January 7, 2023 By Matthew Rathbone *

A deadlock in SQL occurs when two or more transactions are waiting for each other to release locks on database resources, causing a standstill in the execution of the transactions. This can happen when each transaction acquires locks on different resources, and then attempts to acquire a lock on a resource that the other transaction has already locked. As a result, neither transaction can proceed, and the system becomes “deadlocked”.

Avoiding deadlocks

To avoid deadlocks, it is important for database designers to carefully consider the order in which transactions acquire locks on resources. This can be done through the use of lock escalation, where a transaction acquires locks on multiple resources at once, rather than acquiring locks on individual resources one at a time. Additionally, the use of timeouts can be effective in preventing deadlocks, as they allow a transaction to give up waiting for a locked resource after a certain amount of time, and move on to another task.

Detecting deadlocks

There are several ways to detect and resolve deadlocks in SQL. One method is to use a deadlock detection algorithm, which monitors the state of transactions and their locks on resources, and identifies situations where a deadlock is likely to occur. Once a deadlock is detected, the algorithm can take steps to resolve the deadlock, such as rolling back one of the transactions and releasing its locks on resources.

Lock escalation

Another approach to dealing with deadlocks is to use lock escalation. This involves acquiring locks on multiple resources at once, rather than acquiring locks on individual resources one at a time. This can help to prevent deadlocks by reducing the number of times that a transaction needs to acquire locks on different resources.

SQL deadlocks summary

In summary, a deadlock in SQL is a situation where two or more transactions are waiting for each other to release locks on database resources, causing a standstill in the execution of the transactions. To avoid deadlocks, it is important to carefully consider the order in which transactions acquire locks on resources, and to use techniques such as lock escalation and timeouts. Deadlocks can be detected and resolved using deadlock detection algorithms and other methods.