Efficiency depends on the processor, and you're going to need a processor like
Saxon-EE that does join optimization.
My best attempt in XPath 2.0 is
//row[some $q in //row satisfies (./navaid eq $q/navaid and not(. is $q))]
but I haven't checked whether it gets suitably optimized.
Move to more powerful technology. In XQuery 3.1 you can do
for $r in //rows
group by $id := navaid
where count($r) gt 1
return $r
which is almost certain to be implemented efficiently by any self-respecting
processor
Michael Kay
Saxonica
On 13 Dec 2018, at 18:07, Costello, Roger L. costello(_at_)mitre(_dot_)org
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:
Hi Folks,
I have a large XML document containing data about airports around the world:
<airports>
<row>
<navaid>A</navaid>
</row>
<row>
<navaid>B</navaid>
</row>
<row>
<navaid>A</navaid>
</row>
</airports>
Notice that there is only one <row> element having the B navaid, but two
<row> elements having the A navaid.
I want an XPath 2.0 expression to return each <row> element for which there
are other <row> elements having the same navaid. For the above example, I
want the XPath expression to return the first and third <row> elements.
Here is one way to do it:
//row[navaid = (preceding-sibling::row/navaid, following-sibling::row/navaid)]
Eek! That is horribly inefficient. I ran that XPath expression on my XML
document and it took a long time to finish.
Is there an efficient XPath 2.0 expression to solve this problem?
Note: I am running the XPath expression from Oxygen's XPath evaluator, not
from an XSLT program.
/Roger
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