Visible and Invisible Indexes in Oracle Database
Introduction
In Oracle Database, indexes can be created as either visible or invisible. Visible indexes are those that the database engine uses by default to optimize query execution. Invisible indexes, on the other hand, are not automatically used by the database engine but can be made visible if needed.
Visible Indexes
Creating a Visible Index
By default, when you create an index, it is visible. This means it is considered by the query optimizer for query execution plans.
Syntax:
CREATE INDEX index_name ON table_name (column_name);
Example:
CREATE INDEX idx_emp_salary ON employees (salary);
In this example, idx_emp_salary is a visible index created on the salary column of the employees table.
Using Visible Indexes
The query optimizer uses visible indexes to enhance query performance. It selects the most appropriate indexes for queries based on statistics and cost.
Example Query:
SELECT * FROM employees WHERE salary > 50000;
The optimizer might use the idx_emp_salary index to speed up this query.
Invisible Indexes
Creating an Invisible Index
An invisible index is created in a similar manner to a visible index, but with the INVISIBLE keyword. Invisible indexes are not considered by the query optimizer in the execution plans by default but can be made visible if necessary.
Syntax:
CREATE INDEX index_name ON table_name (column_name) INVISIBLE;
Example:
CREATE INDEX idx_emp_department ON employees (department_id) INVISIBLE;
In this example, idx_emp_department is an invisible index created on the department_id column of the employees table.
Using Invisible Indexes
Invisible indexes are not used by the query optimizer for query execution. This feature allows database administrators to test or remove indexes without affecting current query performance.
To view invisible indexes in the database:
SELECT INDEX_NAME, VISIBILITY FROM DBA_INDEXES WHERE TABLE_NAME = 'EMPLOYEES';
Managing Invisible Indexes
Making an Invisible Index Visible
If you want an invisible index to be considered by the query optimizer, you can make it visible using the ALTER INDEX command.
Syntax:
ALTER INDEX index_name MODIFY VISIBLE;
Example:
ALTER INDEX idx_emp_department MODIFY VISIBLE;
This command makes the idx_emp_department index visible, so it will now be used by the query optimizer.
Making a Visible Index Invisible
To make a visible index invisible, use the ALTER INDEX command.
Syntax:
ALTER INDEX index_name MODIFY INVISIBLE;
Example:
ALTER INDEX idx_emp_salary MODIFY INVISIBLE;
This command makes the idx_emp_salary index invisible, so it will no longer be automatically used by the query optimizer.
Advantages of Invisible Indexes
Testing and Validation
Invisible indexes allow administrators to test the impact of an index on query performance without affecting the current performance. This is useful for testing new indexing strategies without immediately influencing live queries.
Maintenance Cost Reduction
By making certain indexes invisible, you can reduce maintenance and optimization costs without affecting overall database performance. Invisible indexes can be used for debugging or analysis purposes.
Considerations
Performance Impact
Although invisible indexes are not used by the optimizer, they still need to be maintained during data modification operations (inserts, updates, deletes). This can introduce some overhead in terms of performance and storage.
Index Management
Managing both visible and invisible indexes requires careful attention to avoid inconsistencies in query performance and maintenance costs. Regular monitoring and evaluation of invisible indexes are necessary to ensure they are used effectively.
Use Cases
Invisible indexes are particularly useful for situations where you want to test the impact of a new index without making it active immediately, or when you want to keep indexes for historical or debugging purposes without affecting current queries.
Conclusion
Visible and invisible indexes offer significant flexibility in managing and optimizing database performance in Oracle. By understanding how to create, manage, and utilize these indexes, you can effectively optimize query performance while maintaining the ability to test and adjust indexing strategies as needed.