Concurrency Problems
1. Dirty Read
Reading uncommitted changes from another transaction:
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LearnStep 1 of 2
Isolation Levels
Learning Objectives
- •Understand concurrency problems
- •Learn isolation levels
Lesson Outline
LearnStep 1/2
Isolation Levels
2. Non-Repeatable Read
Same query returns different results:
3. Phantom Read
New rows appear between reads:
Isolation Levels
| Level | Dirty Read | Non-Repeatable | Phantom |
|---|---|---|---|
| Read Uncommitted | ✓ Possible | ✓ Possible | ✓ Possible |
| Read Committed | ✗ Prevented | ✓ Possible | ✓ Possible |
| Repeatable Read | ✗ Prevented | ✗ Prevented | ✓ Possible |
| Serializable | ✗ Prevented | ✗ Prevented | ✗ Prevented |
Setting Isolation Level
sql
Trade-offs
Higher isolation = More correct but slower
Lower isolation = Faster but may have anomalies