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Change the schema of a system-versioned temporal table

  • Cikk

Applies to: SQL Server 2016 (13.x) and later Azure SQL Database Azure SQL Managed Instance

Use the ALTER TABLE statement to add, alter, or remove a column.

Remarks

CONTROL permission on current and history tables is required to change schema of temporal table.

During an ALTER TABLE operation, the system holds a schema lock on both tables.

Specified schema change is propagated to history table appropriately (depending on type of change).

Adding varchar(max), nvarchar(max), varbinary(max) or XML columns with defaults, is an update data operation on all editions of SQL Server.

If row size after column addition exceeds the row size limit, new columns can't be added online.

Once you extend a table with a new NOT NULL column, consider dropping the default constraint on the history table, as the system automatically populates all columns in that table.

The online option (WITH (ONLINE = ON) has no effect on ALTER TABLE ALTER COLUMN with temporal tables. ALTER column isn't performed as online, regardless of which value was specified for ONLINE option.

You can use ALTER COLUMN to change the IsHidden property for period columns.

You can't use direct ALTER for the following schema changes. For these types of changes, set SYSTEM_VERSIONING = OFF.

  • Adding a computed column
  • Adding an IDENTITY column
  • Adding a SPARSE column or changing existing column to be SPARSE when the history table is set to DATA_COMPRESSION = PAGE or DATA_COMPRESSION = ROW, which is the default for the history table.
  • Adding a COLUMN_SET
  • Adding a ROWGUIDCOL column or changing existing column to be ROWGUIDCOL
  • Altering a NULL column to NOT NULL if the column contains null values in the current or history table

Examples

A. Change the schema of a temporal table

Here are some examples that change the schema of temporal table.

ALTER TABLE dbo.Department
    ALTER COLUMN DeptName varchar(100);

ALTER TABLE dbo.Department
    ADD WebAddress nvarchar(255) NOT NULL
    CONSTRAINT DF_WebAddress DEFAULT 'www.example.com';

ALTER TABLE dbo.Department
    ADD TempColumn INT;
GO

ALTER TABLE dbo.Department
    DROP COLUMN TempColumn;

B. Add period columns using the HIDDEN flag

ALTER TABLE dbo.Department
    ALTER COLUMN ValidFrom ADD HIDDEN;

ALTER TABLE dbo.Department
    ALTER COLUMN ValidTo ADD HIDDEN;

You can use ALTER COLUMN <period_column> DROP HIDDEN to clear the hidden flag on a period column.

C. Change the schema with SYSTEM_VERSIONING set to OFF

The following example demonstrates changing the schema where setting SYSTEM_VERSIONING = OFF is still required (adding an IDENTITY column). This example disables the data consistency check. This check is unnecessary when the schema change is made within a transaction as no concurrent data changes can occur.

BEGIN TRANSACTION

ALTER TABLE [dbo].[CompanyLocation] SET (SYSTEM_VERSIONING = OFF);
ALTER TABLE [CompanyLocation] ADD Cntr INT IDENTITY (1, 1);

ALTER TABLE [dbo].[CompanyLocationHistory]
    ADD Cntr INT NOT NULL
    CONSTRAINT DF_Cntr DEFAULT 0;

ALTER TABLE [dbo].[CompanyLocation] SET
(
    SYSTEM_VERSIONING = ON
    (HISTORY_TABLE = [dbo].[CompanyLocationHistory])
);

COMMIT;

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