Identity Column Skips 1000 and Starts at 20001 When Machine Restarts: The Mystery Solved!

Have you ever encountered a situation where your identity column in SQL Server skips 1000 values and starts from 20001 after a machine restart? If so, you’re not alone! This phenomenon has been puzzling many developers and database administrators for a long time. In this article, we’ll delve into the reasons behind this behavior, and more importantly, provide you with solutions to overcome this issue.

Understanding Identity Columns

An identity column in SQL Server is a special type of column that automatically generates a unique, incremental value for each new row inserted into a table. This feature is particularly useful when you need to create a unique identifier for each row, such as an order ID or customer ID.

CREATE TABLE Customers (
    CustomerID int IDENTITY(1,1) NOT NULL,
    Name varchar(50) NOT NULL,
    Email varchar(100) NOT NULL
);

In the above example, the CustomerID column is defined as an identity column with an initial value of 1 and an increment of 1. This means that each time a new row is inserted, the CustomerID will automatically increment by 1.

The Mystery of the Skipping Identity Column

Now, imagine you’ve created a table with an identity column, and everything seems to be working fine. However, after a machine restart, you notice that the identity column has skipped 1000 values and started from 20001. This can be frustrating, especially if you rely on the identity column for unique identifiers.

So, what’s causing this behavior? The answer lies in how SQL Server handles identity columns.

Seed and Increment Values

When you create an identity column, you specify a seed value and an increment value. The seed value is the initial value of the identity column, and the increment value is the amount by which the column increments for each new row.

CREATE TABLE Orders (
    OrderID int IDENTITY(100,1) NOT NULL,
    OrderDate datetime NOT NULL,
    CustomerID int NOT NULL
);

In the above example, the OrderID column has a seed value of 100 and an increment value of 1. This means that the first row inserted will have an OrderID of 100, the second row will have an OrderID of 101, and so on.

System Restart and the Identity Cache

Here’s where things get interesting. When SQL Server is restarted, the identity cache is lost. The identity cache is a mechanism that stores the last used identity value in memory. When a new row is inserted, the identity cache is updated, and the next available value is retrieved from the cache.

However, when the machine restarts, the identity cache is cleared, and SQL Server needs to reseed the identity column. By default, SQL Server uses a seed value of 0 and an increment value of 1. This means that the next available value will be 1, but since the identity cache is cleared, SQL Server will actually use the next available value in the range, which is 20001 in our case.

Solutions to the Identity Column Skipping Issue

Now that we understand the underlying cause of the issue, let’s explore some solutions to overcome this problem.

RESEED the Identity Column

One solution is to reseed the identity column after a machine restart. You can do this by using the DBCC CHECKIDENT command.

DBCC CHECKIDENT ('Orders', RESEED, 100);

This command will reseed the OrderID column with a value of 100, and subsequent inserts will continue from this value.

Use a Sequence Instead of an Identity Column

Another solution is to use a sequence instead of an identity column. A sequence is a database object that generates a sequence of numeric values, and it’s not affected by machine restarts.

CREATE SEQUENCE OrderSequence AS int START WITH 100 INCREMENT BY 1;

You can then use the sequence to generate unique identifiers for your rows.

INSERT INTO Orders (OrderID, OrderDate, CustomerID)
VALUES (NEXT VALUE FOR OrderSequence, GETDATE(), 1);

Use a Stored Procedure to Generate IDs

Another approach is to create a stored procedure that generates unique identifiers using a combination of the current maximum ID and a increment value.

CREATE PROCEDURE GenerateOrderID
    @OrderID int OUTPUT
AS
BEGIN
    DECLARE @MaxID int;
    SELECT @MaxID = ISNULL(MAX(OrderID), 0) FROM Orders;
    SET @OrderID = @MaxID + 1;
END;

You can then call the stored procedure to generate a new ID for each row.

DECLARE @OrderID int;
EXEC GenerateOrderID @OrderID OUTPUT;
INSERT INTO Orders (OrderID, OrderDate, CustomerID)
VALUES (@OrderID, GETDATE(), 1);

Conclusion

In conclusion, the issue of an identity column skipping 1000 values and starting from 20001 after a machine restart is a common problem that can be frustrating, but it’s not unsolvable. By understanding the underlying causes and using one of the solutions outlined above, you can ensure that your identity column behaves as expected, even after a machine restart.

Remember, it’s essential to choose the solution that best fits your specific use case and requirements. Whether you choose to reseed the identity column, use a sequence, or implement a stored procedure, the key is to ensure that your unique identifiers are generated consistently and reliably.

Solution	Description	Pros	Cons
RESEED the Identity Column	Reseeds the identity column with a specified value	Easy to implement, quick fix	Requires manual intervention, may not be suitable for large-scale applications
Use a Sequence Instead of an Identity Column	Uses a sequence to generate unique identifiers	More flexible, not affected by machine restarts	May require changes to existing code, requires additional database object
Use a Stored Procedure to Generate IDs	Generates unique identifiers using a stored procedure	More flexible, allows for custom logic	May require additional development effort, may be slower than other solutions

By following the guidelines and solutions outlined in this article, you’ll be well-equipped to handle the issue of an identity column skipping 1000 values and starting from 20001 after a machine restart. Remember to choose the solution that best fits your specific needs and requirements, and don’t hesitate to reach out if you have any further questions or concerns.

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