Why Are My SQL Server IDENTITY Increments Jumping After a Restart?

SQL Server IDENTITY Column Increment Gaps: Understanding the Cache

SQL Server's IDENTITY column, used for generating unique row identifiers, sometimes exhibits unexpected behavior: after a server restart, the increment jumps by hundreds, instead of sequentially increasing by one. This isn't a bug, but a consequence of performance optimizations introduced in SQL Server 2012.

SQL Server caches identity values to boost performance. This cache typically stores 1,000 int values or 10,000 bigint/numeric values. Once the cache is depleted, a new batch is allocated. Restarting the server, however, discards any unused cached values.

The observed jump, with the last three digits remaining constant (e.g., always ending in 306), reflects the remaining cache values after the initial allocation of 1,000.

While this caching mechanism creates gaps in the IDENTITY sequence, it primarily enhances performance. Unless frequent restarts are common, the impact is minimal. However, several options exist to mitigate this:

• Use Sequences: Replace IDENTITY columns with SEQUENCE objects, allowing for finer control over cache size.
• Enable Trace Flag 272: Activates logging of identity value allocations for detailed analysis.
• Disable Identity Caching: Execute ALTER DATABASE SCOPED CONFIGURATION SET IDENTITY_CACHE = OFF; to disable caching for a specific database.

It's crucial to remember that even with these workarounds, concurrent inserts and transaction rollbacks can still produce gaps. For applications demanding absolute sequence continuity, consider alternatives like UUID generators or custom sequence implementations.

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