url - Universität zu Lübeck

Chapter 4
Introduction to Recent
Approaches in XML Indexing
In this section we classify and describe recent approaches indexing XML and
semistructured data. Some approaches were published before XML gained the
current importance and generally operate on semistructured data. We transferred
these approaches to XML.
The basic idea of an index for semistructured data and XML is to accelerate the
execution of path expressions, for instance XPath. The more complex XQuery
expressions benefit from an index, too, because XQuery relies on the execution of
XPath expressions for addressing the nodes of the sequences.
All indexing approaches have in common that they try to avoid the linear inspection
of XML nodes when performing node tests or checking predicates. For
instance, when evaluating the XPath expression //item[/name=’MP3 Player’]
every element is treated as if it has the label item or not. Second, for each item
element all children are checked whether they have the label name. Third, for all
name elements the corresponding text value is compared with the given string. For
larger databases this evaluation method leads to unacceptable processing times.
Although all indexing approaches have the same goal, their methodology, the
internal data structures, and the query processing vary significantly. For this
reason we establish some criteria in order to classify and compare the related
work on XML indexing.
Some index approaches index the structure of the XML data without regarding the
value of elements or attributes. These approaches are called structural indexes or
pure-path indexes. On the other hand some indexes cover only the value of elements
and attributes without reflecting the leading path to these values; these
approaches are called value indexes. Advanced approaches cover both structure
and values leading to an acceleration of more general and realistic path expres-