Subject: [xsl] XPath expr optimization (was Re: monotonous...) From: Mike Brown <mike@xxxxxxxx> Date: Thu, 14 Nov 2002 13:03:38 -0700 (MST) |
Greg Faron wrote: > At 05:47 PM 11/13/2002, you wrote: > >Greg Faron wrote: > > > normalize-space(string-length(.)) = 0 > > > >That's a lot of extra work. > > isn't this one of those things that the better XSLT engines will > optimize to the best form? Not unless the people writing the engine have reason to believe it will be faster to look ahead into and analyze an expression and find ways it can be optimized than to just evaluate it as-is. > I also suppose that the "best form" could be > XSLT engine-dependent, but probably it is "not(string())". Hmm. In order to determine if not() is applicable, you have to evaluate the right subexpression to see if it is the number zero or boolean false. You were going to do a "is the number on the left equal to the number on the right?" but now you're asking "if the thing on the right turns out to be a number, is it equal to zero? or if the thing on the right turns out to be a boolean, is it equal to false?". Even if you don't evaluate the right subexpression and just look for the token '0' in the expression string itself, you're asking "is it the string '0'?" which is another equality comparison. If all such equality comparisons are equally expensive, where is the optimization? - Mike ____________________________________________________________________________ mike j. brown | xml/xslt: http://skew.org/xml/ denver/boulder, colorado, usa | resume: http://skew.org/~mike/resume/ XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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