Why Are There Gaps in My SQL Server IDENTITY Column Values?

Understanding and Resolving Gaps in SQL Server IDENTITY Columns

Managing tables with IDENTITY columns requires understanding that this feature doesn't automatically guarantee certain behaviors:

1. Value Uniqueness Isn't Guaranteed:

Unlike PRIMARY KEY or UNIQUE constraints, IDENTITY doesn't inherently ensure unique values.

2. Non-Consecutive Values in Concurrent Transactions:

Simultaneous inserts from multiple transactions can interrupt the sequential order of values within a single transaction. For guaranteed sequential values, employ exclusive table locks or SERIALIZABLE transaction isolation levels.

3. Gaps After Server Failures:

SQL Server's identity value caching can lead to lost assignments during failures or restarts, causing gaps. Consider using sequence generators with NOCACHE or a custom value generation strategy.

4. Unreusable Assigned Values:

Assigned identity values are not reused. Rollbacks or failed inserts consume these values, potentially creating gaps.

Further Points to Note:

• Frequent row deletions in tables with identity columns can magnify the gap problem.
• Check the Identity Column properties; confirm the Identity Increment is set to 1.
• Use SET IDENTITY_INSERT ON to manually insert specific values, helping to fill or analyze existing gaps.

By recognizing these limitations and implementing suitable strategies, such as enforcing exclusive access or employing alternative value generation techniques, you can effectively minimize gaps in your IDENTITY columns, maintaining data integrity and consistency.

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