Information About Waits Stats in SQL Server Waits – Part 5


SQL Server internally uses worker threads to execute SQL queries sent by various applications. While threads are executing they may be either running on CPU processing the request or would be waiting on a certain resource or waiting in waiting for its chance to run on CPU. SQL Server assigns a certain wait type to the worker thread that is waiting.

If a query is taking long time to complete, we can look at the query sessions wait type to get an understanding of what kind of resource it is the query waiting for and take appropriate action to avoid the queries to wait, thus making the queries complete fast.

Below are some of the SQL Server Wait Types related to Deadlock waits in SQL Server, Waits related to SQL Server Versioning.

Occurs when the deadlock monitor and sys.dm_os_waiting_tasks try to make sure that SQL Server is not running multiple deadlock searches at the same time.

Large waiting time on this resource indicates that the server is executing queries on top of sys.dm_os_waiting_tasks, and these queries are blocking deadlock monitor from running deadlock search. This wait type is used by deadlock monitor only. Queries on top of sys.dm_os_waiting_tasks use DEADLOCK_ENUM_MUTEX.

Occurs during Transact-SQL and CLR debugging for internal synchronization.

Occurs when SQL Server polls the version transaction manager to see whether the timestamp of the earliest active transaction is later than the timestamp of when the state started changing. If this is this case, all the snapshot transactions that were started before the ALTER DATABASE statement was run have finished. This wait state is used when SQL Server disables versioning by using the ALTER DATABASE statement.

Occurs when SQL Server waits for all update transactions in this database to finish before declaring the database ready to transition to snapshot isolation allowed state. This state is used when SQL Server enables snapshot isolation by using the ALTER DATABASE statement.

Occurs when a task is waiting to access a file when an external backup is active. This is reported for each waiting user process. A count larger than five per user process may indicate that the external backup is taking too much time to finish.

Occurs when a thread from the dispatcher pool is waiting for more work to process. The wait time for this wait type is expected to increase when the dispatcher is idle.

Occurs once while waiting for the XML parser DLL to load.

Occurs between attempts to drop a temporary object if the previous attempt failed. The wait duration grows exponentially with each failed drop attempt.

This is applicable on below versions of SQL Server

SQL Server 2005
SQL Server 2008 R2
SQL Server 2012
SQL Server 2014

Hope this was helpful.

SQLServerF1 Team
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