url - Universität zu Lübeck

44 CHAPTER 2. FUNDAMENTALS
significant importance in real applications in industrial environments. However,
the object-oriented paradigm lead to the extension of relational DBMS to objectrelational
DBMS supporting a multitude of the object-oriented features, like userdefined
types and methods. The same may apply to native XDMBS so that future
relational DBMS may provide a native support of XML storage and access.
A first prototype of a hybrid database can be seen in the System RX [11] from
IBM Almaden Research Center and the IBM Silicon Valley Lab. SystemRX unifies
a native XML storage and indexing and query processing technologies with the
existing relational approaches of the DB2 implementation. The main idea is a
new column type that keeps the XML data in a native form. Conceptually, the entry
in each row can be interpreted as a reference to the root of a persistent DOM
tree. Path expressions are evaluated directly over the native format; Therefore,
the node of an XML document can be accessed and inserted without reading and
manipulating the whole document. Like in every DBMS the persistent data (here
the nodes) are distributed over multiple pages on the harddisk because the whole
data cannot be kept in the main memory. For an accelerated access to relevant
child-nodes of an element-node SystemRX provides an encoded XML storage and
compression that is comparable to the Natix approach [29, 30].
Basically, because the XML data is stored in its native form any XML index can
be used. SystemRX provides an indexing mechanism that is based on keys stored
in a B+ tree. This approach is discussed more extensively in section 4.
The architecture is presented in figure 2.23. The figure is taken from [11].
Figure 2.23: System Architecture of the hybrid DBMS System RX
With SystemRX existing SQL applications can be augmented with additional XML
data. At this time is is too early to judge the impact of the System RX approach.