How does SQL Server handle deadlocks?

SQL Server handles deadlocks in the following ways:

1. Timeout handling: When a transaction is unable to obtain the necessary resources within a certain time frame, SQL Server will automatically terminate it, release the resources, and provide the user with an appropriate error message. This approach can prevent deadlock from occurring, but may impact transactions that are currently in progress.
2. Deadlock detection and rollback: SQL Server uses a deadlock detection algorithm to identify when deadlocks occur, and it will choose one transaction to rollback in order to resolve the deadlock. The transaction selected for rollback is typically the “victim”, meaning the transaction that has the least impact on system load.
3. Priority adjustment for deadlock: SQL Server assigns a priority to each transaction, and in the event of a deadlock, it chooses the transaction with the lower priority to roll back. By adjusting the priority of transactions, the occurrence and resolution order of deadlocks can be influenced.
4. Lock Timeout: SQL Server has the ability to establish a timeout period for locks. If a transaction exceeds the set timeout period while waiting for a lock, SQL Server will automatically terminate the transaction and release the lock resources.
5. Lock escalation and de-escalation: SQL Server can reduce the occurrence of deadlocks by allowing transactions to initially acquire lower level locks and then upgrade to higher level locks when needed. Transactions can also de-escalate locks to lower levels when resources are no longer needed, thus releasing them.

In general, SQL Server uses a series of deadlock handling mechanisms to reduce the occurrence of deadlocks and resolve them as much as possible. However, deadlocks are a complex and common issue that needs to be carefully avoided in both database design and application development.