url - Universität zu Lübeck

Chapter 5
The Key-Oriented XML Index
KeyX
In this chapter we introduce a new approach for indexing XML data formally and
by examples. Our approach - called KeyX - is motivated by the selective index
structures used within the relational world. Relational indexes are defined upon
a specific table and one (or multiple) columns. Only queries that operate on these
columns can be accelerated with this index. Therefore, a relational index is selective
to specific queries.
Like relational indexes, KeyX is based on keys - the values of elements and attributes
which are accessed by a specific path expression. The path expression
can be part of an XQuery or XUpdate operation.
For a set of frequent queries 1 the relevant keys are extracted from the original
XML data and stored in a search structure optimized for efficient key retrieval.
Those search structures include hashtables, tries, binary search trees, B + Trees
for disk resident indexes, or any other data structure that is capable of storing
and retrieving keys.
An index is defined by the ’shape’ of the path expression to be optimized. After
materializing the index, further queries with a matching shape are processed
by the index - with logarithmic instead of linear complexity. For real databases
with a size of several megabytes a set of suitable indexes implies an acceleration
factor of many magnitudes.
KeyX can also be used to accelerate specific navigational queries. In contrast to
structural summaries like Strong DataGuides and APEX our indexing approach
is defined for a set of frequent navigational queries 1 . A selective structure index
consumes less space and can be tuned for update issues.
In the following we introduce KeyX formally and by examples. We prove the quality
of this approach by performance measurements.
1 Frequent queries can be defined by a database administrator or by tools that analyze the workload
of the database.