url - Universität zu Lübeck

Chapter 6
The Index Selection Problem
Today’s larger business applications with an underlying database cannot exist
without indexes covering the most frequent queries. Recently, relational database
management systems (RDBMS) are dominant although XML database management
systems (XDBMS) are becoming more and more relevant for real world applications.
Defining suitable indexes is a major task when optimizing the database. Usually,
a human database administrator (DBA) defines a set of indexes in the design
phase of the database. This can be done manually or with the help of so-called
index wizards or index adviser tools that analyze predefined or collected database
operations. In both cases the typical usage of the database has to be known in
advance.
But, even having an optimal set of indexes when setting up a database there
is no guarantee that these indexes will suit future demands. Rather, it is realistic
that the typical usage of the database will change after a while because new
queries appear, for instance. In consequence, the existing indexes are suboptimal.
The typical approach to face this problem is that a database administrator maintains
the database permanently: she logs the workload, analyzes the performance
of the database and the existing indexes, and redefines indexes when necessary.
These tasks are time-consuming and require a skilled expert.
In XML database management systems (XDBMS) this problem becomes even
worse. In relational databases (in first normal form 1NF) only the atomic values
of specified columns are reflected in an index; in contrast, XML indexes have
to cover both the structure of the data and the values of elements and attributes;
queries and modifying operations may contain structural and content properties.
Therefore, the number of possible queries and indexes is significantly higher than
in relational databases.
Additionally, for XML data without a schema like a DTD or XML Schema, queries
and indexes cannot be defined finally in advance because elements may appear