Why Does My SQL Server Identity Increment Suddenly Skip Values?

SQL Server Identity Gaps: Understanding and Mitigation

Overview

SQL Server's IDENTITY property simplifies primary key management by automatically incrementing integer values. However, unexpected gaps in this sequence can occur, leaving users puzzled.

Root Cause: Identity Cache

Microsoft's documentation explains that SQL Server (2012 and later) employs an identity cache to optimize performance. This cache, typically set to 1,000 for int IDENTITY columns, pre-allocates a block of values.

• Allocation Process: When an identity value is needed, SQL Server reserves a range from the cache. For instance, if the current value is 1,205,446, it might reserve 1,205,447 through 1,206,446.
• Gap Creation: Server restarts or rolled-back transactions can release these reserved, unused values, resulting in noticeable gaps in the sequence.

Impact of Cache Size

The example shows identity increments jumping by approximately 100 because of the 1,000-value cache. When a value like 1,206,321 is assigned, the next 1,000 are reserved, creating the gaps. The consistent last three digits (e.g., 321) reflect the end of the reserved block.

Strategies for Gap Reduction

While identity gaps are not inherently problematic, they might be undesirable. Consider these options:

• Sequences (NOCACHE): SQL Server sequences offer a caching-free alternative, minimizing gaps to only uncommitted transactions.
• Trace Flag 272: This trace flag provides detailed logging of identity allocation, helping identify gap-causing events. Note that this affects all databases.
• Database-Level Caching Disable: For newer SQL Server versions, ALTER DATABASE SCOPED CONFIGURATION with IDENTITY_CACHE = OFF disables caching for a specific database.

Important Considerations

No method completely eliminates identity gaps. Concurrent inserts or transaction failures can still create them. For applications requiring absolute sequence integrity, consider GUIDs or custom sequence generation mechanisms.

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