MediumExperience level: 3-5 years · 8-15 years1 min read

Dirty Read, Non-Repeatable Read & Phantom Read

Reviewed by Gurusankar M. · Updated Aug 24, 2026

Asked inAmazonMicrosoftbankingDeloitte
#dirty read#non-repeatable read#phantom read#isolation#anomalies
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Short Answer

Dirty read = seeing another txn's uncommitted data (prevented by READ COMMITTED). Non-repeatable read = same row returns different values within a txn (prevented by REPEATABLE READ). Phantom read = new rows appear for a re-run range query (prevented by SERIALIZABLE).

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Concept Made Simple

Coffee Chat Question

“Dirty read, non-repeatable read and phantom read — what are they and how do you prevent each?”

Remember It Faster

Mind Map Answer

Dirty read
reads uncommitted data → READ COMMITTED
Non-repeatable
row value changes → REPEATABLE READ
Phantom
new rows in range → SERIALIZABLE

Think Beyond the Expected

What If?

A balance check then debit double-spends under load — which anomaly, and the fix?

A read-then-write race (often a phantom/non-repeatable issue). Fix with SELECT ... FOR UPDATE to lock the row during the transaction, or optimistic locking with a version check, rather than just raising the global isolation level.

Real World

These anomalies underlie real money bugs (double-spend, oversell). Engineers pick the isolation level — or explicit locking — that blocks the specific anomaly their invariant needs.

Interviewer's Expectation

Keywords they're listening for:

  • define all three
  • which level prevents which
  • read-then-write race
  • FOR UPDATE / optimistic locking
  • MVCC nuance

Common Mistakes

  • Confusing non-repeatable read with phantom read
  • Relying on app logic for read-then-write safety
  • Assuming default isolation prevents all anomalies

Best Practices

  • Lock or version read-then-write critical sections
  • Match isolation to the invariant you must protect
  • Test concurrency paths under load

Follow-up Questions

  1. How does MVCC handle non-repeatable reads without locks?
  2. What's a write skew and which level prevents it?
  3. When do you reach for SELECT FOR UPDATE?

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: Transactions (SQL)
Interview question: "Dirty read, non-repeatable read and phantom read — what are they and how do you prevent each?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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