Changing the IDENTITY
property is really a metadata only change. But to update the metadata directly requires starting the instance in single user mode and messing around with some columns in sys.syscolpars
and is undocumented/unsupported and not something I would recommend or will give any additional details about.
For people coming across this answer on SQL Server 2012+ by far the easiest way of achieving this result of an auto incrementing column would be to create a SEQUENCE
object and set the next value for seq
as the column default.
Alternatively, or for previous versions (from 2005 onwards), the workaround posted on this connect item shows a completely supported way of doing this without any need for size of data operations using ALTER TABLE...SWITCH
. Also blogged about on MSDN here. Though the code to achieve this is not very simple and there are restrictions – such as the table being changed can’t be the target of a foreign key constraint.
Example code.
Set up test table with no identity
column.
CREATE TABLE dbo.tblFoo
(
bar INT PRIMARY KEY,
filler CHAR(8000),
filler2 CHAR(49)
)
INSERT INTO dbo.tblFoo (bar)
SELECT TOP (10000) ROW_NUMBER() OVER (ORDER BY (SELECT 0))
FROM master..spt_values v1, master..spt_values v2
Alter it to have an identity
column (more or less instant).
BEGIN TRY;
BEGIN TRANSACTION;
/*Using DBCC CHECKIDENT('dbo.tblFoo') is slow so use dynamic SQL to
set the correct seed in the table definition instead*/
DECLARE @TableScript nvarchar(max)
SELECT @TableScript="
CREATE TABLE dbo.Destination(
bar INT IDENTITY(" +
CAST(ISNULL(MAX(bar),0)+1 AS VARCHAR) + ',1) PRIMARY KEY,
filler CHAR(8000),
filler2 CHAR(49)
)
ALTER TABLE dbo.tblFoo SWITCH TO dbo.Destination;
'
FROM dbo.tblFoo
WITH (TABLOCKX,HOLDLOCK)
EXEC(@TableScript)
DROP TABLE dbo.tblFoo;
EXECUTE sp_rename N'dbo.Destination', N'tblFoo', 'OBJECT';
COMMIT TRANSACTION;
END TRY
BEGIN CATCH
IF XACT_STATE() <> 0 ROLLBACK TRANSACTION;
PRINT ERROR_MESSAGE();
END CATCH;
Test the result.
INSERT INTO dbo.tblFoo (filler,filler2)
OUTPUT inserted.*
VALUES ('foo','bar')
Gives
bar filler filler2
----------- --------- ---------
10001 foo bar
Clean up
DROP TABLE dbo.tblFoo