SQL Server一個特殊的阻塞案例分析2

最近發現一個非常奇怪的阻塞問題，如下截圖所示（來自監控工具DPA），會話583被會話1036阻塞，而且阻塞發生在tempdb，被阻塞的SQL如下截圖所示，會話等待類型為LCK_M_S

因為DPA工具不好截取全部信息，使用下麵SQL語句獲取了阻塞會話的詳細信息如下，來自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;

而會話1036處於睡眠（sleeping）狀態，有一個Open的事務。通過wait_resource的值“KEY: 2:1125899909070848 (79c68d10aa42)”，我們定位到是一個系統表sysschobj.它是sys.triggers這個系統視圖的基表。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

使用下麵腳本可以查到對應的鎖信息（其實，正確的做法應該是加上條件過濾spid），從而可以判斷，會話1036可能因為事務中，間接涉及對基表sysschobj的DML操作（有可能是衍生的系統視圖），而由於事務沒有提交也沒有釋放，導致在基表上的鎖沒有釋放，從而導致出現這樣一個阻塞。個人也很好奇是什麼樣的SQL會導致這樣一個阻塞出現，不過這種阻塞源頭是Sleeping狀態的會話，已經無法定位阻塞源頭SQL，只能通過Profile去跟蹤捕獲。但是這種阻塞出現時機和場景不清楚，很難一下子捕獲到。

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()