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Re: [xsl] General rule for designing XPath expressions to return items in document order?


Subject: Re: [xsl] General rule for designing XPath expressions to return items in document order?
From: David Carlisle <davidc@xxxxxxxxx>
Date: Wed, 08 Jan 2014 12:51:18 +0000

On 08/01/2014 12:40, Costello, Roger L. wrote:
use this one /descendant::head[parent::section] Can you provide a
general rule?

It is best not to even try, The processor is mandated to do this rewrite and it can rewrite it using internal facilities not available in the top level syntax if it wishes. So it is better to write a simple clear xpath expression that the each processor optimiser can write to an optimal form _for that processor_ rather than try to second guess the optimisation strategy and do the rewrites by hand.


You see a similar thing with //foo :


Going back to the beginning of time we used to tell people that that was
an expensive full document search and it is better to go /a/b/c/foo if
you know all the foo elements are at that level, to save the processor
searching arbitrarily deep. But now some database backed processors can
map //foo to a pre-computed index so it is fast but /a/b/c/foo does an
iterative walk over that level of the document so is relatively slow.

David


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