Performance Tuning with SQL Server Dynamic Management Views

Chapter 4: Transactions
• commit_sequence_num – indicates the order that the transaction was committed;
will be NULL if the transaction is still active
• is_snapshot
• 1 if this is a SNAPSHOT isolation level transaction
• 0 if it is in here for a different reason, such as a READ_COMMITTED_SNAPSHOT
transaction.
• first_snapshot_sequence_num – the lowest active transaction_sequence_
num value of any of the transactions that were active when the current SNAPSHOT
transaction was started (any subsequent XSNs would need to be preserved for this
transaction's use; column displays 0 for non-SNAPSHOT transactions
• elapsed_time_seconds – elapsed time since this transaction acquired XSN.
sys.dm_tran_current_snapshot
This single-column DMV returns the transaction_sequence_num of all the
active transactions in the version store relevant to the current transaction at the time
the current SNAPSHOT transaction started. No results are returned if the current
transaction is not a SNAPSHOT transaction.
In and of itself, this DMV does not provide much information for the DBA. I find it pertinent
only for running an IF EXISTS query to determine if there is valid row versioning
occurring when I am about to execute a query against a snapshot-enabled database.
sys.dm_tran_transactions_snapshot
The final non-aggregate DMV pertaining to snapshot isolation is sys.dm_tran_
transactions_snapshot. The results are a superset of those that are returned for
sys.dm_tran_current_snapshot. It returns the active snapshots for all sessions, not
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