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Re: [xsl] General rule for designing XPath expressions to return items in document order?
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Subject: Re: [xsl] General rule for designing XPath expressions to return items in document order? From: Michael Kay <mike@xxxxxxxxxxxx> Date: Wed, 8 Jan 2014 12:53:42 +0000 |
The general rule is: (a) if you're using a smart XPath processor, don't worry about it, the processor will do the work for you (b) if you're using a stupid XPath processor, then find out something about the nature of its stupidity, because its stupidities are probably not the same as those of other processors. Michael Kay Saxonica On 8 Jan 2014, at 12:40, Costello, Roger L. <costello@xxxxxxxxx> wrote: > This is a fascinating and enlightening discussion. I am getting a glimpse of the difference between what is returned from an XPath query versus what is happening under the hood by the XPath-processor. > > Let me recap: > > 1. This XPath expression //section/head always returns items in document order. > > 2. Under the hood, a XPath-processor might proceed like so: > > a. Fetch all the <head> elements > b. Sort them into document order > c. Return the sorted <head> elements > > Is that accurate? > > If so, please allow me to rephrase my initial question: > > I am seeking a general rule for designing XPath > expressions so that under the hood the XPath- > processor never needs to sort the fetched items into > document order because the XPath expression > is designed to always select items in document > order. For example, instead of the above XPath > expression, use this one /descendant::head[parent::section] > Can you provide a general rule? > > /Roger > > -----Original Message----- > From: Michael Kay [mailto:mike@xxxxxxxxxxxx] > Sent: Wednesday, January 08, 2014 6:54 AM > To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx > Subject: Re: [xsl] General rule for designing XPath expressions to return items in document order? > > > 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
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