What are Isolation Levels?
| Level | Speed | Safety | When to Use |
|---|---|---|---|
| Read Uncommitted | Fastest | Risky | Rarely |
| Read Committed | Fast | Safe | Default (most common) |
| Repeatable Read | Slower | Very Safe | Important data |
| Serializable | Slowest | Safest | Banking, money |
Isolation levels control how transactions interact when multiple people use the database at the same time.
Simple analogy: Like rules for sharing a bathroom - how much privacy do you need?
The Problem
| Level | Speed | Safety | When to Use |
|---|---|---|---|
| Read Uncommitted | Fastest | Risky | Rarely |
| Read Committed | Fast | Safe | Default (most common) |
| Repeatable Read | Slower | Very Safe | Important data |
| Serializable | Slowest | Safest | Banking, money |
Two people updating same data at same time:
- Person A reads: stock = 10
- Person B reads: stock = 10
- Person A updates: stock = 9 (sold 1)
- Person B updates: stock = 8 (sold 2)
- Final stock should be 7, but might be 8 or 9!
Isolation levels solve this problem.
Four Isolation Levels (Simple)
| Level | Speed | Safety | When to Use |
|---|---|---|---|
| Read Uncommitted | Fastest | Risky | Rarely |
| Read Committed | Fast | Safe | Default (most common) |
| Repeatable Read | Slower | Very Safe | Important data |
| Serializable | Slowest | Safest | Banking, money |
1. Read Uncommitted (Least Strict)
Can see changes other people have not saved yet.
Analogy: Peeking at someone's unfinished homework.
Risky! Might see data that gets cancelled.
2. Read Committed (Common Default)
Can only see changes that are saved (COMMITTED).
Analogy: Only read homework after it is submitted.
Safe! Most databases use this by default.
3. Repeatable Read
Data you read stays the same throughout your transaction.
Analogy: Taking a photo - people in photo do not change even if they move.
4. Serializable (Most Strict)
Transactions run one at a time (like a queue).
Analogy: One person uses bathroom at a time.
Safest! But slowest (must wait in line).
Simple Example
| Level | Speed | Safety | When to Use |
|---|---|---|---|
| Read Uncommitted | Fastest | Risky | Rarely |
| Read Committed | Fast | Safe | Default (most common) |
| Repeatable Read | Slower | Very Safe | Important data |
| Serializable | Slowest | Safest | Banking, money |
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
BEGIN;
SELECT * FROM products WHERE id = 1;
-- Other people's uncommitted changes are not visible
COMMIT;When to Use Each Level
| Level | Speed | Safety | When to Use |
|---|---|---|---|
| Read Uncommitted | Fastest | Risky | Rarely |
| Read Committed | Fast | Safe | Default (most common) |
| Repeatable Read | Slower | Very Safe | Important data |
| Serializable | Slowest | Safest | Banking, money |
Read Committed: Most common, good balance Repeatable Read: When consistency is very important Serializable: Banking, money transfers (must be perfect) Read Uncommitted: Rarely used (risky)
Summary
| Level | Speed | Safety | When to Use |
|---|---|---|---|
| Read Uncommitted | Fastest | Risky | Rarely |
| Read Committed | Fast | Safe | Default (most common) |
| Repeatable Read | Slower | Very Safe | Important data |
| Serializable | Slowest | Safest | Banking, money |
Isolation Levels = Rules for multiple users
- Read Committed: Most common (default)
- Serializable: Safest but slowest
- Controls what users see during transactions
Most of the time, default settings work fine!