What is the difference between Non-Repeatable Read and Phantom Read?

Understanding the Difference between Non-Repeatable Read and Phantom Read

If you're familiar with database systems, you may have come across the terms "non-repeatable read" and "phantom read" before. But what exactly do these terms mean, and how do they differ from each other? In this blog post, we'll dive into the world of database isolation levels to help you understand the nuances between non-repeatable read and phantom read.

Non-Repeatable Read

📚 Definition: Non-repeatable read occurs when a transaction reads the same row multiple times within the same transaction, but the data values change between the reads.

Imagine a scenario where Transaction A retrieves a row from the USERS table, makes a modification, and then re-retrieves the same row. However, before Transaction A commits its changes, Transaction B modifies the same row, resulting in a different value being read during the second retrieval in Transaction A.

Here's an example to illustrate this:

Transaction A

SELECT ID, USERNAME, accountno, amount FROM USERS WHERE ID=1

OUTPUT:

1----MIKE------29019892---------5000

Transaction B

UPDATE USERS SET amount=amount+5000 where ID=1 AND accountno=29019892;
COMMIT;

Transaction A

SELECT ID, USERNAME, accountno, amount FROM USERS WHERE ID=1

In this scenario, if non-repeatable read is allowed, the second SELECT statement in Transaction A would return a different amount value due to the modification made by Transaction B.

Phantom Read

📚 Definition: Phantom read occurs when a transaction re-executes a query and retrieves a different set of rows between the multiple executions.

Unlike non-repeatable read, which involves changes to individual rows, phantom read involves changes to the entire result set. Imagine a scenario where Transaction A performs a SELECT statement that returns a set of rows, but before it completes, Transaction B inserts or deletes rows that would affect the result set of Transaction A's query when re-executed.

Here's an example to illustrate this:

Transaction A

SELECT ID, USERNAME, accountno, amount FROM USERS WHERE amount > 5000

OUTPUT:

1----MIKE------29019892---------6000
2----JOHN------48374920---------7500

Transaction B

INSERT INTO USERS (ID, USERNAME, accountno, amount) VALUES (3, 'LISA', '87239408', 4000)
COMMIT;

Transaction A

SELECT ID, USERNAME, accountno, amount FROM USERS WHERE amount > 5000

In this scenario, if phantom read is allowed, the second SELECT statement in Transaction A would return an additional row (LISA's row) due to the insertion made by Transaction B.

Isolation Levels and Recommendations

Now that we understand the difference between non-repeatable read and phantom read, you might be wondering which isolation level should be used in the above example to prevent these problems.

The isolation level that should be used depends on your specific requirements and the level of data consistency needed in your application. In this example, using the Repeatable Read isolation level would prevent both non-repeatable read and phantom read problems.

Repeatable Read isolation level ensures that a transaction sees a consistent snapshot of the data throughout its entire duration. This means that any changes made by other transactions after the start of the current transaction will not be visible.

To specify the Repeatable Read isolation level in various database systems, you can refer to their respective documentation or consult with your database administrator.

Conclusion

In conclusion, non-repeatable read and phantom read are common problems that can occur in database systems. Understanding the difference between the two is crucial in designing and implementing robust and consistent database applications. By using the appropriate isolation level, such as Repeatable Read, you can mitigate these problems and ensure data consistency.

💡 Action Tip: If you're currently facing non-repeatable read or phantom read issues in your application, consider reviewing your isolation level settings. Consult with your development team or database administrator to determine the best approach for your specific scenario.

We hope this blog post has shed some light on the difference between non-repeatable read and phantom read. If you found this information helpful, feel free to share it with your fellow developers and database enthusiasts!

🤝 Engage with Us: Have you experienced non-repeatable read or phantom read issues in your database applications? Share your thoughts and experiences in the comments below. Let's discuss and learn from each other!