Count items in a key ?

Hi List.
I would like to know if it is possible to get all "key" item from an
xsl:key ?
I mean:
when you define a key, say:
	<xsl:key name="mykey" match="NODE" use="@id"/>

You can retrieve all NODE nodes with a given @id attribute.
But is it possible to retrieve all distinct @id values from the key ?
(I know it is possible to get them from the XML file, but wouldn't it be
faster to get what is already retrieved ?)

Thanks a lot in advance,
Best regards,
Fabien