Sessions can exists without an active request, but still block other sessions.
Consider if you have one window open in SSMS where you run this:
BEGIN TRANSACTION
INSERT INTO dbo.SomeTable DEFAULT VALUES;
Then in another window you run:
BEGIN TRANSACTION
SELECT * FROM dbo.SomeTable;
The first session will be holding locks on dbo.SomeTable
, without it showing up in sys.dm_exec_requests
. You will see the session in sys.dm_exec_sessions
, and you'll be able to see the open transaction in sys.dm_tran_session_transactions
.
I'd recommend Adam Machanic's sp_whoisactive, however if you aren't allowed to download scripts to run against your server, you may consider something like the following code that can identify open transactions without an active request that are blocking other sessions:
/*
This query shows sessions that are blocking other sessions, including sessions that are
not currently processing requests (for instance, they have an open, uncommitted transaction).
By: Max Vernon, 2017-03-20
*/
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; --reduce possible blocking by this query.
USE tempdb;
IF OBJECT_ID('tempdb..#dm_tran_session_transactions') IS NOT NULL
DROP TABLE #dm_tran_session_transactions;
SELECT *
INTO #dm_tran_session_transactions
FROM sys.dm_tran_session_transactions;
IF OBJECT_ID('tempdb..#dm_exec_connections') IS NOT NULL
DROP TABLE #dm_exec_connections;
SELECT *
INTO #dm_exec_connections
FROM sys.dm_exec_connections;
IF OBJECT_ID('tempdb..#dm_os_waiting_tasks') IS NOT NULL
DROP TABLE #dm_os_waiting_tasks;
SELECT *
INTO #dm_os_waiting_tasks
FROM sys.dm_os_waiting_tasks;
IF OBJECT_ID('tempdb..#dm_exec_sessions') IS NOT NULL
DROP TABLE #dm_exec_sessions;
SELECT *
INTO #dm_exec_sessions
FROM sys.dm_exec_sessions;
IF OBJECT_ID('tempdb..#dm_exec_requests') IS NOT NULL
DROP TABLE #dm_exec_requests;
SELECT *
INTO #dm_exec_requests
FROM sys.dm_exec_requests;
;WITH IsolationLevels AS
(
SELECT v.*
FROM (VALUES
(0, 'Unspecified')
, (1, 'Read Uncomitted')
, (2, 'Read Committed')
, (3, 'Repeatable')
, (4, 'Serializable')
, (5, 'Snapshot')
) v(Level, Description)
)
, trans AS
(
SELECT dtst.session_id
, blocking_sesion_id = 0
, Type = 'Transaction'
, QueryText = dest.text
FROM #dm_tran_session_transactions dtst
LEFT JOIN #dm_exec_connections dec ON dtst.session_id = dec.session_id
OUTER APPLY sys.dm_exec_sql_text(dec.most_recent_sql_handle) dest
)
, tasks AS
(
SELECT dowt.session_id
, dowt.blocking_session_id
, Type = 'Waiting Task'
, QueryText = dest.text
FROM #dm_os_waiting_tasks dowt
LEFT JOIN #dm_exec_connections dec ON dowt.session_id = dec.session_id
OUTER APPLY sys.dm_exec_sql_text(dec.most_recent_sql_handle) dest
WHERE dowt.blocking_session_id IS NOT NULL
)
, requests AS
(
SELECT des.session_id
, der.blocking_session_id
, Type = 'Session Request'
, QueryText = dest.text
FROM #dm_exec_sessions des
INNER JOIN #dm_exec_requests der ON des.session_id = der.session_id
OUTER APPLY sys.dm_exec_sql_text(der.sql_handle) dest
WHERE der.blocking_session_id IS NOT NULL
AND der.blocking_session_id > 0
)
, Agg AS (
SELECT SessionID = tr.session_id
, ItemType = tr.Type
, CountOfBlockedSessions = (SELECT COUNT(*) FROM requests r WHERE r.blocking_session_id = tr.session_id)
, BlockedBySessionID = tr.blocking_sesion_id
, QueryText = tr.QueryText
FROM trans tr
WHERE EXISTS (
SELECT 1
FROM requests r
WHERE r.blocking_session_id = tr.session_id
)
UNION ALL
SELECT ta.session_id
, ta.Type
, CountOfBlockedSessions = (SELECT COUNT(*) FROM requests r WHERE r.blocking_session_id = ta.session_id)
, BlockedBySessionID = ta.blocking_session_id
, ta.QueryText
FROM tasks ta
UNION ALL
SELECT rq.session_id
, rq.Type
, CountOfBlockedSessions = (SELECT COUNT(*) FROM requests r WHERE r.blocking_session_id = rq.session_id)
, BlockedBySessionID = rq.blocking_session_id
, rq.QueryText
FROM requests rq
)
SELECT agg.SessionID
, ItemType = STUFF((SELECT ', ' + COALESCE(a.ItemType, '') FROM agg a WHERE a.SessionID = agg.SessionID ORDER BY a.ItemType FOR XML PATH ('')), 1, 2, '')
, agg.BlockedBySessionID
, agg.CountOfBlockedSessions
, agg.QueryText
, des.host_name
, des.login_name
, des.is_user_process
, des.program_name
, des.status
, TransactionIsolationLevel = il.Description
FROM agg
LEFT JOIN #dm_exec_sessions des ON agg.SessionID = des.session_id
LEFT JOIN IsolationLevels il ON des.transaction_isolation_level = il.Level
GROUP BY agg.SessionID
, agg.BlockedBySessionID
, agg.CountOfBlockedSessions
, agg.QueryText
, des.host_name
, des.login_name
, des.is_user_process
, des.program_name
, des.status
, il.Description
ORDER BY
agg.BlockedBySessionID
, agg.CountOfBlockedSessions
, agg.SessionID;
st.text IS NOT NULL
? Even better, why not use sp_WhoIsActive? – Erik Darling Oct 5 '17 at 19:28