<xsl:copy-of select="id($id)"/>
well id() will only work if the attribute is of type ID which it wasn't
in your sample document. If it is then this is probably quickest
It depends a bit on your processor, if it indexes all IDs on input then
id() is going to be easily faster than anything else, if (as is possibly
more likely) it only indexes IDs if you actually use id() then the first
time you use it you have to pay the prices of indexing the document, but
if you use it often enough that usually is a price worth paying.
Is key() quicker?
key() would _work_ on your input document, as it doesn't require
a DOCTYPE, other than that it is presumably identical in speed to id()
as the main point of key is that it gives the ID functionality
to documents without requiring IDs specified in a DTD.
Both of these will make some kind of index/hash table of the document
so that finding an element by id will be quick, but it has to walk the
entire tree to index it and costs space to store the index.
If you are only looking up one element you might be better off with
//*[(_at_)id='foo']
as with any speed question this is _highly_ dependent on which processor
you use (some versions of msxsl had an acknowledged bug that using keys
was almost always slow for example) the real answer is just to try it
and time it.
David
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