SQL Server is a powerful relational database management system that is widely used in various industries. One common issue that database administrators often encounter is blocking, which occurs when one transaction holds a lock on a resource that another transaction needs, causing the second transaction to wait.

In order to identify and resolve blocking issues, there are several methods and scripts available. One popular script is the Blocking Tree TSQL script, which provides a visual representation of the blocking relationships between different sessions.

Here is an example of how the Blocking Tree TSQL script can be used:

SET NOCOUNT ON

SELECT SPID, BLOCKED, REPLACE(REPLACE(T.TEXT, CHAR(10), ' '), CHAR(13), ' ') AS BATCH
INTO #T
FROM sys.sysprocesses R
CROSS APPLY sys.dm_exec_sql_text(R.SQL_HANDLE) T

WITH BLOCKERS (SPID, BLOCKED, LEVEL, BATCH) AS (
    SELECT SPID, BLOCKED, CAST(REPLICATE('0', 4 - LEN(CAST(SPID AS VARCHAR))) + CAST(SPID AS VARCHAR) AS VARCHAR(1000)) AS LEVEL, BATCH
    FROM #T R
    WHERE (BLOCKED = 0 OR BLOCKED = SPID)
    AND EXISTS (
        SELECT *
        FROM #T R2
        WHERE R2.BLOCKED = R.SPID
        AND R2.BLOCKED <> R2.SPID
    )
    UNION ALL
    SELECT R.SPID, R.BLOCKED, CAST(BLOCKERS.LEVEL + RIGHT(CAST((1000 + R.SPID) AS VARCHAR(100)), 4) AS VARCHAR(1000)) AS LEVEL, R.BATCH
    FROM #T AS R
    INNER JOIN BLOCKERS ON R.BLOCKED = BLOCKERS.SPID
    WHERE R.BLOCKED > 0
    AND R.BLOCKED <> R.SPID
)

SELECT N'    ' + REPLICATE(N'|         ', LEN(LEVEL)/4 - 1) + CASE WHEN (LEN(LEVEL)/4 - 1) = 0 THEN 'HEAD -  ' ELSE '|------  ' END + CAST(SPID AS NVARCHAR(10)) + N' ' + BATCH AS BLOCKING_TREE
FROM BLOCKERS
ORDER BY LEVEL ASC

DROP TABLE #T

The output of the script will display a hierarchical representation of the blocking relationships, with each session represented by its SPID (Session ID) and the corresponding SQL batch that is being executed. The script uses a recursive CTE (Common Table Expression) to traverse the blocking relationships and generate the tree structure.

By analyzing the blocking tree, database administrators can easily identify the sessions that are causing the blocking and take appropriate actions to resolve the issue. This can include optimizing queries, adjusting transaction isolation levels, or implementing proper indexing strategies.

It’s important to note that the Blocking Tree TSQL script is just one of many tools available for troubleshooting and resolving blocking issues in SQL Server. Depending on the specific scenario, other methods such as SQL Server Profiler, Extended Events, or dynamic management views (DMVs) may also be useful.

In conclusion, understanding and effectively managing blocking in SQL Server is crucial for maintaining optimal performance and ensuring smooth operation of your database applications. The Blocking Tree TSQL script provides a valuable visualization of the blocking relationships, making it easier to identify and resolve blocking issues.

Have you used the Blocking Tree TSQL script before? Share your experiences and any modifications you made to enhance its functionality in the comments below!