DTD vs. XML Schema (not exactly regarding DocBook)

Hello all,

I have a problem, not exactly with DocBook, but closely related to it.
I've created a repository of components and terms used in a software
package. It's an XML file based on a custom XML schema. The source XML
file became way too big to handle, since the number of items exceeds
1000. By using a XQuery and XSL transformation, I get the result set
formatted as a valid DocBook article.
But the problem I have is that I don't know how to split this huge
file. I failed using the Xinclude , and entities either.
Any thoughts how to use Xinclude or entities in XML Schema-based XML files?

I appreciate your feedback.

Rgds,
Zbyszek