Database Reference
In-Depth Information
21
Indexes and Clusters
In this chapter:
What is an index and what is the purpose of an index?
What types of indexes are there, and how do they work?
What are the special attributes of indexes?
What is a cluster?
Recent chapters have discussed various database objects such as tables,
views, and constraints. This fourth chapter on database objects covers
indexing and clustering. Understanding database objects is essential to a
proper understanding of Oracle SQL, particularly with respect to building
efficient SQL code; tuning is another subject.
1
It is important to under-
stand different database objects, indexes and clusters included.
21.1
Indexes
Let's start by briefly discussing what exactly an index is, followed by some
salient facts about indexing.
21.1.1
What Is an Index?
An index is a database object, similar to a table, that is used to increase read
access performance. A reference topic, for instance, having an index, allows
rapid access to a particular subject area on a specific page within that topic.
Database indexes serve the same purpose, allowing a process in the database
quick access directly to a row in the table.
An index contains copies of specific columns in a table where those col-
umns make up a very small part of the table row length. The result is an
