Causes of Deadlocks in DB2

Oct 13, 2025 by talynt.org 17 views

Introduction

In a recent scenario, a DBA encountered a DB2 table deadlock when creating a large index on an unrelated table. The DBA had created a new non-unique index on a table containing approximately 800 million rows using the following command:

CREATE INDEX DBA_PERF_TIMESTMP ON ...

However, this seemingly innocuous operation resulted in a deadlock, causing significant delays and impacting the overall performance of the database. In this article, we will delve into the reasons behind this deadlock and explore strategies to prevent such issues in the future.

Understanding Deadlocks

A deadlock is a situation where two or more processes are unable to proceed due to a mutual hold on resources. In the context of DB2, a deadlock can occur when two or more transactions are competing for the same resources, such as locks on tables or indexes.

Causes of Deadlocks in DB2

There are several reasons why deadlocks can occur in DB2:

Lock contention: When multiple transactions are competing for the same locks on tables or indexes, a deadlock can occur.
Resource starvation: When a transaction is unable to acquire the necessary resources due to a lack of available locks, a deadlock can occur.
Cyclic wait: When two or more transactions are waiting for each other to release a lock, a deadlock can occur.

The Role of Index Creation in Deadlocks

Index creation can be a resource-intensive operation, especially when dealing with large tables. When creating an index, DB2 must acquire locks on the underlying table and potentially other related tables. If another transaction is also competing for these locks, a deadlock can occur.

The Specific Scenario

In the scenario described, the DBA created a new non-unique index on a table containing approximately 800 million rows. The index creation operation required locks on the underlying table, which in turn caused a deadlock with another transaction that was also competing for these locks.

Analyzing the Situation

To understand the root cause of the deadlock, we need to analyze the situation further. The following factors contributed to the deadlock:

Large table size: The table containing approximately 800 million rows was a significant contributor to the deadlock.
Index creation: The index creation operation required locks on the underlying table, which in turn caused a deadlock with another transaction.
Lock contention: The two transactions were competing for the same locks on the table, resulting in a deadlock.

Preventing Deadlocks During Index Creation

To prevent deadlocks during index creation, we can employ several strategies:

Use the NOWAIT option: When creating an index, we can use the NOWAIT option to prevent the operation from waiting for locks. Instead, the operation will fail immediately if the necessary locks are not available.
Use the WAIT option with a timeout: We can use the WAIT option with a timeout to specify the maximum amount of time the operation should wait for locks. If the locks are not available within the specified time, the operation will fail.
Create the index in parallel: We can create the index in parallel by using the PARALLEL option. This can help reduce the lock contention and prevent deadlocks.
Use a different index creation method: We can use a different index creation method, such as creating the index in stages or using a different indexing algorithm.

Conclusion

In conclusion, the DB2 table deadlock when creating a large index on an unrelated table is a complex issue that requires a thorough understanding of the underlying causes. By analyzing the situation and employing strategies to prevent deadlocks, we can minimize the impact of such issues and ensure the smooth operation of our database.

Best Practices for Index Creation

To avoid deadlocks during index creation, we should follow these best practices:

Create indexes in stages: Instead of creating a single large index, we can create smaller indexes in stages. This can help reduce the lock contention and prevent deadlocks.
Use parallel indexing: We can use parallel indexing to create indexes in parallel, which can help reduce the lock contention and prevent deadlocks.
Monitor lock contention: We should monitor lock contention and adjust our indexing strategy accordingly.
Test indexing operations: We should test indexing operations in a controlled environment to ensure that they do not cause deadlocks.

DB2 Indexing Best Practices

To ensure optimal performance and minimize the risk of deadlocks, we should follow these DB2 indexing best practices:

Use the correct indexing algorithm: We should use the correct indexing algorithm for our specific use case.
Choose the right index type: We should choose the right index type based on our specific use case.
Monitor index performance: We should monitor index performance and adjust our indexing strategy accordingly.
Test indexing operations: We should test indexing operations in a controlled environment to ensure that they do not cause deadlocks.

Conclusion

Q: What is a deadlock in DB2?

A: A deadlock is a situation where two or more processes are unable to proceed due to a mutual hold on resources. In the context of DB2, a deadlock can occur when two or more transactions are competing for the same resources, such as locks on tables or indexes.

Q: What are the common causes of deadlocks in DB2?

A: The common causes of deadlocks in DB2 include:

Lock contention: When multiple transactions are competing for the same locks on tables or indexes, a deadlock can occur.
Resource starvation: When a transaction is unable to acquire the necessary resources due to a lack of available locks, a deadlock can occur.
Cyclic wait: When two or more transactions are waiting for each other to release a lock, a deadlock can occur.

Q: How can I prevent deadlocks during index creation in DB2?

A: To prevent deadlocks during index creation in DB2, you can employ several strategies:

Use the NOWAIT option: When creating an index, you can use the NOWAIT option to prevent the operation from waiting for locks. Instead, the operation will fail immediately if the necessary locks are not available.
Use the WAIT option with a timeout: You can use the WAIT option with a timeout to specify the maximum amount of time the operation should wait for locks. If the locks are not available within the specified time, the operation will fail.
Create the index in parallel: You can create the index in parallel by using the PARALLEL option. This can help reduce the lock contention and prevent deadlocks.
Use a different index creation method: You can use a different index creation method, such as creating the index in stages or using a different indexing algorithm.

Q: What are some best practices for index creation in DB2?

A: Some best practices for index creation in DB2 include:

Create indexes in stages: Instead of creating a single large index, you can create smaller indexes in stages. This can help reduce the lock contention and prevent deadlocks.
Use parallel indexing: You can use parallel indexing to create indexes in parallel, which can help reduce the lock contention and prevent deadlocks.
Monitor lock contention: You should monitor lock contention and adjust your indexing strategy accordingly.
Test indexing operations: You should test indexing operations in a controlled environment to ensure that they do not cause deadlocks.

Q: What are some DB2 indexing best practices?

A: Some DB2 indexing best practices include:

Use the correct indexing algorithm: You should use the correct indexing algorithm for your specific use case.
Choose the right index type: You should choose the right index type based on your specific use case.
Monitor index performance: You should monitor index performance and adjust your indexing strategy accordingly.
Test indexing operations: You should test indexing operations in a controlled environment to ensure that they do not cause deadlocks.

Q: How can I troubleshoot deadlocks in DB2?

A: To troubleshoot deadlocks in DB2, you can:

Use the DB2ADVISOR tool: The DB2ADVISOR tool can help you identify and troubleshoot deadlocks in your database.
Monitor lock contention: You should monitor lock contention and adjust your indexing strategy accordingly.
Analyze deadlock logs: You should analyze deadlock logs to identify the root cause of the deadlock.
Test indexing operations: You should test indexing operations in a controlled environment to ensure that they do not cause deadlocks.

Q: What are some common mistakes to avoid when creating indexes in DB2?

A: Some common mistakes to avoid when creating indexes in DB2 include:

Creating too many indexes: Creating too many indexes can lead to increased lock contention and deadlocks.
Creating indexes on columns with low selectivity: Creating indexes on columns with low selectivity can lead to increased storage requirements and decreased performance.
Not monitoring lock contention: Not monitoring lock contention can lead to increased deadlocks and decreased performance.
Not testing indexing operations: Not testing indexing operations can lead to unexpected deadlocks and decreased performance.