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Re: [xsl] Which is faster: count(/Document/A/B) eq 1 ... or ... /Document/A/(count(B) eq 1) ?

Subject: Re: [xsl] Which is faster: count(/Document/A/B) eq 1 ... or ... /Document/A/(count(B) eq 1) ?
From: Michael Kay <mike@xxxxxxxxxxxx>
Date: Thu, 20 Sep 2012 00:11:17 +0100

Execution speed is a property of the XSLT implementation, not a property of the language. You can't ask questions about performance without saying what processor you are interested in.

With a trivial source document like this, it is very unlikely that any performance difference will be measurable.

If there were a million B elements, some processors might (with either expression) count all the elements and then ask whether a million is equal to one. Other processors might (with either expression) stop at the second element and realize that the count is not going to be equal to one. So different processors might give very different performance from each other (with either expression), varying by a factor of a million.

But then, if you parsed the source document containing a million B elements, it's likely that parsing the document would take a lot longer than counting the elements, so the performance difference including parsing cost might not be so big after all.

If there were more than one A element, the two expressions would not be equivalent, so asking whether one is faster than the other is irrelevant. One of them will give the wrong answer.

Assuming you know there will always be one A, I would think that the most consistent performance across processors would be achieved by writing not(Document/A/B[2]). Of course, there's still a possibility that a very naive processor would construct a node-set containing all the nodes in Document/A/B, and then select the second of these nodes. There are no limits to human stupidity.

Michael Kay

On 19/09/2012 14:34, Costello, Roger L. wrote:
Hi Folks,

Consider this XML document:


Here are two XPath expressions to test that there is exactly one <B> element in the <A> element (assume there is only one <A> element):

1. count(/Document/A/B) eq 1

2. /Document/A/(count(B) eq 1)

Which is faster?

Is there an XPath expression, besides the ones listed, that is even better and faster?


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