Create a system-versioned temporal table
Applies to:
SQL Server 2016 (13.x) and later
Azure SQL Database
Azure SQL Managed Instance
There are three ways to create a system-versioned temporal table when considering how the history table is specified:
Temporal table with an anonymous history table: you specify the schema of the current table and let the system create a corresponding history table with autogenerated name.
Temporal table with a default history table: you specify the history table schema name and table name and let the system create a history table in that schema.
Temporal table with a user-defined history table created beforehand: you create a history table that fits best your needs and then reference that table during temporal table creation.
Create a temporal table with an anonymous history table
Creating a temporal table with an anonymous history table is a convenient option for quick object creation, especially in prototypes and test environments. It's also the simplest way to create a temporal table since it doesn't require any parameter in the SYSTEM_VERSIONING
clause. In the following example, a new table is created with system-versioning enabled without defining the name of the history table.
CREATE TABLE Department
(
DeptID INT NOT NULL PRIMARY KEY CLUSTERED,
DeptName VARCHAR(50) NOT NULL,
ManagerID INT NULL,
ParentDeptID INT NULL,
ValidFrom DATETIME2 GENERATED ALWAYS AS ROW START NOT NULL,
ValidTo DATETIME2 GENERATED ALWAYS AS ROW END NOT NULL,
PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo)
)
WITH (SYSTEM_VERSIONING = ON);
Remarks
A system-versioned temporal table must have a primary key defined and have exactly one PERIOD FOR SYSTEM_TIME
defined with two datetime2 columns, declared as GENERATED ALWAYS AS ROW START
or GENERATED ALWAYS AS ROW END
.
The PERIOD
columns are always assumed to be non-nullable, even if nullability isn't specified. If the PERIOD
columns are explicitly defined as nullable, the CREATE TABLE
statement fails.
The history table must always be schema-aligned with the current or temporal table, with respect to the number of columns, column names, ordering, and data types.
An anonymous history table is automatically created in the same schema as current or temporal table.
The anonymous history table name has the following format: MSSQL_TemporalHistoryFor_<current_temporal_table_object_id>_<suffix>
. The suffix is optional, and is added only if the first part of the table name isn't unique.
The history table is created as a rowstore table. PAGE
compression is applied if possible, otherwise the history table is uncompressed. For example, some table configurations, such as SPARSE
columns, don't allow compression.
A default clustered index is created for the history table with an autogenerated name in format IX_<history_table_name>
. The clustered index contains the PERIOD
columns (end, start).
To create the current table as a memory-optimized table, see System-versioned temporal tables with memory-optimized tables.
Create a temporal table with a default history table
Creating a temporal table with a default history table is a convenient option when you want to control naming, and still rely on the system to create the history table with the default configuration. In the following example, a new table is created with system-versioning enabled with the name of the history table explicitly defined.
CREATE TABLE Department
(
DeptID INT NOT NULL PRIMARY KEY CLUSTERED,
DeptName VARCHAR(50) NOT NULL,
ManagerID INT NULL,
ParentDeptID INT NULL,
ValidFrom DATETIME2 GENERATED ALWAYS AS ROW START NOT NULL,
ValidTo DATETIME2 GENERATED ALWAYS AS ROW END NOT NULL,
PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo)
)
WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.DepartmentHistory));
Remarks
The history table is created using the same rules as apply to creating an "anonymous" history table, with the following rules that apply specifically to the named history table.
The schema name is mandatory for the
HISTORY_TABLE
parameter.If the specified schema doesn't exist, the
CREATE TABLE
statement fails.If the table specified by the
HISTORY_TABLE
parameter already exists, it validates against the newly created temporal table in terms of schema consistency and temporal data consistency. If you specify an invalid history table, theCREATE TABLE
statement fails.
Create a temporal table with a user-defined history table
Creating a temporal table with user-defined history table is a convenient option when the user wants to specify a history table with specific storage options and different indexes tuned to historical queries. In the following example, a user-defined history table is created with a schema that is aligned with the temporal table that is created. To this user-defined history table, a clustered columnstore index and an extra nonclustered rowstore (B-tree) index is created for point lookups. After this user-defined history table is created, the temporal table is created specifying the user-defined history table as the default history table.
Note
SQL Server documentation uses the term B-tree generally in reference to indexes. In rowstore indexes, SQL Server implements a B+ tree. This does not apply to columnstore indexes or in-memory data stores. For more information, see the SQL Server and Azure SQL index architecture and design guide.
CREATE TABLE DepartmentHistory
(
DeptID INT NOT NULL,
DeptName VARCHAR(50) NOT NULL,
ManagerID INT NULL,
ParentDeptID INT NULL,
ValidFrom DATETIME2 NOT NULL,
ValidTo DATETIME2 NOT NULL
);
GO
CREATE CLUSTERED COLUMNSTORE INDEX IX_DepartmentHistory ON DepartmentHistory;
CREATE NONCLUSTERED INDEX IX_DepartmentHistory_ID_Period_Columns
ON DepartmentHistory (ValidTo, ValidFrom, DeptID);
GO
CREATE TABLE Department
(
DeptID int NOT NULL PRIMARY KEY CLUSTERED,
DeptName VARCHAR(50) NOT NULL,
ManagerID INT NULL,
ParentDeptID INT NULL,
ValidFrom DATETIME2 GENERATED ALWAYS AS ROW START NOT NULL,
ValidTo DATETIME2 GENERATED ALWAYS AS ROW END NOT NULL,
PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo)
)
WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.DepartmentHistory));
Remarks
If you plan to run analytic queries on the historical data that employs aggregates or windowing functions, creating a clustered columnstore as a primary index is highly recommended for compression and query performance.
