SQL Isolation Levels and Their Effect on Concurrency
💡 Concept Name
SQL Isolation Levels – Set the rules that govern how transactions interact with each other and the data, balancing data consistency with concurrency.
📘 Quick Intro
Isolation levels define how and when the changes made by one operation become visible to others. They help prevent anomalies like dirty reads, non-repeatable reads, and phantom reads during concurrent transactions.
🧠 Analogy / Short Story
Think of a library with multiple readers and writers. The isolation level determines how much each visitor can see or be interrupted by others. At higher isolation, it's like a private room — fewer disruptions but slower service. At lower levels, it's like reading in a busy hall — faster, but you might catch glimpses of unfinished stories.
🔧 Technical Explanation
- Read Uncommitted – Allows dirty reads; highest concurrency, least safety.
- Read Committed – Prevents dirty reads; default in most DBMS.
- Repeatable Read – Prevents dirty and non-repeatable reads; allows phantom reads.
- Serializable – Strictest; prevents all anomalies but with lowest concurrency.
- Snapshot (optional) – Uses versioning to provide consistent reads without locks.
💻 Code Examples
-- Set isolation level
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
SELECT * FROM Orders WHERE CustomerID = 1;
-- No other transaction can insert/update these rows until commit
COMMIT;
❓ Interview Q&A
Q1: What is an isolation level in SQL?
A: It controls how visible the changes of one transaction are to other concurrent transactions.
Q2: What is a dirty read?
A: Reading uncommitted changes from another transaction.
Q3: Which level allows dirty reads?
A: Read Uncommitted.
Q4: What issues does Repeatable Read prevent?
A: Dirty and non-repeatable reads, but not phantom reads.
Q5: What is the safest isolation level?
A: Serializable – it emulates serial execution.
Q6: What is a phantom read?
A: When a row appears/disappears between reads in the same transaction.
Q7: What does Read Committed prevent?
A: Only dirty reads.
Q8: What are the trade-offs between isolation and concurrency?
A: Higher isolation reduces concurrency and can lead to blocking.
Q9: What is Snapshot isolation?
A: It uses versioning to avoid locks and anomalies.
Q10: Is Serializable always the best choice?
A: Not always; it’s safest but may hurt performance under high load.
📝 MCQs
Q1. Which isolation level allows dirty reads?
- Read Committed
- Repeatable Read
- Read Uncommitted
- Serializable
Q2. Which isolation level is the default in most databases?
- Serializable
- Read Uncommitted
- Snapshot
- Read Committed
Q3. What is a phantom read?
- Reading dirty data
- Row updates between reads
- New rows appear/disappear between reads
- Syntax error
Q4. Which level avoids both dirty and non-repeatable reads?
- Read Uncommitted
- Read Committed
- Repeatable Read
- Snapshot
Q5. What does Serializable isolation level guarantee?
- Dirty reads only
- Full concurrency
- No anomalies; full consistency
- No locks used
Q6. What is the effect of higher isolation?
- Faster writes
- Lower concurrency
- More locks released
- Better UI
Q7. What mechanism does Snapshot isolation use?
- Locking
- Triggers
- Versioning
- Replication
Q8. Which isolation level prevents only dirty reads?
- Read Uncommitted
- Serializable
- Read Committed
- Repeatable Read
Q9. What is a dirty read?
- Reading NULLs
- Reading from temp table
- Reading uncommitted data
- Reading foreign key rows
Q10. Which isolation level has highest safety?
- Read Uncommitted
- Read Committed
- Serializable
- Snapshot
💡 Bonus Insight
Use isolation levels wisely based on your workload. In OLTP systems, defaulting to Read Committed often offers a good balance. For reporting, Snapshot or Serializable may be needed. Always test performance under realistic concurrency conditions.
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