Why Do Gaps Appear in SQL Server's IDENTITY Column Values?

Gaps in SQL Server IDENTITY column values: Understanding the IDENTITY property

Inserting records into a table with an auto-incrementing IDENTITY column does not always result in consecutive ID values. This difference is a result of the inherent characteristics of the IDENTITY property.

Limitations of the IDENTITY attribute:

• Non-uniqueness: IDENTITY columns themselves do not enforce uniqueness of values. Additional constraints, such as PRIMARY KEY or UNIQUE, must be used to ensure a single, non-duplicate value.
• Non-contiguous transaction values: Concurrent inserts may disrupt the ordering of IDENTITY values ​​within a transaction.
• Gaps after server failure: SQL Server may cache IDENTITY values ​​for performance, which may cause values ​​to be lost on server restarts or failures, creating gaps in the sequence.
• Value reuse prevention: IDENTITY values ​​are not reused within a specific seed/increment combination even after an insert failure or rollback. This may cause gaps.

Management gap:

• Avoid gap formation: Frequent deletions in tables with IDENTITY columns may cause gaps. If this is a problem, consider using an alternative key generation mechanism.
• Explicit value insertion: Use SET IDENTITY_INSERT ON to specify an explicit value for the IDENTITY column, but first check the existing value to ensure no gaps are created.
• Identity column property: Make sure the Identity column's delta value is set to 1 to maintain a consistent sequence.

Remember that while the IDENTITY property provides auto-incrementing values, it does not guarantee uniqueness, continuous values, or handle server failures seamlessly.

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