Lesson Learned #22: How to identify blocking issues?

Today, we were working on a service request where our customer reported that, some TRUNCATE executions are taking more time that they expected. Normally, these TRUNCATE commands just only take 3 or 5 seconds to complete, however, this time, the problem is that they never finish.

Using the following TSQL command:

select conn.session_id as blockerSession,conn2.session_id as BlockedSession,req.wait_time as Waiting_Time_ms,cast((req.wait_time/1000.) as decimal(18,2)) as Waiting_Time_secs,
cast((req.wait_time/1000./60.) as decimal(18,2)) as Waiting_Time_mins,t.text as BlockerQuery,t2.text as BlockedQuery, req.wait_type from sys.dm_exec_requests as req
inner join sys.dm_exec_connections as conn on req.blocking_session_id=conn.session_id
inner join sys.dm_exec_connections as conn2 on req.session_id=conn2.session_id
cross apply sys.dm_exec_sql_text(conn.most_recent_sql_handle) as t
cross apply sys.dm_exec_sql_text(conn2.most_recent_sql_handle) as t2


We identified that these TRUNCATE commands are waiting for another query that is reading the data/is hanging. Basically, killing the session using KILL TSQL command all those TRUNCATE commands were completed successfully.

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