Why Do Gaps Appear in SQL Server IDENTITY Column Values?

Understanding Gaps in SQL Server IDENTITY Columns

SQL Server's IDENTITY property automatically increments column values during row insertion. However, this doesn't guarantee a perfectly sequential, gap-free series of numbers.

Why Gaps Appear

Several factors can lead to gaps in IDENTITY column values:

• Uniqueness isn't inherent: The IDENTITY property alone doesn't ensure unique values. A UNIQUE constraint or index is essential.
• Concurrent Inserts: Multiple simultaneous inserts can create gaps if the database doesn't use exclusive locking or a SERIALIZABLE isolation level.
• Server Restarts: The server's cache may be reset on restart, potentially leading to lost identity values and gaps.
• Rollback Transactions: Failed or rolled-back inserts reserve identity values that aren't recycled.
• Data Deletion: Frequent deletions leave gaps in the sequence.

Minimizing Gaps

Here's how to reduce the occurrence of gaps:

• Enforce Uniqueness: Always use PRIMARY KEY or UNIQUE constraints.
• Control Concurrency: Employ exclusive locks or the SERIALIZABLE isolation level for transactions.
• Avoid Value Reuse: Use a sequence generator with the NOCACHE option, or consider a custom key generation strategy.

Best Practices

• Maintain an identity increment of 1.
• Carefully assess existing values before manually assigning an identity value to prevent further gaps.
• If deletions are frequent, explore alternative approaches to data management.

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