sys.dm_tran_active_snapshot_database_transactions (Transact-SQL)

Άρθρο
02/28/2023

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

In a SQL Server instance, this dynamic management view returns a virtual table for all active transactions that generate or potentially access row versions. Transactions are included for one or more of the following conditions:

When either or both ALLOW_SNAPSHOT_ISOLATION and READ_COMMITTED_SNAPSHOT database options are set to ON:
- There is one row for each transaction that is running under snapshot isolation level, or read-committed isolation level that is using row versioning.
- There is one row for each transaction that causes a row version to be created in the current database. For example, the transaction generates a row version by updating or deleting a row in the current database.
When a trigger is fired, there is one row for the transaction under which the trigger is executing.
When an online indexing procedure is running, there is one row for the transaction that is creating the index.
When Multiple Active Results Sets (MARS) session is enabled, there is one row for each transaction that is accessing row versions.

This dynamic management view does not include system transactions.

Note

To call this from Azure Synapse Analytics or Analytics Platform System (PDW), use the name sys.dm_pdw_nodes_tran_active_snapshot_database_transactions. This syntax is not supported by serverless SQL pool in Azure Synapse Analytics.

Syntax

  
sys.dm_tran_active_snapshot_database_transactions

Table Returned

Column name	Data type	Description
transaction_id	bigint	Unique identification number assigned for the transaction. The transaction ID is primarily used to identify the transaction in locking operations.
transaction_sequence_num	bigint	Transaction sequence number. This is a unique sequence number that is assigned to a transaction when it starts. Transactions that do not generate version records and do not use snapshot scans will not receive a transaction sequence number.
commit_sequence_num	bigint	Sequence number that indicates when the transaction finishes (commits or stops). For active transactions, the value is NULL.
is_snapshot	int	0 = Is not a snapshot isolation transaction. 1 = Is a snapshot isolation transaction.
session_id	int	ID of the session that started the transaction.
first_snapshot_sequence_num	bigint	Lowest transaction sequence number of the transactions that were active when a snapshot was taken. On execution, a snapshot transaction takes a snapshot of all of the active transactions at that time. For nonsnapshot transactions, this column shows 0.
max_version_chain_traversed	int	Maximum length of the version chain that is traversed to find the transactionally consistent version.
average_version_chain_traversed	real	Average number of row versions in the version chains that are traversed.
elapsed_time_seconds	bigint	Elapsed time since the transaction obtained its transaction sequence number.
pdw_node_id	int	Applies to: Azure Synapse Analytics, Analytics Platform System (PDW) The identifier for the node that this distribution is on.

Permissions

On SQL Server and SQL Managed Instance, requires VIEW SERVER STATE permission.

On SQL Database Basic, S0, and S1 service objectives, and for databases in elastic pools, the server admin account, the Microsoft Entra admin account, or membership in the ##MS_ServerStateReader## server role is required. On all other SQL Database service objectives, either the VIEW DATABASE STATE permission on the database, or membership in the ##MS_ServerStateReader## server role is required.

Permissions for SQL Server 2022 and later

Requires VIEW SERVER PERFORMANCE STATE permission on the server.

Remarks

sys.dm_tran_active_snapshot_database_transactions reports transactions that are assigned a transaction sequence number (XSN). The XSN is assigned when the transaction first accesses the version store. In a database that is enabled for snapshot isolation or read committed isolation using row versioning, the examples show when an XSN is assigned to a transaction:

If a transaction is running under serializable isolation level, an XSN is assigned when the transaction first executes a statement, such as an UPDATE operation, that causes a row version to be created.
If a transaction is running under snapshot isolation, an XSN is assigned when any data manipulation language (DML) statement, including a SELECT operation, is executed.

Transaction sequence numbers are serially incremented for each transaction that is started in an instance of the Database Engine.

Examples

The following example uses a test scenario in which four concurrent transactions, each identified by a transaction sequence number (XSN), are running in a database that has the ALLOW_SNAPSHOT_ISOLATION and READ_COMMITTED_SNAPSHOT options set to ON. The following transactions are running:

XSN-57 is an update operation under serializable isolation.
XSN-58 is the same as XSN-57.
XSN-59 is a select operation under snapshot isolation
XSN-60 is same as XSN-59.

The following query is executed.

SELECT   
    transaction_id,  
    transaction_sequence_num,  
    commit_sequence_num,  
    is_snapshot session_id,  
    first_snapshot_sequence_num,  
    max_version_chain_traversed,  
    average_version_chain_traversed,  
    elapsed_time_seconds  
  FROM sys.dm_tran_active_snapshot_database_transactions;

Here's the result set.

transaction_id  transaction_sequence_num  commit_sequence_num  
--------------  ------------------------  -------------------  
9295            57                        NULL  
9324            58                        NULL  
9387            59                        NULL  
9400            60                        NULL  
  
is_snapshot  session_id   first_snapshot_sequence_num  
-----------  -----------  ---------------------------  
0            54           0  
0            53           0  
1            52           57  
1            51           57  
  
max_version_chain_traversed  average_version_chain_traversed  
---------------------------  -------------------------------  
0                            0  
0                            0  
1                            1  
1                            1  
  
elapsed_time_seconds  
--------------------  
419  
397  
359  
333

The following information evaluates the results from sys.dm_tran_active_snapshot_database_transactions:

XSN-57: Because this transaction is not running under snapshot isolation, the is_snapshot value and first_snapshot_sequence_num are 0. transaction_sequence_num shows that a transaction sequence number has been assigned to this transaction, because one or both ALLOW_SNAPSHOT_ISOLATION or READ_COMMITTED_SNAPSHOT database options are ON.
XSN-58: This transaction is not running under snapshot isolation and the same information for XSN-57 applies.
XSN-59: This is the first active transaction that is running under snapshot isolation. This transaction reads data that is committed before XSN-57, as indicated by first_snapshot_sequence_num. The output for this transaction also shows the maximum version chain that is traversed for a row is 1 and has traversed an average of 1 version for each row that is accessed. This means that transactions XSN-57, XSN-58, and XSN-60 have not modified rows and committed.
XSN-60: This is the second transaction running under snapshot isolation. The output shows the same information as XSN-59.

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