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RE: Re: . in for

Subject: RE: Re: . in for
From: Dimitre Novatchev <dnovatchev@xxxxxxxxx>
Date: Sun, 6 Jan 2002 12:03:31 -0800 (PST)
 RE: Re: . in for
Mike wrote:
> Dimitre wrote:
> > While your mapping operator will perform a series of
> > mappings, each producing an
> > intermediate sequence and may require too much memory, the
> > last function applies the
> > map function only once. The composition of all functions is
> > applied on every element
> > of $sequence and the resulting sequence is produced. No
> > additional memory for
> > intermediate sequences is necessary.
> >
> > This shows that it is better to have a map() function and a
> > composition operator for
> > expressions (in case XPath 2.0 will not fully support
> > higher-order functions).
> >
> As a point of information, the implementation of "for" expressions in Saxon
> 7.0 is fully pipelined, so there will never be a need to store intermediate
> sequences in memory.

We were discussing with Jeni the problem for optimisation if certain functions like
position() (or any other function, including user-defined ones, that uses the result
of the inner for as a sequence) are used in an outer 'for' and the inner 'for'
decreases the cardinality of its sequence by producing one or more empty sequences
(). This is fully described in:


It is interesting to hear your solution -- where were Jeni and I wrong?

Dimitre Novatchev.

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