Re: General rule for designing XPath expressions to r

On 8 Jan 2014, at 11:41, Costello, Roger L. <costello@xxxxxxxxx> wrote:

> David Carlisle wrote:
>
> 	Path expressions constructed with  / always
> 	returns nodes in document order.
>
> Section 19.1 of the XSLT 3.0 specification says:
>
> ... evaluation of the path expression //section/head will not always deliver
nodes in document order; this can be seen by examining the source document
below, where a child of the first section appears in document order after a
child of the second section:
>

You're quoting selectively. It actually says:

"The reason for this is that a naive nested loop evaluation of the path
expression //section/head will not always deliver nodes in document order"

and then:

"An implementation that literally followed the semantics of path expressions
as defined in [XPath 3.0] would therefore require to sort the nodes into
document order"

In other words, a nested loop evaluation will not deliver the correct results
(i.e. the sequence nodes in document order) unless it is followed by a sort,
and the sort would prevent streaming.

Michael Kay
Saxonica