a node. Streamed execution of the second
> expression requires much less analysis.

A fully-streamed merge, does that mean the processor starts scanning for
nodes and stops on encountering the first one satisfying the predicate
[self::h1 or self::h2] instead of *first* assembling node-sets for both
h1 and h2, *then* merging them into a union and *then* applying the
predicate [1]?

> Saxon copes well with it, however: [...]
> The expression tree generated by the Saxon optimizer is in effect
>
> first(following-sibling::(h1|h2))
>
> which is roughly what I suggested you should write. This will do a
> single scan of the following sibling axis looking for h1 and h2
> elements, and return the first one it finds: no merging needed.

Okay, I think this is the (affirmative) answer to my above question.

Thanks a lot,

Michael Ludwig