pointers, solutions and scripts for the SQL DBA
not intended to replace common sense

10/19/2011

Using sys.dm_exec_requests to find a blocking chain

A query that calls sys.dm_exec_requests and pulls the statement text out of the request is a handy script for any DBA's toolbox.  A simple version of the query performs a cross apply to dm_exec_sql_text and pulls out the request's TSQL statement-in-progress using the unicode character offsets.

This is simple enough and does a good job as a queryable sp_who2 and DBCC INPUTBUFFER replacement to an extent.

In an environment that suffers from frequent spells of query blocking, the ability to identify which query was leading the way becomes important in finding the query or queries to troubleshoot.

Whether the end result of his query was a simple KILL command, and/or a walk down the hall to investigate the "business case" would be a situational decision.

I wanted a blocking chain - a list of all other spids that a session was blocking.

Here's a query that dumps the output of dm_exec_requests into a temporary table, then uses the XML PATH to conveniently transform the blocked_by list into a comma-delimited list in the row of the blocker.  The  sort order is the length of this comma-delimited list, putting the cause of a pileup right at the top.

Doing this in a temp table was necessary because the blocking_session_id is a highly volatile column.  As a result, the output of this query is several ms behind "live".  After a few executions, the spid hanging out at the top is easy enough to spot.

Here's an example of the nightmare kind of scenario that prompted this script into existence:




 declare @showallspids bit 
 select @showallspids =1

 create table #ExecRequests  (
  id int IDENTITY(1,1) PRIMARY KEY
 , session_id smallint not null 
 , request_id int null
 , request_start_time datetime null
 , login_time datetime not null
 , login_name nvarchar(256) null
 , client_interface_name nvarchar(64)
 , session_status nvarchar(60) null
 , request_status nvarchar(60) null
 , command nvarchar(32) null
 , sql_handle varbinary(64) null
 , statement_start_offset int null
 , statement_end_offset int null
 , plan_handle varbinary (64) null
 , database_id smallint null
 , user_id int null
 , blocking_session_id smallint null
 , wait_type nvarchar (120) null
 , wait_time_s int null
 , wait_resource nvarchar(120) null
 , last_wait_type nvarchar(120) null
 , cpu_time_s int null
 , tot_time_s int null
 , reads bigint null
 , writes bigint null
 , logical_reads bigint null
 , [host_name] nvarchar(256) null
 , [program_name] nvarchar(256) null
 , blocking_these varchar(1000) NULL
 , percent_complete int null
 , session_transaction_isolation_level varchar(20) null
 , request_transaction_isolation_level varchar(20) null
 )

 insert into #ExecRequests (session_id,request_id, request_start_time, login_time, login_name, client_interface_name, session_status, request_status, command,sql_handle,statement_start_offset,statement_end_offset,plan_handle,database_id,user_id,blocking_session_id,wait_type,last_wait_type,wait_time_s,wait_resource,cpu_time_s,tot_time_s,reads,writes,logical_reads,[host_name], [program_name] , session_transaction_isolation_level , request_transaction_isolation_level )
       select s.session_id,request_id, r.start_time, s.login_time, s.login_name, s.client_interface_name, s.status, r.status,command,sql_handle,statement_start_offset,statement_end_offset,plan_handle,r.database_id,user_id,blocking_session_id,wait_type,r.last_wait_type, r.wait_time/1000.,r.wait_resource ,r.cpu_time/1000.,r.total_elapsed_time/1000.,r.reads,r.writes,r.logical_reads,s.[host_name], s.[program_name], s.transaction_isolation_level, r.transaction_isolation_level
 from sys.dm_exec_sessions s 
 left outer join sys.dm_exec_requests r on r.session_id = s.session_id
 where 1=1
 and s.session_id >= 50 --retrieve only user spids
 and s.session_id <> @@SPID --ignore myself
 and  (@showallspids = 1 or r.session_id is not null) 

 update #ExecRequests 
 set blocking_these = LEFT((select isnull(convert(varchar(5), er.session_id),'') + ', ' 
       from #ExecRequests er
       where er.blocking_session_id = isnull(#ExecRequests.session_id ,0)
       and er.blocking_session_id <> 0
       FOR XML PATH('') 
       ),1000)
 select * from 
 (
  select  
   timestamp = getdate()
  , r.session_id , r.host_name , r.program_name
  , r.session_status
  , r.request_status
  , r.blocking_these
  , blocked_by =  r.blocking_session_id
  , r.wait_type 
  , r.wait_resource
  , r.last_wait_type
  , DBName = db_name(r.database_id)
  , r.command
  , login_time
  , login_name
  , client_interface_name
  , request_start_time
  , r.tot_time_s, r.wait_time_s, r.cpu_time_s, r.reads, r.writes, r.logical_reads
  --, [fulltext] = est.[text]
  , offsettext = CASE WHEN r.statement_start_offset = 0 and r.statement_end_offset= 0 THEN NULL
         ELSE SUBSTRING (  est.[text]
             , r.statement_start_offset/2 + 1, 
              CASE WHEN r.statement_end_offset = -1 THEN LEN (CONVERT(nvarchar(max), est.[text])) 
               ELSE r.statement_end_offset/2 - r.statement_start_offset/2 + 1
              END )
       END
  , r.statement_start_offset, r.statement_end_offset
  , cacheobjtype = LEFT (p.cacheobjtype + ' (' + p.objtype + ')', 35)
  , QueryPlan  = qp.query_plan 
  , request_transaction_isolation_level = case request_transaction_isolation_level 
    when 0 then 'Unspecified'
    when 1 then 'ReadUncommitted'
    when 2 then 'ReadCommitted'
    when 3 then 'Repeatable'
    when 4 then 'Serializable'
    when 5 then 'Snapshot' end 
  , session_transaction_isolation_level = case session_transaction_isolation_level 
    when 0 then 'Unspecified'
    when 1 then 'ReadUncommitted'
    when 2 then 'ReadCommitted'
    when 3 then 'Repeatable'
    when 4 then 'Serializable'
    when 5 then 'Snapshot' end 
  , p.plan_handle
  , stat.execution_count, stat.total_worker_time, stat.last_worker_time, stat.total_elapsed_time, stat.last_elapsed_time, stat.total_physical_reads, stat.total_logical_writes, stat.total_logical_reads
  --next two lines are SQL 2012 only!
  --, stat.total_rows, stat.last_rows
  from #ExecRequests r
  LEFT OUTER JOIN sys.dm_exec_cached_plans p ON p.plan_handle = r.plan_handle 
  OUTER APPLY sys.dm_exec_query_plan (r.plan_handle) qp
  OUTER APPLY sys.dm_exec_sql_text (r.sql_handle) est
  LEFT OUTER JOIN sys.dm_exec_query_stats stat on stat.plan_handle = r.plan_handle
  and r.statement_start_offset = stat.statement_start_offset  
  and r.statement_end_offset = stat.statement_end_offset
  
 
 ) a
 order by LEN(blocking_these) desc, blocking_these desc, blocked_by desc, session_id asc
 go
 drop table #ExecRequests  

Updated 20140813: latest tweaks and changes Updated 20140929: latest tweaks and changes

1 comment:

  1. See this blog post and comments at SQLServerCentral.com:
    http://www.sqlservercentral.com/blogs/sqltact/archive/2011/10/19/using-sys.dm_5F00_exec_5F00_requests-to-find-a-blocking-chain.aspx

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