X-WACoDa - Data Warehousing Design and Advanced Engineering Applications - page 46

Figure 2. XML data warehouse model

to the four architectures we have enumerated.
These XML warehouses have been stored on the
eXist XML-native DBMS (Meier, 2002). Then,
the authors measured the response time of a
fixed XQuery workload over these warehouses
and showed that representing facts in one single
XML document and each dimension in one
XML document allowed the best performance.
They especially conclude that storing facts in
one single XML document helps decrease their
scanning cost.
Moreover, this representation also allows to
model constellation schemas without duplicating
dimension information. Several fact documents
can indeed share the same dimensions.Also, since
each dimension and its hierarchical levels are
stored in one XML document, dimension updates
are more easily and efficiently performed than if
dimensions were either embedded with the facts
or all stored in one single document.
Hence, we adopt the architecture model from
Figure 2 to represent our XML data warehouse. It
is actually the translation of a classical snowflake
schema from the relational model to the XML
model, i.e., tables become XML documents. Note,

however, that XML warehouses can bear irregu-
lar structures that are not possible in relational
warehouses. On the physical level, our reference
data warehouse is composed of the following
XML documents:

• DW-model.xml represents the warehouse-

metadata, basically the warehouse schema:
fact document(s) structure (related dimen-
sions, numeric measures), dimension docu-
ments structure (including any hierarchical
level information, each level being charac-
terized by descriptive member attributes);
A set of • facts f . xml documents that each

help store information related to set of
facts f , i.e., dimension references and mea-
sure values;
A set of • dimension d . xml documents that

each help store a given dimension d 's
member values.

Figure 3 represents the dw-model.xml docu-
ment's graph structure. The equivalent XML
Schema definition we actually use in practice
is available on-line to accommodate space con-