ora-12838: cannot read/modify an object after modifying it in parallel

Guri Bee

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01-10-2024

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If you're working with Oracle databases, you may encounter the error ORA-12838: "cannot read/modify an object after modifying it in parallel." This error can be particularly frustrating, especially when you're running queries that involve parallel processing. Let's delve into the details of this error, its causes, and practical examples to help you avoid or resolve it.

What Causes ORA-12838?

The ORA-12838 error typically arises when an operation attempts to read or modify an object that is currently being modified in a parallel execution context. Here’s a breakdown of potential causes:

1. Concurrent DML Operations: If two sessions attempt to perform Data Manipulation Language (DML) operations on the same object simultaneously, you might see this error.

2. Parallel DML Operations: When a session modifies a table using parallel DML and then attempts to read or write to the same table within the same transaction, the database can throw ORA-12838.

3. Transactional Integrity: Oracle enforces strict transactional integrity, and when it detects that a transaction might read stale data or conflict with another transaction, it raises this error.

Example Scenario

Imagine you have a table called employees, and you're executing a parallel update statement while attempting to read from it at the same time:

-- Session 1
ALTER SESSION ENABLE PARALLEL DML;
UPDATE /*+ PARALLEL(employees, 4) */ employees
SET salary = salary * 1.1
WHERE department_id = 10;

-- Session 2
SELECT * FROM employees WHERE department_id = 10;

In this scenario, if Session 2 tries to read the employees table while Session 1 is still performing a parallel update, ORA-12838 may occur.

How to Resolve ORA-12838

To resolve this error, consider the following strategies:

1. Disable Parallel DML: If parallel processing is not essential for your operation, you can disable parallel DML. This can be done by executing ALTER SESSION DISABLE PARALLEL DML; before your DML operation.

2. Sequential Execution: If you have control over how sessions execute, ensure that DML operations do not overlap. Wait for one operation to complete before starting another.

3. Use Appropriate Isolation Levels: Adjusting the transaction isolation levels can sometimes mitigate concurrency issues. For example, using READ COMMITTED isolation can help in some cases.

4. Error Handling: Implement error handling in your code. Catch the ORA-12838 error and reattempt the operation after a brief pause or interval.

5. Monitoring Long-Running Transactions: Keep an eye on long-running transactions that might be holding locks on objects.

Additional Tips and Insights

• Debugging Parallel Execution: When working with parallel execution, always monitor the processes and their resource usage. Use the V$PX_PROCESS view to get insights into the parallel processes.

• Understanding Execution Plans: Utilize the EXPLAIN PLAN command to visualize how Oracle executes your queries. This will give you clues about whether parallel execution is beneficial or if it may lead to contention issues.

• Performance Tuning: Remember that while parallel execution can improve performance, it can also lead to issues like ORA-12838 if not managed correctly. Profile your queries and analyze if parallelism is necessary.

• Documentation: Always refer to the official Oracle documentation for details about your specific version of the database as handling parallel DML may vary across versions.

Conclusion

The ORA-12838 error can disrupt your database operations, especially when parallel execution is involved. Understanding the causes, utilizing effective strategies, and implementing best practices can help mitigate this issue. Always ensure that your operations are well-structured to maintain concurrency without compromising data integrity. By following these guidelines, you can achieve a smoother experience in working with Oracle databases.

Attribution: The explanations and concepts in this article are informed by contributions and discussions from various developers and database administrators on GitHub. For more technical details, refer to Oracle's official documentation and community forums.