I want to check each spid/session isolation level in SQL Server. My question is specific to READ COMMITTED SNAPSHOT - sql-server-2014

Question

How to check where all my currently running sessions's isolation level. what we set at database level is READ COMMITTED SNAPSHOT. I want to make sure each session is using that to make sure something is not getting changed through application while creating session-connection to SQL Server
I have tried below query but it shows only READ COMMITTED.
SELECT transaction_sequence_num,
commit_sequence_num,
is_snapshot,
t.session_id,
first_snapshot_sequence_num,
max_version_chain_traversed,
elapsed_time_seconds,
host_name,
login_name,
CASE transaction_isolation_level
WHEN '0' THEN
'Unspecified'
WHEN '1' THEN
'ReadUncomitted'
WHEN '2' THEN
'ReadCommitted'
WHEN '3' THEN
'Repeatable'
WHEN '4' THEN
'Serializable'
WHEN '5' THEN
'Snapshot'
END AS transaction_isolation_level
FROM sys.dm_tran_active_snapshot_database_transactions t
JOIN sys.dm_exec_sessions s
ON t.session_id = s.session_id;

Answer 1

Since the query you posted inner joins to dm_tran_active_snapshot_database_transactions only sessions using row versioning will by returned. To return sessions using the Read Commited Snapshot Isolation level (RSCI) and sessions with other isolation levels left join to dm_tran_active_snapshot_database_transactions and check for a non-match (null join column). I wasn't sure if you needed all the columns you listed from dm_tran_active_snapshot_database_transactions so a condensed version is below, and any columns can be added back as needed. The total_elapsed_time in dm_exec_sessions is in milliseconds while the elapsed_time_seconds from dm_tran_active_snapshot_database_transactions is in seconds and this example returns the result in seconds regardless of which DMV the session originates from. I'd also recommend always aliasing columns, especially when joins are involved.
SELECT
S.session_id,
S.host_name,
S.login_name,
--divide milliseconds in total_elapsed_time by 1000
--to get time in seconds if session not using Read Committed SnapShot
COALESCE(T.elapsed_time_seconds, (S.total_elapsed_time / 1000.0)) as TimeElapsedInSeconds,
S.transaction_isolation_level,
CASE WHEN T.session_id is not null then 'ReadCommittedSnapShot' ELSE
CASE S.transaction_isolation_level
WHEN 0 THEN 'Unspecified'
WHEN 1 THEN 'ReadUncomitted'
WHEN 2 THEN 'ReadCommitted'
WHEN 3 THEN 'Repeatable'
WHEN 4 THEN 'Serializable'
WHEN 5 THEN 'Snapshot' END
END as transaction_isolation_level
FROM SYS.DM_EXEC_SESSIONS S
LEFT JOIN SYS.DM_TRAN_ACTIVE_SNAPSHOT_DATABASE_TRANSACTIONS T on S.session_id = T.session_id

Answer 2

I think this may work
SELECT transaction_sequence_num,
commit_sequence_num,
is_snapshot,
t.session_id,
first_snapshot_sequence_num,
max_version_chain_traversed,
elapsed_time_seconds,
host_name,
login_name,
CASE transaction_isolation_level
WHEN '0' THEN
'Unspecified'
WHEN '1' THEN
'ReadUncomitted'
WHEN '2' THEN
CASE
WHEN
(
SELECT [is_read_committed_snapshot_on]
FROM sys.databases
WHERE [database_id] = s.[database_id]
) = 1 THEN
'ReadCommittedSnapShot'
ELSE
'ReadCommitted'
END
WHEN '3' THEN
'Repeatable'
WHEN '4' THEN
'Serializable'
WHEN '5' THEN
'Snapshot'
END AS transaction_isolation_level
FROM sys.dm_tran_active_snapshot_database_transactions t
JOIN sys.dm_exec_sessions s
ON t.session_id = s.session_id;