url - Universität zu Lübeck

2.5. XML AND DATABASES 43
an early step towards an XDBMS.
At least a native XDBMS provides an efficient storage of the XML data, support
of a query language like XPath or XQuery, and an interface for programming languages
like the DOM API [118] or XML:DB [133]. Advanced XDBMS may support
an XML modification language like XUpdate, indexes to improve the query performance,
and locking mechanisms to support multi-user interaction.
The technical functionality of native XDBMS is described here on the basis of
Natix [29, 30]: Subtrees of the original XML data are stored together in a single
(physical) record (and, hence, are clustered). Thereby, the inner structure of the
subtrees is retained. The XML segment’s interface allows to access an unordered
set of trees. New nodes can be inserted as children or siblings of existing nodes,
and any node (including its induced subtree) can be removed. The individual
documents are represented as ordered trees with non-leaf-nodes labeled with a
symbol taken from an alphabet. Elements are mapped one-to-one to tree nodes
of the logical data model. Attributes are mapped to child-nodes of an additional
attribute container child node, which is always the first child of the element-node
the attributes belong to. Attributes, PCDATA, CDATA nodes and comments are
stored as leaf-nodes.
Figure 2.22 illustrates how the logical tree (an XML data) is mapped to the physical
tree. The relationships between elements are preserved. The image is taken
from [30].
Figure 2.22: One possibility for the distribution of logical nodes onto records.
2.5.4 Hybrid Approaches
The example of figure 2.22 shows that the storage technique differs significantly
from the relational approach. Therefore, existing implementations of relational
DBMS can be reused (if at all) to a limited extent only. It is feared that native
XDBMS will meet the fate of pure object-oriented databases that never gained