Oracle Reverse Key Index FAQs with examples

1. What is a Reverse Key Index in Oracle?

A Reverse Key Index is a B-Tree index that stores the bytes of the indexed values in reverse order.

It helps reduce index block contention when many users insert sequential values, such as sequence-generated IDs, at the same time.

Example:

CREATE INDEX idx_emp_rev
ON employees(employee_id)
REVERSE;

2. Why do we use a Reverse Key Index?

Reverse Key Indexes are used to:

• Reduce index block contention
• Improve concurrent INSERT performance
• Distribute index entries across multiple index blocks
• Improve scalability in high-concurrency OLTP systems

3. How does a Reverse Key Index work?

Suppose employee IDs are inserted in sequence:

1001
1002
1003
1004

Normally, Oracle inserts them into the same end of the index, which can create contention. With a Reverse Key Index, Oracle internally stores them in reverse-byte order, helping spread inserts across different index blocks.

4. How do you create a Reverse Key Index?

Syntax:

CREATE INDEX index_name
ON table_name(column_name)
REVERSE;

Example:

CREATE INDEX idx_empid_rev
ON employees(employee_id)
REVERSE;

5. When should you use a Reverse Key Index?

Use a Reverse Key Index when:

• The indexed column uses a sequence
• Many users insert data simultaneously
• The application experiences index hot spots
• The system is an OLTP application with heavy INSERT activity

Example:

INSERT INTO employees
VALUES (emp_seq.NEXTVAL, 'John', 50000);

If thousands of users insert rows concurrently, a Reverse Key Index can reduce contention.

6. When should you avoid a Reverse Key Index?

Avoid Reverse Key Indexes when queries frequently perform:

• Range searches
• BETWEEN operations
• Greater than (>)
• Less than (<)
• ORDER BY using the indexed column

These operations are generally not efficient because the key values are stored in reverse order.

7. Difference between a Normal Index and a Reverse Key Index

Normal Index	Reverse Key Index
Stores keys in normal order	Stores keys in reverse-byte order
Supports range scans	Does not efficiently support range scans
Suitable for general queries	Suitable for high-concurrency inserts
Default index type	Special B-Tree index option

8. Can a Reverse Key Index be Unique?

Yes.

Example:

CREATE UNIQUE INDEX idx_emp_rev
ON employees(employee_id)
REVERSE;

This enforces uniqueness while also reducing insert contention.

9. Can a Reverse Key Index be Composite?

Yes.

Example:

CREATE INDEX idx_emp_rev
ON employees(employee_id, department_id)
REVERSE;

The composite key is stored using reverse-key indexing.

10. Can Oracle perform Range Scans on a Reverse Key Index?

Generally, no.

Example:

SELECT *
FROM employees
WHERE employee_id BETWEEN 1000 AND 2000;

Oracle usually performs a Full Table Scan or uses another suitable index because Reverse Key Indexes are not designed for efficient range scans.

11. How do you view Reverse Key Indexes?

SELECT index_name,
       index_type
FROM user_indexes;

The INDEX_TYPE column indicates the index type. You can also review the index DDL using DBMS_METADATA.GET_DDL to confirm that it was created with the REVERSE option.

12. How do you drop a Reverse Key Index?

DROP INDEX idx_emp_rev;

13. What are the advantages of a Reverse Key Index?

• Reduces index block contention
• Improves concurrent INSERT performance
• Better scalability for sequence-generated keys
• Useful in high-volume OLTP applications

14. What are the disadvantages of a Reverse Key Index?

• Does not efficiently support range scans
• Not suitable for BETWEEN, <, or > searches
• Less suitable for queries requiring ordered index access

15. Interview Example

Create a table:

CREATE TABLE employees (
  employee_id NUMBER,
  first_name  VARCHAR2(50)
);

Create a Reverse Key Index:

CREATE INDEX idx_emp_rev
ON employees(employee_id)
REVERSE;

Insert data:

INSERT INTO employees
VALUES (1001, 'John');

INSERT INTO employees
VALUES (1002, 'David');

INSERT INTO employees
VALUES (1003, 'Scott');

The Reverse Key Index helps distribute these sequential key inserts across the index, reducing contention.

16. Real-World Example

Suppose an online banking system generates transaction IDs using a sequence.

INSERT INTO transactions
VALUES (txn_seq.NEXTVAL, 5000, SYSDATE);

Create the index:

CREATE INDEX idx_txn_rev
ON transactions(transaction_id)
REVERSE;

Thousands of users can insert new transactions simultaneously with less index block contention than a normal B-Tree index.

17. Can Oracle use a Reverse Key Index for Equality Searches?

Yes.

Example:

SELECT *
FROM employees
WHERE employee_id = 105;

Oracle can efficiently use a Reverse Key Index for equality (=) searches.

18. How do you check if Oracle is using a Reverse Key Index?

Use an execution plan.

EXPLAIN PLAN FOR
SELECT *
FROM employees
WHERE employee_id = 105;

SELECT *
FROM TABLE(DBMS_XPLAN.DISPLAY);

If the execution plan shows an index access path, Oracle is using the Reverse Key Index.

Key Point:
Reverse Key Indexes are best for high-concurrency inserts with sequential keys, but they are not suitable for range queries.