How Can I Avoid Gaps in My SQL Server IDENTITY Column Values?

Understanding Gaps in SQL Server IDENTITY Columns

Auto-incrementing primary keys, often implemented with SQL Server's IDENTITY columns, don't inherently guarantee consecutive values. Several factors can introduce gaps:

• Non-Consecutive Values: Concurrent insertions from multiple processes can disrupt the sequence. Exclusive locks or SERIALIZABLE transaction isolation levels can mitigate this.
• Server Restart Issues: Caching mechanisms can lead to lost identity values during server restarts or failures, resulting in gaps. Using NOCACHE sequences or alternative key generation methods can help.
• Value Loss: Rolled-back transactions consume identity values without assigning them, creating gaps.
• Lack of Uniqueness Guarantee: IDENTITY columns alone don't ensure uniqueness; PRIMARY KEY constraints or UNIQUE indexes are essential.

Addressing and Preventing Gaps

If gaps already exist:

• Pre-Insertion Checks: Before inserting data, check the current identity value to avoid creating further gaps.
• Increment Value Verification: Ensure the identity increment value is set to 1 for consecutive numbering.

Best Practices

• Deletion Impact: Frequent deletions can cause significant gaps. Consider alternative key generation strategies if this is a frequent occurrence.
• External Inserts: External inserts or concurrent operations with rollbacks are other potential sources of gaps.

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