What are the potential problems with the filtering approach?  What is the "stopping problem"? If I understand this correctly it involves converting an xslt file into a stateful sax event processor.  The end result would seem to be similar to the best possible results achievable with the pruning approach.

Edward Middleton

>>> Xalan is capable of "streaming processing".
>>The interesting challenge is to work out when you can discard parts of
>>the tree that won't be needed again. I think this could be done quite
>>easily for a small class of very simple stylesheets, but the general
>>problem is quite hard.
>
>That's been our conclusion. XSLT's semantics require at least the 
>appearance of having the whole document in memory at once. Figuring out 
>how to reduce
>
>The terminology Xalan uses for these issues:
>
>Filtering: An optimization consisting of not building portions of the 
>source model which stylesheet analysis proves will never be referenced by 
>the stylesheet. Conceptually straightforward, but runs into the "stopping 
>problem" to some extent; may be hard to apply generally. May require some 
>rewriting of the stylesheet and/or retaining of "stub" branches of the 
>tree to avoid breaking XPaths. NOT IMPLEMENTED at this time.

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