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RE: XQuery -- Reinventing the Wheel?

• From: Evan Lenz <elenz@x...>
• To: Michael Rys <mrys@m...>
• Date: Fri, 23 Feb 2001 18:39:21 -0800

> However rule-based systems have not achieved the
> level of performance and scalability of query-based systems,
> neither in the
> research area (as far as I know) nor in the commercial world.

I'm coming to realize that there's no technical reason why XQuery should not
have a template rule mechanism like that of XSLT. Perhaps what you mean by
"rule-based systems" is some sort of magic. XSLT's template rules, on the
other hand, have an exact functional equivalent already in XQuery, though it
is not nearly as convenient to specify in XQuery (even in comparison to
XSLT's XML syntax, imagine that.) The difference between the two, as I
mentioned in my paper, is that one uses *implicit* recursive processing and
the other uses *explicit* recursive processing.

Let me demonstrate. Take a small set of template rules such as the
following:

<xsl:template match="heading">
  <title>
    <xsl:value-of select="."/>
  </title>
</xsl:template>

<xsl:template match="chapter">
  <section>
    <xsl:apply-templates/>
  </section>
</xsl:template>

A stylesheet containing these will recursively process these template rules
*implicitly*. In XQuery (or in XSLT minus template rules), you can specify
the same query, but you have to use *explicit* recursion, as in the
following XQuery example (the XPath self:: axis is used, because I'm not yet
sure how to test a node's node type in XQuery yet):

FUNCTION applyTemplates(NODE $n) RETURNS LIST(NODE)
{
    IF name($n) = 'heading'
    THEN
      <title>
        $n/text()
      </title>
    ELSE IF name($n) = 'chapter'
    THEN
      <section>
        applyTemplates($n/node())
      </section>
    ELSE IF self::text()
      $e
    ELSE
      applyTemplates($n/node())
}
applyTemplates(/)

The above query will evaluate to the same result as produced by the set of
XSLT template rules above. This could apply for any set of template rules.
Since a template-rule-based query a la XSLT could always be algorithmically
converted to a recursive XQuery function, why should the convenience of
template rules be thrown out? Since I can express any set of template rules
in XQuery using recursive functions, (though quite awkwardly as I add more
rules), your argument that their inclusion would inherently hamper
performance simply holds no water.

And that is why I would be sad to see an XML query language developed that
makes no use of template rules. As I said in a prior post, such a mechanism
would not always have to be used. Some queries will always be slow. These
are documentable. Template rules might be among those, or maybe even not.
Regardless, their perfect fit for XML processing, i.e. processing recursive,
flexible, unexpected tree structures, should *not* be sacrificed because
certain queries might be slow.

An XML query language, as the WG Requirements document states clearly, is
about handling XML--not just relational data in XML, or even OO data in XML,
but also *documents*.

Excluding template rules from an XML query language would be a terrible
mistake, since there is no good technical reason to do so, and XML was made
for template rules, IMHO.

Evan Lenz
XYZFind Corp.

• References:
  • RE: XQuery -- Reinventing the Wheel?
    • From: Michael Rys <mrys@m...>

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