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How to: Determine Which Queries Are Holding Locks

Database administrators often need to identify the source of locks that are hindering database performance.

For example, you suspect that a performance issue on your server could be caused by blocking. When you query the sys.dm_exec_requests, you find several sessions in a suspended mode with a wait type indicating that a lock is the resource being waited on.

You query sys.dm_tran_locks and the results show that there are many locks outstanding, but the sessions that were granted the locks do not have any active requests showing in sys.dm_exec_requests.

This example demonstrates a method of determining what query took the lock, the plan of the query, and the Transact-SQL stack at the time the lock was taken. This example also illustrates how the pairing target is used in an Extended Events session.

Accomplishing this task involves using Query Editor in SQL Server Management Studio to carry out the following procedure.

Note

This example uses the AdventureWorks2008R2 database.

To determine which queries are holding locks

  1. In Query Editor, issue the following statements.

    -- Perform cleanup. 
IF EXISTS(SELECT * FROM sys.server_event_sessions WHERE name='FindBlockers')
    DROP EVENT SESSION FindBlockers ON SERVER
GO
-- Use dynamic SQL to create the event session and allow creating a -- predicate on the AdventureWorks2008R2 database id.
--
DECLARE @dbid int

SELECT @dbid = db_id('AdventureWorks2008R2')

IF @dbid IS NULL
BEGIN
    RAISERROR('AdventureWorks2008R2 is not installed. Install AdventureWorks2008R2 before proceeding', 17, 1)
    RETURN
END

DECLARE @sql nvarchar(1024)
SET @sql = '
CREATE EVENT SESSION FindBlockers ON SERVER
ADD EVENT sqlserver.lock_acquired 
    (action 
        ( sqlserver.sql_text, sqlserver.database_id, sqlserver.tsql_stack,
         sqlserver.plan_handle, sqlserver.session_id)
    WHERE ( database_id=' + cast(@dbid as nvarchar) + ' AND resource_0!=0) 
    ),
ADD EVENT sqlserver.lock_released 
    (WHERE ( database_id=' + cast(@dbid as nvarchar) + ' AND resource_0!=0 ))
ADD TARGET package0.pair_matching 
    ( SET begin_event=''sqlserver.lock_acquired'', 
            begin_matching_columns=''database_id, resource_0, resource_1, resource_2, transaction_id, mode'', 
            end_event=''sqlserver.lock_released'', 
            end_matching_columns=''database_id, resource_0, resource_1, resource_2, transaction_id, mode'',
    respond_to_memory_pressure=1)
WITH (max_dispatch_latency = 1 seconds)'

EXEC (@sql)
-- 
-- Create the metadata for the event session
-- Start the event session
--
ALTER EVENT SESSION FindBlockers ON SERVER
STATE = START

  2. After execution of a workload on the server, issue the following statements in Query Editor to find queries still holding locks.

    --
-- The pair matching targets report current unpaired events using 
-- the sys.dm_xe_session_targets dynamic management view (DMV)
-- in XML format.
-- The following query retrieves the data from the DMV and stores
-- key data in a temporary table to speed subsequent access and
-- retrieval.
--
SELECT 
objlocks.value('(action[@name="session_id"]/value)[1]', 'int')
        AS session_id,
    objlocks.value('(data[@name="database_id"]/value)[1]', 'int') 
        AS database_id,
    objlocks.value('(data[@name="resource_type"]/text)[1]', 'nvarchar(50)' ) 
        AS resource_type,
    objlocks.value('(data[@name="resource_0"]/value)[1]', 'bigint') 
        AS resource_0,
    objlocks.value('(data[@name="resource_1"]/value)[1]', 'bigint') 
        AS resource_1,
    objlocks.value('(data[@name="resource_2"]/value)[1]', 'bigint') 
        AS resource_2,
    objlocks.value('(data[@name="mode"]/text)[1]', 'nvarchar(50)') 
        AS mode,
    objlocks.value('(action[@name="sql_text"]/value)[1]', 'varchar(MAX)') 
        AS sql_text,
    CAST(objlocks.value('(action[@name="plan_handle"]/value)[1]', 'varchar(MAX)') AS xml) 
        AS plan_handle,    
    CAST(objlocks.value('(action[@name="tsql_stack"]/value)[1]', 'varchar(MAX)') AS xml) 
        AS tsql_stack
INTO #unmatched_locks
FROM (
    SELECT CAST(xest.target_data as xml) 
        lockinfo
    FROM sys.dm_xe_session_targets xest
    JOIN sys.dm_xe_sessions xes ON xes.address = xest.event_session_address
    WHERE xest.target_name = 'pair_matching' AND xes.name = 'FindBlockers'
) heldlocks
CROSS APPLY lockinfo.nodes('//event[@name="lock_acquired"]') AS T(objlocks)

--
-- Join the data acquired from the pairing target with other 
-- DMVs to return provide additional information about blockers
--
SELECT ul.*
    FROM #unmatched_locks ul
    INNER JOIN sys.dm_tran_locks tl ON ul.database_id = tl.resource_database_id AND ul.resource_type = tl.resource_type
    WHERE resource_0 IS NOT NULL
    AND session_id IN 
        (SELECT blocking_session_id FROM sys.dm_exec_requests WHERE blocking_session_id != 0)
    AND tl.request_status='wait'
    AND REPLACE(ul.mode, 'LCK_M_', '' ) = tl.request_mode

  3. After identifying the issues, drop any temporary tables and the event session.

    DROP TABLE #unmatched_locks
DROP EVENT SESSION FindBlockers ON SERVER