The XQuery spec describes the semantics of sorting in terms of a stream of
tuples - that is, it requires data structures outside the scope of the
XPath/XQuery/XSLT data model. If it had been possible to describe the
semantics without straying outside the data model, I think someone would
have found a way of doing so. And if it requires extensions to the data
model, then I suspect that mechanical translation into XSLT is quite
difficult.
Am I right in thinking that your translation models the stream of N tuples
of width M as a sequence of length N*M, and then computes the sort key of
each item in a way that ensures that all the M items corresponding to a
single tuple get the same sort key, and therefore remain together when
sorted?
Another way of doing this sort might be to use xsl:for-each-group to treat
each tuple as a group, so that the sort key is only computed for the first
item in the group.
The question I'm more interested in is: is this feature useful to anyone?
-----Original Message-----
From: David Carlisle [mailto:davidc(_at_)nag(_dot_)co(_dot_)uk]
Sent: 29 April 2005 12:43
To: xsl-list(_at_)lists(_dot_)mulberrytech(_dot_)com
Subject: [xsl] order-by vs xsl:sort
I've been toying to see how to express xquery's order by in terms of
xsl. (There's a question at the end, bonus points for anyone
not called
Michael who even gets that far:-)
here's an example xquery
let $data1:=
<a>
<z id="a"><y id="1"/></z>
<z id="b"><y id="2"/></z>
<z id="c"><y id="3"/></z>
<z id="d"><y id="4"/></z>
<z id="e"><y id="5"/></z>
</a>
return
let $data2:=
<a>
<z id="s"><y id="6"/></z>
<z id="t"><y id="7"/></z>
<z id="u"><y id="8"/></z>
</a>
return
for $i in $data1/z/y, $j in $data2/z/y
order by ($j/@id - $i/@id)
return
concat($i/../@id,$j/../@id)
and here's its output (linebreak added after the xml delcn by hand)
$ saxon8q order.xq
<?xml version="1.0" encoding="UTF-8"?>
es ds et cs dt eu bs ct du as bt cu at bu au
Things to note about this are
a) the sort key cuts across the diagonal, you can't just sort
the i and j
axes separately. So you can't simply make this into two nested
xsl:for-each each with separate xsl:sort's.
b) the final result uses data accessed by the parent axis
from the items
in the sorted sequence so you can't simply build a temporary tree
that holds a sequence of elements modelling the tuple with
copies of
the items in the sequence as then stuff in the parent axis is lost.
Current best plan is the following XSL
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet
xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
version="2.0"
<xsl:variable name="data1" as="item()">
<a>
<z id="a"><y id="1"/></z>
<z id="b"><y id="2"/></z>
<z id="c"><y id="3"/></z>
<z id="d"><y id="4"/></z>
<z id="e"><y id="5"/></z>
</a>
</xsl:variable>
<xsl:variable name="data2" as="item()">
<a>
<z id="s"><y id="6"/></z>
<z id="t"><y id="7"/></z>
<z id="u"><y id="8"/></z>
</a>
</xsl:variable>
<xsl:template name="x">
<xsl:variable name="is" select="$data1/z/y"/>
<xsl:variable name="ci" select="count($is)"/>
<xsl:variable name="js" select="$data2/z/y"/>
<xsl:variable name="cj" select="count($js)"/>
<xsl:for-each select="0 to ($ci * $cj)-1">
<xsl:sort select="$js[(current() idiv $ci) +1]/@id -
$is[(current() mod $ci)+1]/@id"/>
<xsl:sequence select="concat($is[(current() mod
$ci)+1]/../@id,$js[(current() idiv $ci) +1]/../@id)"/>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
which works:
$ saxon8 -it x order.xsl
<?xml version="1.0" encoding="UTF-8"?>
es ds et cs dt eu bs ct du as bt cu at bu au
and I think would be automatically generatable in an xquery to xslt
conversion program, although the indexes get messy if there are more
sequences than two involved, and the constraints on xsl:sort
being first
mean that you can't make it look nicer by using variables to hold the
calculated indices.
Is this approach plausible? and if so can it be made a bit less ugly?
If not, is there a more plausible approach?
David
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