An index associated with a constraint because this constraint is required to have an index. There are two types of constraints are required to have indexes: UNIQUE and PRIMARY KEY. When you defines a UNIQUE or PRIMARY KEY constraint in a table, Oracle will automatically create an index for that constraint. The following script shows you an example:

CREATE TABLE student (id NUMBER(5) PRIMARY KEY,

first_name VARCHAR(80) NOT NULL,

last_name VARCHAR(80) NOT NULL,

birth_date DATE NOT NULL,

social_number VARCHAR(80) UNIQUE NOT NULL);

Table created.

SELECT index_name, table_name, uniqueness

FROM USER_INDEXES WHERE table_name = 'STUDENT';

INDEX_NAME   TABLE_NAME  UNIQUENES

----------------------- --------------------- ---------

SYS_C004123   STUDENT   UNIQUE

SYS_C004124   STUDENT   UNIQUE

The result confirms that Oracle automatically created two indexes for you.