If you plan to use temporal tables for data auditing (that is, searching for historical changes for a single row from the current table), you should create a rowstore history table with a clustered index.
The history table can't have a primary key, foreign keys, unique indexes, table constraints, or triggers. It can't be configured for change data capture, change tracking, transactional replication, or merge replication.
Alter non-temporal table to be a system-versioned temporal table
You can enable system-versioning on an existing non-temporal table, such as when you wish to migrate a custom temporal solution to built-in support.
For example, you might have a set of tables where versioning is implemented with triggers. Using temporal system-versioning is less complex and provides other benefits including:
- Immutable history
- New syntax for time-traveling queries
- Better DML performance
- Minimal maintenance costs
When converting an existing table, consider using the HIDDEN
clause to hide the new PERIOD
columns (the datetime2 columns ValidFrom
and ValidTo
) to avoid affecting existing applications that don't explicitly specify column names (for example, SELECT *
or INSERT
without a column list) aren't designed to handle new columns.
Add versioning to non-temporal tables
If you want to start tracking changes for a non-temporal table that contains the data, you need to add the PERIOD
definition and optionally provide a name for the empty history table that SQL Server creates for you:
CREATE SCHEMA History;
GO
ALTER TABLE InsurancePolicy ADD
ValidFrom DATETIME2 GENERATED ALWAYS AS ROW START HIDDEN
CONSTRAINT DF_InsurancePolicy_ValidFrom DEFAULT SYSUTCDATETIME(),
ValidTo DATETIME2 GENERATED ALWAYS AS ROW END HIDDEN
CONSTRAINT DF_InsurancePolicy_ValidTo DEFAULT CONVERT(DATETIME2, '9999-12-31 23:59:59.9999999'),
PERIOD FOR SYSTEM_TIME(ValidFrom, ValidTo);
GO
ALTER TABLE InsurancePolicy
SET (SYSTEM_VERSIONING = ON (HISTORY_TABLE = History.InsurancePolicy));
GO
Important
The precision for DATETIME2
must align with the precision for the underlying table.
Remarks
Adding non-nullable columns with defaults to an existing table with data is a size of data operation on all editions other than SQL Server Enterprise edition (on which it's a metadata operation). With a large existing history table with data on SQL Server Standard edition, adding a non-null column can be an expensive operation.
Constraints for period start and period end columns must be carefully chosen:
Default for start column specifies from which point in time you consider existing rows to be valid. It can't be specified as a datetime point in the future.
End time must be specified as the maximum value for a given datetime2 precision, for example
9999-12-31 23:59:59
or9999-12-31 23:59:59.9999999
.
Adding PERIOD
performs a data consistency check on current table to make sure that the existing values for period columns are valid.
When an existing history table is specified when enabling SYSTEM_VERSIONING
, a data consistency check is performed across both the current and the history table. It can be skipped if you specify DATA_CONSISTENCY_CHECK = OFF
as an extra parameter.
Migrate existing tables to built-in support
This example shows how to migrate from an existing solution based on triggers to built-in temporal support. For this example, we assume that the current custom solution splits the current and historical data in two separate user tables (ProjectTaskCurrent
and ProjectTaskHistory
).
If your existing solution uses single table to store actual and historical rows, then you should split the data into two tables before the migration steps shown in following example. First, drop the trigger on the future temporal table. Then, ensure the PERIOD
columns are non-nullable.
/* Drop trigger on future temporal table */
DROP TRIGGER ProjectCurrent_OnUpdateDelete;
/* Make sure that future period columns are non-nullable */
ALTER TABLE ProjectTaskCurrent
ALTER COLUMN [ValidFrom] DATETIME2 NOT NULL;
ALTER TABLE ProjectTaskCurrent
ALTER COLUMN [ValidTo] DATETIME2 NOT NULL;
ALTER TABLE ProjectTaskHistory
ALTER COLUMN [ValidFrom] DATETIME2 NOT NULL;
ALTER TABLE ProjectTaskHistory
ALTER COLUMN [ValidTo] DATETIME2 NOT NULL;
ALTER TABLE ProjectTaskCurrent ADD PERIOD
FOR SYSTEM_TIME([ValidFrom], [ValidTo]);
ALTER TABLE ProjectTaskCurrent SET (
SYSTEM_VERSIONING = ON (
HISTORY_TABLE = dbo.ProjectTaskHistory,
DATA_CONSISTENCY_CHECK = ON
)
);
Remarks
Referencing existing columns in PERIOD
definition implicitly changes generated_always_type
to AS_ROW_START
and AS_ROW_END
for those columns.
Adding PERIOD
performs a data consistency check on current table to make sure that the existing values for period columns are valid.
We highly recommend that you set SYSTEM_VERSIONING
with DATA_CONSISTENCY_CHECK = ON
, to enforce data consistency checks on existing data.
If hidden columns are preferred, use the command ALTER TABLE [tableName] ALTER COLUMN [columnName] ADD HIDDEN;
.
Related content
- Temporal tables
- Get started with system-versioned temporal tables
- Manage retention of historical data in system-versioned temporal tables
- System-versioned temporal tables with memory-optimized tables
- CREATE TABLE (Transact-SQL)
- Modify data in a system-versioned temporal table
- Query data in a system-versioned temporal table
- Change the schema of a system-versioned temporal table
- Stop system-versioning on a system-versioned temporal table
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