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An index can be either unique (which means that no two rows in the table or view
can have the same index value) or nonunique. If the column or columns on which an
index is based contain NULL values, the row isn??™t included in an index.
An index in Oracle refers to the physical structure used within the database. A key is
a term for a logical entity, typically the value stored within the index. In most places
in the Oracle documentation, the two terms are used interchangeably, with the notable
exception of the foreign key constraint, which is discussed later in this chapter.
Four different types of index structures, which are described in the following sections,
are used in Oracle: standard B*-tree indexes; reverse key indexes; bitmap
indexes; and function-based indexes, which were introduced in Oracle8i. Oracle
Database 11g delivers the ability to use invisible indexes, which are described below.
Oracle also gives youthe ability to cluster the data in the tables, which can improve
performance. This is described later, in the section ???Clusters.


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