diff options
Diffstat (limited to 'lib/sqlalchemy/dialects/mssql/base.py')
| -rw-r--r-- | lib/sqlalchemy/dialects/mssql/base.py | 37 |
1 files changed, 15 insertions, 22 deletions
diff --git a/lib/sqlalchemy/dialects/mssql/base.py b/lib/sqlalchemy/dialects/mssql/base.py index 84be8d0e3..5f936fd76 100644 --- a/lib/sqlalchemy/dialects/mssql/base.py +++ b/lib/sqlalchemy/dialects/mssql/base.py @@ -560,17 +560,20 @@ This option can also be specified engine-wide using the Rowcount Support / ORM Versioning --------------------------------- -The SQL Server drivers have very limited ability to return the number -of rows updated from an UPDATE or DELETE statement. In particular, the -pymssql driver has no support, whereas the pyodbc driver can only return -this value under certain conditions. - -In particular, updated rowcount is not available when OUTPUT INSERTED -is used. This impacts the SQLAlchemy ORM's versioning feature when -server-side versioning schemes are used. When -using pyodbc, the "implicit_returning" flag needs to be set to false -for any ORM mapped class that uses a version_id column in conjunction with -a server-side version generator:: +The SQL Server drivers may have limited ability to return the number +of rows updated from an UPDATE or DELETE statement. + +As of this writing, the PyODBC driver is not able to return a rowcount when +OUTPUT INSERTED is used. This impacts the SQLAlchemy ORM's versioning feature +in many cases where server-side value generators are in use in that while the +versioning operations can succeed, the ORM cannot always check that an UPDATE +or DELETE statement matched the number of rows expected, which is how it +verifies that the version identifier matched. When this condition occurs, a +warning will be emitted but the operation will proceed. + +The use of OUTPUT INSERTED can be disabled by setting the +:paramref:`.Table.implicit_returning` flag to ``False`` on a particular +:class:`.Table`, which in declarative looks like:: class MyTable(Base): __tablename__ = 'mytable' @@ -585,14 +588,10 @@ a server-side version generator:: 'implicit_returning': False } -Without the implicit_returning flag above, the UPDATE statement will -use ``OUTPUT inserted.timestamp`` and the rowcount will be returned as --1, causing the versioning logic to fail. - Enabling Snapshot Isolation --------------------------- -Not necessarily specific to SQLAlchemy, SQL Server has a default transaction +SQL Server has a default transaction isolation mode that locks entire tables, and causes even mildly concurrent applications to have long held locks and frequent deadlocks. Enabling snapshot isolation for the database as a whole is recommended @@ -606,12 +605,6 @@ following ALTER DATABASE commands executed at the SQL prompt:: Background on SQL Server snapshot isolation is available at http://msdn.microsoft.com/en-us/library/ms175095.aspx. -Known Issues ------------- - -* No support for more than one ``IDENTITY`` column per table -* reflection of indexes does not work with versions older than - SQL Server 2005 """ import datetime |