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   Fw: XML query language

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  • From: "Oren Ben-Kiki" <oren@capella.co.il>
  • To: "XML List" <xml-dev@ic.ac.uk>
  • Date: Tue, 30 Mar 1999 17:14:15 +0200

Paul Janssens <paul.janssens@skynet.be> wrote:
>In my opinion, an xml query language should only describe a set of
>equations, an xml query language implementation should only solve these
>equations, and whatever is done with the result is NO business of the
>query language.

Just to make sure I follow: you'd prefer that there would be a standard
<xql:result> DTD, so that results would always be created in an XML format
containing references to the matched XML elements (XLink/XPointer?). The
user would then filter this through XSL or whatever to display the results.

Nice separation of concerns, but I see several objections:

- Efficiency. Suppose I'm querying a very large DB, and I'm getting a list
of matches scattered all over the place. In the current approach, the DB
would both resolve the matches and extract the necessary data, potentially
at the same pass using a lot of locality-of-reference optimizations. In your
method a second tool would re-fetch the references in a second phase, which
would probably double the cost of doing the query.

- Power. Assume that I hypnotize all the W3C members to adopt the XSL
transformational part as XQL version 1.0 :-) This is more powerful then
current ?QL proposals because it allows for an <xsl:template> to call
<xsl:apply-templates> - that is, to perform nested queries (and therefore,
BTW, offers a natural way to do joins without variables, and solves other
?QL problems). All this works because XSL has a rich language for
constructing the results. In your approach, you won't be able to do a lot of
that; you'd end up adding special constructs for them, duplicating XSL's
capabilities in an incompatible language. Of course you'd be in good
company - that is what all the other ?QL language proposals do :-)

- Convenience. It is easier to specify a query as just "one thing" instead
of two. Note that even if ?QL == XSL transformation, it still makes a lot of
sense to filter its results through another XSL stylesheet for presentation
in most cases. Even lazy users will do so - if, for example, they had
already available XSL sheets for displaying certain types of results.

So all in all I prefer my approach: XQL = XSL - FO.

Share & Enjoy,

    Oren Ben-Kiki

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