RE: XPath/XSLT 2.0 concerns
> I second Jeni's opinion. More than one of our developers has > questioned the claim that optimizations based on schema based > static & dynamic typing in XQuery et al will buy any > significant gains perfromance-wise and in fact may hurt it in > simple cases. I've seen a few cases where schema information could be useful, but in general I agree that the benefits are often overstated, at least in an XSLT context where you're typically parsing the document once for each XSLT execution. There are some things in the dynamically-typed nature of XPath 1.0 that make optimization very difficult, for example the overloading of , but in most cases you can do all the static inferencing you need without a strongly-typed language. In fact I would say that type inferencing is only a small part of the analysis that an optimizer needs to do, far more important is analysing the dependencies of expressions on the context and on variables, so that subexpressions can be moved out of predicates, sort keys, etc. The most obvious thing that an optimizer can get from a schema is the graph of which elements can contain which others, but arguably that can be built just as easily - and more accurately - on the fly while parsing the source document. Optimization in a typical XQuery scenario is of course very different: you have a much wider choice of access paths based on pre-constructed indexes. (But of course, there are some people who see XSLT 2.0 being used as a database query language). Michael Kay Software AG home: Michael.H.Kay@n... work: Michael.Kay@s...
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