  Linked Open Data (LOD) has been proliferated over various domains, however,
there are still lots of open data in various format other than RDF, a standard
data description framework in LOD. These open data can also be connected to
entities in LOD when they are associated with URIs. Document-centric XML data
are such open data that are connected with entities in LOD as supplemental
documents for these entities, and to convert these XML data into RDF requires
various techniques such as information extraction, ontology design and ontology
mapping with human prior knowledge. To utilize document-centric XML data linked
from entities in LOD, in this paper, a SPARQL-based seamless access method on
RDF and XML data is proposed. In particular, an extension to SPARQL,
XQueryFILTER, which enables XQuery as a filter in SPARQL is proposed. For
efficient query processing of the combination of SPARQL and XQuery, a database
theory-based query optimization is proposed. Real-world scenario-based
experiments in this paper showcase that effectiveness of XQueryFILTER and
efficiency of the optimization.
