Unique Indexes in Oracle Database
Introduction
A unique index is an index that ensures that the values in the indexed column(s) are unique across the table. This type of index enforces data integrity by preventing duplicate values in the specified columns.
Creating Unique Indexes
Unique indexes are created using the CREATE UNIQUE INDEX statement. This index type ensures that no two rows have the same value in the indexed column(s).
Syntax:
CREATE UNIQUE INDEX index_name ON table_name (column_name1, column_name2, ...);
Example:
CREATE UNIQUE INDEX idx_emp_id ON employees (emp_id);
In this example, idx_emp_id is a unique index created on the emp_id column of the employees table, ensuring that each emp_id is unique across the table.
Benefits of Unique Indexes
Enforcing Data Integrity
Unique indexes are primarily used to enforce the uniqueness of values in one or more columns. This prevents the insertion of duplicate rows based on the columns indexed.
Example:
If emp_id is a unique index, attempting to insert another row with an emp_id that already exists will result in an error:
INSERT INTO employees (emp_id, emp_name) VALUES (123, 'Jane Doe');
If emp_id = 123 already exists, this insert will fail due to the unique constraint enforced by the index.
Improved Query Performance
Unique indexes can also improve query performance for lookups involving the unique columns. Since the index ensures uniqueness, the database can quickly locate the specific row.
Example Query:
SELECT * FROM employees WHERE emp_id = 123;
The query performance will benefit from the unique index on emp_id as the database engine can efficiently find the exact row.
Managing Unique Indexes
Dropping a Unique Index
You can drop a unique index if it is no longer needed or if it needs to be recreated with different properties.
Syntax:
DROP INDEX index_name;
Example:
DROP INDEX idx_emp_id;
Rebuilding a Unique Index
Over time, unique indexes can become fragmented. Rebuilding the index can help improve its performance and maintain its efficiency.
Syntax:
ALTER INDEX index_name REBUILD;
Example:
ALTER INDEX idx_emp_id REBUILD;
Monitoring Index Usage
Monitor the usage of unique indexes to ensure they are being used effectively and to maintain data integrity.
Example:
SELECT * FROM DBA_INDEXES WHERE INDEX_NAME = 'IDX_EMP_ID';
Unique Indexes vs. Primary Keys
Primary Keys
A primary key is a special case of a unique index. It uniquely identifies each row in a table and implicitly creates a unique index. Additionally, a primary key column cannot have NULL values.
Example:
ALTER TABLE employees ADD CONSTRAINT pk_emp_id PRIMARY KEY (emp_id);
This creates a primary key constraint, which also creates a unique index on the emp_id column.
Unique Constraints
A unique constraint is similar to a unique index but is defined at the table level. It ensures that the values in the column(s) are unique but also allows for additional constraint options.
Example:
ALTER TABLE employees ADD CONSTRAINT uq_emp_id UNIQUE (emp_id);
This creates a unique constraint on emp_id, which has similar effects as a unique index but is managed as a constraint.
Considerations
Performance Overhead
Unique indexes introduce additional overhead for data modification operations such as inserts, updates, and deletes, as the database must maintain the uniqueness constraint.
Data Integrity vs. Performance
While unique indexes ensure data integrity by preventing duplicate values, they can affect performance during data modifications due to the need to maintain uniqueness.
Composite Unique Indexes
You can also create unique indexes on multiple columns. This ensures that the combination of values in these columns is unique across the table.
Example:
CREATE UNIQUE INDEX idx_emp_dept ON employees (emp_id, department_id);
In this example, idx_emp_dept ensures that the combination of emp_id and department_id is unique across the employees table.
Conclusion
Unique indexes are essential for maintaining data integrity and can also enhance query performance by ensuring unique values in one or more columns. By understanding how to create and manage unique indexes, you can effectively ensure data consistency while optimizing performance.