Why Do Identity Values Skip Numbers After a Rollback (and What You Can Do About It)

However, things can get tricky when you combine identity columns with transactions. Transactions allow you to group multiple database operations into a single unit. If any operation within the transaction fails, the entire transaction is rolled back, meaning all changes are undone.

Here's the problem: Even though a transaction is rolled back due to an error, the identity value assigned during the attempt is not reused. This can lead to gaps in the sequence of generated IDs, which might seem counterintuitive.

Example:

-- Start a transaction
BEGIN TRANSACTION;

-- Try inserting a new record
INSERT INTO Books (Title, Author) VALUES ('The Hitchhiker\'s Guide to the Galaxy', 'Douglas Adams');

-- Simulate an error (e.g., invalid data)
RAISE ERROR 'Invalid book title';

-- Rollback the transaction (changes are undone)
ROLLBACK TRANSACTION;

-- Now, try inserting another record
INSERT INTO Books (Title, Author) VALUES ('The Lord of the Rings', 'J.R.R. Tolkien');

-- You might expect the ID to be 1, but it could be 2!

In this example, even though the first insert was rolled back due to the error, the identity value "1" is not reused. When the second insert happens, it might get assigned the value "2," causing a gap in the sequence.

Why does this happen?

• Efficiency: Reusing identity values after a rollback would require complex locking mechanisms, potentially impacting database performance.
• Data Integrity: Maintaining a continuous sequence can be detrimental in some scenarios. For example, gaps in IDs might raise unnecessary flags or cause issues with external systems relying on sequential numbering.

• Gaps in Identity Sequence: While gaps might seem undesirable, identity columns are primarily for unique identification, not necessarily sequential numbering. Consider using other mechanisms like sequences or triggers if you require strict sequential ordering.
• Accidental Data Loss: Ensure proper error handling and validation within your transactions to minimize the need for rollbacks and potential data loss.