Menu
Once in a while I get following error in production enviornment which goes away on running the same stored procedure again.
Transaction (Process ID 86) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction
Someone told me that if I use NOLOCK hint in my stored procedures, it will ensure it will never be deadlocked. Is this correct? Are there any better ways of handling this error?
511
The benefits of querying data using the NOLOCK table hint is that it requires less memory and prevents deadlocks from occurring with any other queries that may be reading similar data.
The WITH (NOLOCK) table hint is used to override the default transaction isolation level of the table or the tables within the view in a specific query, by allowing the user to retrieve the data without being affected by the locks, on the requested data, due to another process that is changing it.
Deadlock can be prevented by eliminating any of the four necessary conditions, which are mutual exclusion, hold and wait, no preemption, and circular wait. Mutual exclusion, hold and wait and no preemption cannot be violated practically. Circular wait can be feasibly eliminated by assigning a priority to each resource.
Useful ways to avoid and minimize SQL Server deadlocksTry to keep transactions short; this will avoid holding locks in a transaction for a long period of time. Access objects in a similar logical manner in multiple transactions. Create a covering index to reduce the possibility of a deadlock.
Occasional deadlocks on an RDBMS that locks like SQL Server/Sybase are expected.
You can code on the client to retry as recommended my MSDN "Handling Deadlocks". Basically, examine the SQLException and maybe a half second later, try again.
Otherwise, you should review your code so that all access to tables are in the same order. Or you can use SET DEADLOCK_PRIORITY to control who becomes a victim.
On MSDN for SQL Server there is "Minimizing Deadlocks" which starts
Although deadlocks cannot be completely avoided
This also mentions "Use a Lower Isolation Level" which I don't like (same as many SQL types here on SO) and is your question. Don't do it is the answer... :-)
Note: MVCC type RDBMS (Oracle, Postgres) don't have this problem. See http://en.wikipedia.org/wiki/ACID#Locking_vs_multiversioning but MVCC has other issues.
180
While adding NOLOCK can prevent readers and writers from blocking each other (never mind all of the negative side effects it has), it is not a magical fix for deadlocks. Many deadlocks have nothing at all to do with reading data, so applying NOLOCK to your read queries might not cause anything to change at all. Have you run a trace and examined the deadlock graph to see exactly what the deadlock is? This should at least let you know which part of the code to look at. For example, is the stored procedure deadlocking because it is being called by multiple users concurrently, or is it deadlocking with a different piece of code?
43
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
