What is the best way to handle a deadlock in SQL?

A deadlock is a situation in which two or more users are waiting for data locked by each other. Deadlocks prevent some transactions from continuing to work. Oracle Database automatically detects deadlocks (ORA-00060: deadlock detected while waiting for resource) and resolves them by rolling back one statement involved in the deadlock, releasing one set of the conflicting row locks.

The best way to handle deadlock in SQL is to design your database transactions in a way that minimizes the likelihood of deadlocks occurring. This can be achieved by following best practices such as keeping transactions short and avoiding long-running transactions, using appropriate isolation levels, and ensuring that transactions access resources in a consistent order. Additionally, implementing retry logic in your application code can help to automatically resolve deadlocks when they occur. It is also important to monitor and analyze deadlock occurrences to identify and address any underlying issues in the database design or application logic.

Deadlock is an unwanted situation in which two or more transactions are waiting indefinitely for one another to give up locks. Each transaction needs a resource that another transaction holds, creating a circular dependency. Example: Transaction T1 holds a lock on some rows in the Students table and needs to update rows in the Grades table. Transaction T2 holds locks on the same rows in the Grades table (which T1 needs to update) but needs to update rows in the Students table held by T1. Both transactions wait for each other to release the lock, resulting in a halt of all activity until the deadlock is resolved.

Deadlock occurs when two or more transactions are blocked indefinitely, each waiting for a resource held by the other to be released. This situation creates a deadlock cycle, where no transaction can proceed further, leading to a stalemate. Deadlocks typically arise in systems that use locking mechanisms to control access to resources such as database rows or tables.

Think of a traffic jam, you are surrounded by other cars that are completely stopped, the cars in front of you need the car behind you to move so it can move, but those cars need you to move first. Creating a cyclic dependency. That's a deadlock, its dead because it won't be resolved naturally. One car needs to be removed so the others can move. the DB engine takes care of these simple deadlocks by killing one of the sessions so the others can run.

Oracle, when it detects a deadlock and has to terminate one of the two sessions, does write to the alert log and generates a trace file with detailed information about the two sessions. So, you could setup monitoring to look for the ORA-0060 error message and then grab the trace files.

1. Deadlock can be detected by checking alert log file with ORA-00060 error message. 2. By creating shell script in cronjob for monitoring deadlock. 3. The LMD process also handles deadlock detection Global Enqueue Service requests. 4. We can check for holder and waiter session by dynamic performance views. SELECT DECODE(request, 0, 'Holder: ', 'Waiter: ') || l.sid sess, s.serial#, l.type, s.USERNAME, s.SCHEMANAME, s.status, s.OSUSER, l.CTIME, l.id1, l.id2, l.lmode, l.request FROM gV$LOCK l, gV$session s WHERE (l.id1, l.id2, l.type) IN (SELECT l1.id1, l1.id2, type FROM gV$LOCK l1 WHERE request > 0) and l.sid = s.sid ORDER BY l.id1, l.request;

Detecting deadlocks in SQL typically involves monitoring for specific error messages or using system views and tools provided by the DBMS. Here are some common methods: 1. SQL Server Profiler 2. Extended Events 3. Error Log 4. System Views 5. Database management tools 6. Application Logging By employing these methods, you can effectively detect and diagnose deadlocks in your SQL database environment.

Deadlock detection algorithms: Periodically scan the transaction log for deadlock cycles and choose a transaction to abort.

In SQL Server you can create extended event which ll keep with rollover all deadlocks occurring in the instance after a specific threshold.

Identify which sessions are involved in the deadlock and determine if it's a single deadlock or multiple ones. If there are multiple deadlocks, start by solving the simplest ones first to free resources. For a locking cascade, clearing the longest running one often clears up the subsequent locks. Look at the statements executed. If it's just a simple SELECT, kill the session. For other statements, evaluate the database's condition. Make sure killing the session won't cause more trouble. Keep in mind that resources are only freed after the rollback finishes. Be cautious in clustered setups. Mistakes can lead to bigger issues, especially if secondary databases aren't synchronized properly or storage sizes don't match.

Oracle is smart and it is able to find deadlock situation within few seconds. Below are the options to avoid deadlocks inside the database 1. Ask the session getting deadlock error ORA-00060 to issue either COMMIT or ROLLBACK 2. Ask the waiting session to kill the SQL / transaction 3. Look inside alert log / trace file for the sessions involved in deadlock and inform application team to improve the code 4. Tune the application - Single-threading related updates and other application changes can often remove deadlocks. Re-scheduling batch update jobs to low-update times can also help. As a DBA, whenever you get a Deadlock alert, immediately contact application team and inform them.

In most cases, a deadlock results from two sessions holding (locking) and attempting to hold (lock) the same resources out of sequence. If this happens frequently, an application change (to re-order the "lock the resource" attempt) may be required. This could be a user-workflow change or a code change.

One way to handle deadlocks in SQL is to implement retry logic. When a deadlock occurs, catch the error, wait for a brief period, and then retry the transaction. This approach helps alleviate deadlocks by allowing transactions to proceed after a delay. Now you must be wondering what is retry logic? Retry logic is a technique used to handle transient failures or errors by automatically retrying an operation that failed. In the context of handling deadlocks in SQL, retry logic involves catching deadlock errors, waiting for a short period, and then attempting to re-execute the transaction. This process is repeated for a certain number of times or until the transaction succeeds, helping to mitigate the impact of deadlocks on the application.

Deadlock avoidance: Preferable for smaller databases. Use application-consistent logic to avoid circular dependencies. Deadlock prevention: Suitable for larger databases. Design transactions to acquire locks in a consistent order.Release locks as soon as possible. Remember, handling deadlocks is crucial for maintaining a reliable and efficient database system!

Keep transactions short. Access objects in the same order. Use indexes wisely. Avoid lock escalation. Optimize isolation levels. Use lock hints. Implement retry logic. Monitor and analyze deadlocks regularly. By implementing these strategies and continuously monitoring and optimizing your SQL Server environment, you can reduce the likelihood of deadlocks and ensure better performance and reliability.

Keep Transactions Short:Minimize the duration of transactions.Gather all necessary information before starting a transaction. Choose the Right Isolation Level:For example, if you only need to temporarily display values to the user, avoid setting the isolation level to “serializable”. Access Objects in the Same Order:Define a consistent order for accessing database objects (tables, rows, etc.). Avoid User Interaction Within Transactions: Transactions should not wait for user input. Keep Transactions in One Batch:Avoid breaking transactions into smaller parts. Execute related operations within a single transaction to minimize the chances of deadlocks.

The best way to avoid deadlocks isto follow one of most important ACID rules for OLTP: keep your transactions atomic. The other, very important thing is don't allow your application developer to write stupid ad-hoc LINQ queries. Protect your database at least with ownership chaining (sql server) and give them controled access.

I have seen some companies simply increase computing power or other resources in order to solve potential deadlocks. This is not always the best solution since deadlocks are usually caused by bad logic or poor planning in the application that tries to access the resources.