[xsl] How to use xsl:key to make my XSLT program super-efficient?

["Roger L Costello costello@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Hi Folks,

I have a huge (1.2GB) XML document containing air navigation data. I need to
convert the XML document to another XML format, i.e., it is an XML-to-XML
translation task.

Deep within the source XML document is an <ARPT> element that contains <row>
elements, one for each airport in the world:

<Air_Navigation>
    ...
    <ARPT>
         <row>
             airport 1 data (bunch of child elements)
        </row>
         <row>
             airport 2 data (bunch of child elements)
        </row>
        ...
    </ARPT>
    ...
</Air_Navigation>

My XSLT needs to be super-efficient because the XML document is huge, and I
need to perform a lot of processing.

When I think of "efficiency" what comes to mind is the xsl:key/key() pair.

I need to iterate over each airport record, i.e., iterate over each <row>
element within the <ARPT> element. I figured that the following xsl:key/key()
pair will enable me to iterate efficiently:

<xsl:key name="airports" match="ARPT/row" use="''"/>

<xsl:for-each select="key('airports','')"> ... </xsl:for-each>

Is that correct? Is that efficient? Will the loop execute super-fast?

Truthfully, I am a bit confused about the "use" attribute on the <xsl:key>
element. As I understand it, the <xsl:key> will gather up all the <row>
elements that are within <ARPT> and then the "use" attribute may be used to
indicate how to select a subset of the <row> elements; is that correct? But I
have no desire to select a subset, I want to iterate over all the elements
gathered up by <xsl:key>. Notice that I specified the empty string for the
value of the "use" attribute, and I specified the empty string as the second
argument of key(). Is that the right thing to do, given that I have no desire
to select a subset of <row> elements?

/Roger