Explain Snapshot Isolation Pitfalls.

Snapshot isolation ensures each transaction reads from a consistent snapshot of the database at its start, avoiding dirty reads but allowing certain anomalies like write skew.

When to Use

Snapshot isolation works best in read-heavy workloads such as analytics dashboards, reporting systems, or systems where read consistency is critical but occasional conflicts are tolerable.

Example

Two users each reduce inventory for the same item from 1 to 0 simultaneously. Both see the same snapshot and commit, resulting in a negative stock—a write-skew anomaly.

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Why It Is Important

It balances concurrency and consistency—readers don’t block writers. Databases like PostgreSQL and SQL Server use it to improve throughput while maintaining logical isolation.

Interview Tips

• Compare snapshot isolation with serializable and read committed levels.
• Mention write skew as its key pitfall.
• Explain how databases detect and handle update conflicts.

Trade-offs

Pros: High concurrency, non-blocking reads. Cons: Memory overhead from versioning, risk of anomalies, and potential transaction rollbacks.

Pitfalls

• Misunderstanding it as fully serializable.
• Ignoring write skew anomalies.
• Long transactions causing version bloat.
• Overreliance without monitoring conflict rates.

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