Subject: Re: [xsl] Wath is the opposite of the union operator? From: David Carlisle <davidc@xxxxxxxxx> Date: Thu, 22 Sep 2005 16:44:09 +0100 |
> I'm curious about what's going on behind the scenes (that is, in the > processor) that makes this expensive. without having looked behind the scenes, on the face of it count(.|B) is a very expensive operation (if "B" selects a large set of noders) as you have to take the set union | which invloves removing duplicates etc, then count the thing, which means that you need to fully evaluate all of B. All you actually need to know is is . in the set returned by B which you can ask in xpath 2 using the is operator (so it would be a lot more efficient even if except wasn't added as a primitive. With such a test you can stop and return true as soon as you have found the current node, just as B[1] typically only causes a processor to evaluate one node, even though the formal semantics of the expression are that B is fully evaluated, sorted into document order, and then the first item is taken. David ________________________________________________________________________ This e-mail has been scanned for all viruses by Star. The service is powered by MessageLabs. For more information on a proactive anti-virus service working around the clock, around the globe, visit: http://www.star.net.uk ________________________________________________________________________
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