Performance Tuning with SQL Server Dynamic Management Views

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Chapter 4: Transactions
In other words, it returns records for all active transactions that either create or read row
versions from the version store in tempdb. It is an instance-level view, and so will return
rows for transactions using snapshot isolation for all databases on a given instance.
However, version store usage is not limited to just sessions running under snapshot
isolation, but also occurs as a result of triggers, online indexing, or the use of MARS
(Multiple Active Results Sets – see Books Online for details). Triggers were the first SQL
Server construct to make use of row versioning. DML triggers make use of temporary,
non-materialized tables that mimic the structure of the tables upon which they are based.
There is both an inserted and a deleted table. The inserted table stores versions of
the rows affected by an INSERT or UPDATE query; specifically, the new rows added to the
underlying (trigger) table. The deleted table stores a copy of the records deleted from
the underlying table via a DELETE statement. Both tables are hit in the case of an UPDATE
statement, as an UPDATE is nothing more than a DELETE followed by an INSERT. The
rows are first moved from the underlying table to the deleted "virtual table." Then the
new rows are added to the inserted "virtual table" after being added to the underlying
table. In the case of online indexing, available as a benefit of licensing Enterprise
Edition SQL Server 2005 and 2008, row versioning is used to maintain concurrency
during the online indexing process. All these transactions will be noted in a query
against sys.dm_tran_active_snapshot_database_transactions.
In terms of identifier columns, this DMV provides a transaction_id column,
which identifies the transaction at the instance level and can be used to join to other
transaction-related DMOs. It also provides a session_id column, which identifies the
session with which the transaction is associated and can be used to join to executionrelated
DMOs, as well as to spid in sys.sysprocesses. Other relevant columns
include:
• transaction_sequence_num – for transactions that access or modify the version
store; this is a sequence number given to the transaction the first time it reads or
writes to the version store; Transaction Sequence Numbers (XSNs) increment serially
for each transaction