Why Do Gaps Appear in SQL Server IDENTITY Columns?

SQL Server IDENTITY Columns: Addressing Gaps in Auto-Incrementing Values

Using IDENTITY columns in SQL Server often leads to unexpected gaps in the automatically generated sequence. This article clarifies the inherent limitations of the IDENTITY property and offers solutions to mitigate these issues.

Understanding IDENTITY Property Limitations

The IDENTITY property doesn't guarantee:

• Value Uniqueness: Always enforce uniqueness using a primary key or unique constraint.
• Consecutive Values During Concurrent Inserts: Simultaneous insertions can create non-sequential values. Employ exclusive locks or the SERIALIZABLE transaction isolation level to maintain sequence.
• Consecutive Values After Server Restarts: Server restarts might lose identity values, resulting in gaps. The NOCACHE option or a custom key generation strategy can help prevent this.
• Value Reuse After Rollbacks: Failed or rolled-back transactions consume identity values, creating gaps.

Further Considerations for Gap Prevention

• Frequent Deletions: Frequent data deletion significantly contributes to gaps. A unique key might be a more suitable solution in such scenarios.
• Identity Increment Value: Ensure the Identity Increment property is set to 1.
• SET IDENTITY_INSERT ON: Use this command to explicitly control identity values, potentially filling existing gaps or preventing future ones.

By understanding these limitations and implementing the suggested strategies, developers can effectively manage auto-incrementing in SQL Server, ensuring data integrity and minimizing gaps in IDENTITY column values.

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