最近发现一个非常奇怪的阻塞问题，如下截图所示（来自监控工具DPA），会话583被会话1036阻塞，而且阻塞发生在tempdb，被阻塞的SQL如下截图所示，会话等待类型为LCK_M_S
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因为DPA工具不好截取全部信息，使用下面SQL语句获取了阻塞会话的详细信息如下，来自Microsoft SQL Server Management Studio - Transact-SQL IntelliSense的的SQL被阻塞了
SELECT s.session_id ,
r.status ,
r.blocking_session_id 'Blk by' ,
r.wait_type ,
wait_resource ,
r.wait_time / ( 1000 * 60 ) 'Wait M' ,
r.cpu_time ,
r.logical_reads ,
r.reads ,
r.writes ,
r.total_elapsed_time / ( 1000 * 60 ) 'Elaps M' ,
SUBSTRING(st.text, ( r.statement_start_offset / 2 ) + 1,
( ( CASE r.statement_end_offset
WHEN -1 THEN DATALENGTH(st.text)
ELSE r.statement_end_offset
END - r.statement_start_offset ) / 2 ) + 1) AS statement_text ,
COALESCE(QUOTENAME(DB_NAME(st.dbid)) + N'.'
+ QUOTENAME(OBJECT_SCHEMA_NAME(st.objectid, st.dbid)) + N'.'
+ QUOTENAME(OBJECT_NAME(st.objectid, st.dbid)), '') AS command_text ,
r.command ,
s.login_name ,
s.host_name ,
s.program_name ,
s.last_request_end_time ,
s.login_time ,
r.open_transaction_count
FROM sys.dm_exec_sessions AS s
JOIN sys.dm_exec_requests AS r ON r.session_id = s.session_id
CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) AS st
WHERE r.session_id =583
ORDER BY r.cpu_time DESC;
而会话1036处于睡眠（sleeping）状态，有一个Open的事务。通过wait_resource的值“KEY: 2:1125899909070848 (79c68d10aa42)”，我们定位到是一个系统表sysschobj.它是sys.triggers这个系统视图的基表。如下所示：
SET QUOTED_IDENTIFIER ON
SET ANSI_NULLS ON
GO
CREATE VIEW sys.triggers AS
SELECT o.name,
object_id = o.id,
parent_class = o.pclass,
parent_class_desc = pc.name,
parent_id = o.pid,
type = o.type,
type_desc = n.name,
create_date = o.created,
modify_date = o.modified,
is_ms_shipped = sysconv(bit, o.status & 1), -- OBJALL_MSSHIPPED
is_disabled = sysconv(bit, o.status & 256), -- OBJTRG_DISABLED
is_not_for_replication = sysconv(bit, o.status & 512), -- OBJTRG_NOTFORREPL
is_instead_of_trigger = sysconv(bit, o.status & 1024) -- OBJTRG_INSTEADOF
FROM sys.sysschobjs o
LEFT JOIN sys.syspalnames n ON n.class = 'OBTY' AND n.value = o.type
LEFT JOIN sys.syspalvalues pc ON pc.class = 'UNCL' AND pc.value = o.pclass
WHERE o.type IN ('TA','TR') AND o.pclass <> 100
AND has_access('TR', o.id, o.pid, o.nsclass) = 1
GO
使用下面脚本可以查到对应的锁信息（其实，正确的做法应该是加上条件过滤spid），从而可以判断，会话1036可能因为事务中，间接涉及对基表sysschobj的DML操作（有可能是衍生的系统视图），而由于事务没有提交也没有释放，导致在基表上的锁没有释放，从而导致出现这样一个阻塞。个人也很好奇是什么样的SQL会导致这样一个阻塞出现，不过这种阻塞源头是Sleeping状态的会话，已经无法定位阻塞源头SQL，只能通过Profile去跟踪捕获。但是这种阻塞出现时机和场景不清楚，很难一下子捕获到。
USE tempdb
GO
SELECT request_session_id AS spid ,
DB_NAME(resource_database_id) AS dbname ,
CASE WHEN resource_type = 'OBJECT'
THEN OBJECT_NAME(resource_associated_entity_id)
WHEN resource_associated_entity_id = 0 THEN 'n/a'
ELSE OBJECT_NAME(p.object_id)
END AS entity_name ,
index_id ,
resource_type AS resource ,
resource_description AS description ,
request_mode AS mode ,
request_status AS status
FROM sys.dm_tran_locks t
LEFT JOIN sys.partitions p
ON p.partition_id = t.resource_associated_entity_id
WHERE resource_database_id = DB_ID()
今天早上有发现同样的阻塞，猜测是SQL里面有创建临时表或对临时表做DDL或DML操作的迹象。不过依然没有找到源头SQL，只能静候下次机会。找出“罪魁祸首”。不过让我头痛的是，对于这种特殊阻塞情况，即使是使用Profile跟踪也是相当麻烦，因为不知道它什么时间点出现、出现在那个会话。
SELECT tl.resource_type AS [Resource Type] ,
DB_NAME(tl.resource_database_id) AS [DB Name] ,
CASE tl.resource_type
WHEN 'OBJECT'
THEN OBJECT_NAME(tl.resource_associated_entity_id,
tl.resource_database_id)
WHEN 'DATABASE' THEN 'DB'
ELSE CASE WHEN tl.resource_database_id = DB_ID()
THEN ( SELECT OBJECT_NAME(object_id,
tl.resource_database_id)
FROM sys.partitions
WHERE hobt_id = tl.resource_associated_entity_id
)
ELSE '(Run under DB context)'
END
END AS [Object] ,
tl.resource_description AS [Resource] ,
tl.request_session_id AS [Session] ,
tl.request_mode AS [Mode] ,
tl.request_status AS [Status] ,
wt.wait_duration_ms AS [Wait (ms)] ,
qi.sql ,
qi.query_plan
FROM sys.dm_tran_locks tl WITH ( NOLOCK )
LEFT OUTER JOIN sys.dm_os_waiting_tasks wt WITH ( NOLOCK )
ON tl.lock_owner_address = wt.resource_address
AND tl.request_status = 'WAIT'
OUTER APPLY ( SELECT SUBSTRING(s.text,
( er.statement_start_offset / 2 ) + 1,
( ( CASE er.statement_end_offset
WHEN -1 THEN DATALENGTH(s.text)
ELSE er.statement_end_offset
END - er.statement_start_offset ) / 2 )
+ 1) AS sql ,
qp.query_plan
FROM sys.dm_exec_requests er WITH ( NOLOCK )
CROSS APPLY sys.dm_exec_sql_text(er.sql_handle) s
OUTER APPLY sys.dm_exec_query_plan(er.plan_handle) qp
WHERE tl.request_session_id = er.session_id
) qi
WHERE tl.request_session_id =134
