In some scenarios it is possible for a whole tsunami of key lookup deadlocks to proliferate in unison, which is clearly not ideal, even if it is not a complete disaster.Īs you will now see, there is a similar thread connecting all of the major types of deadlocks, which is that they come down to resource management and locking conflicts, while being separate from standard database locking. The final type of SQL Server deadlock that you should keep an eye out for is a key lookup deadlock, which tends to emerge in the case that a pair of sessions run on a single statement and thus butt heads when attempting to gain an exclusive lock to pinpoint the necessary data. The third is UIX (update with intent exclusive) which arises when update and intent exclusive locks exist as distinct from one another but occur simultaneously. The second is SIX (share with intent exclusive) which takes place when shared and exclusive locks are possessed by a single thread. The first is SIU (share with intent update) which sees a thread with a set of shared locks as well as update locks creating the conflict. It is worth noting that there are actually a trio of sub-categories that exist within the broader umbrella of conversion deadlocks. However, because each holds an exclusive lock on the resource it is using, this exchange cannot take place because it is prevented in software.Ĭycle deadlocks are one of the most common forms you are likely to encounter when managing an SQL Server ecosystem, yet that does not mean they can be overlooked.Īnother deadlock type that is simple enough to grasp from a theoretical standpoint, this conundrum will occur when an attempt to convert lock types is made but a separate thread already has a shared lock on the resource that the first thread requires. In this case, the first process and the second process will already have exclusive locks on two different resources, and will be attempting to swap places. The first type of deadlock that you are likely to encounter is known as a cycle deadlock, and as the name suggests it results from a vicious cycle of resource locking being created between two processes. The software will automatically terminate one of the offending processes to keep the ball rolling, so the sooner you can pinpoint the problem and remedy it, the faster you will be able to restore the server to full operational capacity. This will not bring the entire database grinding to a halt, but it will monopolize resources unnecessarily and compromise performance as a result. In short, a deadlock will rear its ugly head in the event that a pair of processes attempt to vie for control of a particular resource at the same time, without either one being able to achieve exclusive access to said resource. Getting to grips with the concept of deadlock issues that can arise within SQL Server is straightforward. To that end, here is a deep dive into the three main types that you are likely to encounter and the steps you can take to prevent them reoccurring in the future. Of course if you are a newcomer to this field or are just interested in finding out why certain conflicts can arise and what you can do to overcome them, you might want to learn more about the tricky issue of deadlocks. Since wee enabled READ_COMMITTED_SNAPSHOT SQL Server will use versioning instead of locks.Identifying and dealing with SQL server deadlocks is all part and parcel of what a DBA has to handle as part of their role. The isolation level uses shared locking or row versioning to prevent dirty reads, depending on whether the READ_COMMITTED_SNAPSHOT database option is enabled. The default database isolation level in SQL Server is READ_COMMITTED which means a query in the current transaction cannot read data modified by another transaction that has not yet committed, thus preventing dirty reads. To solve this issue we enabled READ_COMMITTED_SNAPSHOT option for the database so that it uses READ_COMMITTED with row versioning isolation strategy. This was causing SQL Server to stop other transactions from accessing the table. When we debugged SQL Server traces we discovered that row-level lock was escalated to table lock. While the file was being uploaded user navigated to another page where data from the same table was queried. The database deadlock was caused when user uploads a file that was being written to the database. Exception is : could not execute query SQL nested exception is : could not execute query] with root causeĬom.: Transaction (Process ID 132) was deadlocked on lock resources with another process and has been chosen as the deadlock victim.
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