Why Do SQL Server Identity Gaps Appear After Transaction Rollbacks?

SQL Server Identity Gaps: Understanding Rollback Impacts

Unexpected gaps in SQL Server's identity (auto-increment) sequences can appear after transaction rollbacks, particularly when dealing with inserts. This isn't a bug; it's a design choice prioritizing performance.

Consider a scenario: a .NET transaction inserts into a SQL Server 2005 table with an identity primary key. If an error triggers a Rollback(), the inserted rows are removed. However, the identity value is already incremented, leaving a gap.

This non-transactional behavior prevents performance bottlenecks. Imagine two concurrent transactions using an auto-number ID in table A:

• Transaction 1: Inserts into tables A, B, C, and D, then commits.
• Transaction 2: Inserts into tables A and B, then commits.

If Transaction 2 starts after Transaction 1, it would need to wait for Transaction 1's completion to secure the next auto-number for table A. The non-transactional approach avoids this blocking.

Therefore, if strictly sequential identity values are crucial, alternative methods are recommended. The system's design prioritizes concurrency over strict sequential numbering.

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