Identity key counter increment by one although it is in TRY Catch and Transaction is roll-backed ? SSMS 2008 - sql-server

Question

Identity counter increment by one although it is in TRY Catch and Transaction is roll-backed ? SSMS 2008 is there any way i can stop it +1 or rollback it too.

Answer 1

In order to understand why this happened, Let's execute below sample code first-
USE tempdb
CREATE TABLE dbo.Sales
(ID INT IDENTITY(1,1), Address VARCHAR(200))
GO
BEGIN TRANSACTION
INSERT DBO.Sales
( Address )
VALUES ( 'Dwarka, Delhi' );
ROLLBACK TRANSACTION
Now, Execution plan for above query is-
The second last operator from right Compute Scalar is computing value for [Expr1003]=getidentity((629577281),(2),NULL) which is IDENTITY value for ID column. So this clearly indicates that IDENTITY values are fetched & Incremented prior to Insertion (INSERT Operator). So its by nature that even transaction rollback at later stage once created IDENTITY value is there.
Now, in order to reseed the IDENTITY value to Maximum Identity Value present in table + 1, you need sysadmin permission to execute below DBCC command -
DBCC CHECKIDENT
(
table_name
[, { NORESEED | { RESEED [, new_reseed_value ] } } ]
)
[ WITH NO_INFOMSGS ]
So the final query should include below piece of code prior to rollback statement:-
-- Code to check max ID value, and verify it again IDENTITY SEED
DECLARE #MaxValue INT = (SELECT ISNULL(MAX(ID),1) FROM dbo.Sales)
IF #MaxValue IS NOT NULL AND #MaxValue <> IDENT_CURRENT('dbo.Sales')
DBCC CHECKIDENT ( 'dbo.Sales', RESEED, #MaxValue )
--ROLLBACK TRANSACTION
So it is recommended to leave it on SQL Server.

Answer 2

You are right and the following code inserts record with [Col01] equal to 2:
CREATE TABLE [dbo].[DataSource]
(
[Col01] SMALLINT IDENTITY(1,1)
,[Col02] TINYINT
);
GO
BEGIN TRY
BEGIN TRANSACTION;
INSERT INTO [dbo].[DataSource] ([Col02])
VALUES (1);
SELECT 1/0
END TRY
BEGIN CATCH
IF ##TRANCOUNT > 0
BEGIN
ROLLBACK TRANSACTION
END;
END CATCH;
GO
INSERT INTO [dbo].[DataSource] ([Col02])
VALUES (1);
SELECT *
FROM [dbo].[DataSource]
This is by design (as you can see in the documentation:
Consecutive values after server restart or other failures –SQL Server
might cache identity values for performance reasons and some of the
assigned values can be lost during a database failure or server
restart. This can result in gaps in the identity value upon insert. If
gaps are not acceptable then the application should use its own
mechanism to generate key values. Using a sequence generator with the
NOCACHE option can limit the gaps to transactions that are never
committed.
I try using NOCACHE sequence but it does not work on SQL Server 2012:
CREATE TABLE [dbo].[DataSource]
(
[Col01] SMALLINT
,[Col02] TINYINT
);
CREATE SEQUENCE [dbo].[MyIndentyty]
START WITH 1
INCREMENT BY 1
NO CACHE;
GO
BEGIN TRY
BEGIN TRANSACTION;
INSERT INTO [dbo].[DataSource] ([Col01], [Col02])
SELECT NEXT VALUE FOR [dbo].[MyIndentyty], 1
SELECT 1/0
END TRY
BEGIN CATCH
IF ##TRANCOUNT > 0
BEGIN
ROLLBACK TRANSACTION
END;
END CATCH;
GO
INSERT INTO [dbo].[DataSource] ([Col01], [Col02])
SELECT NEXT VALUE FOR [dbo].[MyIndentyty], 1
SELECT *
FROM [dbo].[DataSource]
DROP TABLE [dbo].[DataSource];
DROP SEQUENCE [dbo].[MyIndentyty];
You can use MAX to solve this:
CREATE TABLE [dbo].[DataSource]
(
[Col01] SMALLINT
,[Col02] TINYINT
);
BEGIN TRY
BEGIN TRANSACTION;
DECLARE #Value SMALLINT = (SELECT MAX([Col01]) FROM [dbo].[DataSource]);
INSERT INTO [dbo].[DataSource] ([Col01], [Col02])
SELECT #Value, 1
SELECT 1/0
END TRY
BEGIN CATCH
IF ##TRANCOUNT > 0
BEGIN
ROLLBACK TRANSACTION
END;
END CATCH;
GO
DECLARE #Value SMALLINT = ISNULL((SELECT MAX([Col01]) FROM [dbo].[DataSource]), 1);
INSERT INTO [dbo].[DataSource] ([Col01], [Col02])
SELECT #Value, 1
SELECT *
FROM [dbo].[DataSource]
DROP TABLE [dbo].[DataSource];
But you must pay attentions to your isolation level for potential issues:
If you want to insert many rows at the same time, do the following:
get the current max value
create table where to store the rows (that are going to be inserted) generating ranking (you can use identity column, you can use ranking function) and adding the max value to it
insert the rows