Isolation levels in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of isolation levels in a database?

Isolation levels control how transaction changes are visible to other transactions, helping to prevent problems like dirty reads, non-repeatable reads, and phantom reads.

beginner

Name the four standard isolation levels defined by SQL.

The four standard isolation levels are:
1. Read Uncommitted
2. Read Committed
3. Repeatable Read
4. Serializable

intermediate

What problem does the Read Uncommitted isolation level allow?

Read Uncommitted allows dirty reads, meaning a transaction can see uncommitted changes made by other transactions.

intermediate

Explain the difference between Repeatable Read and Serializable isolation levels.

Repeatable Read prevents dirty and non-repeatable reads but may allow phantom reads. Serializable is the strictest level, preventing dirty reads, non-repeatable reads, and phantom reads by making transactions appear fully isolated.

beginner

Which isolation level is the default in MySQL InnoDB engine?

The default isolation level in MySQL InnoDB is Repeatable Read.

Which isolation level allows a transaction to read uncommitted changes from another transaction?

A Read Uncommitted

B Read Committed

C Repeatable Read

D Serializable

Read Uncommitted allows dirty reads, meaning it can see uncommitted changes.

Which isolation level prevents dirty reads but allows non-repeatable reads?

A Read Uncommitted

B Read Committed

C Repeatable Read

D Serializable

Read Committed prevents dirty reads but does not prevent non-repeatable reads.

What problem does the Serializable isolation level prevent that Repeatable Read might not?

A Dirty reads

B Non-repeatable reads

C Phantom reads

D Lost updates

Serializable prevents phantom reads, which Repeatable Read may allow.

In MySQL InnoDB, what is the default isolation level?

A Repeatable Read

B Read Committed

C Read Uncommitted

D Serializable

MySQL InnoDB uses Repeatable Read as the default isolation level.

Which isolation level offers the highest data consistency but may reduce concurrency?

A Read Uncommitted

B Read Committed

C Repeatable Read

D Serializable

Serializable is the strictest isolation level, ensuring highest consistency but can reduce concurrency.

Describe the four standard isolation levels and the types of read phenomena they prevent.

Think about how each level controls visibility of changes between transactions.

Explain why the default isolation level in MySQL InnoDB is Repeatable Read and what benefits it provides.

Consider the trade-off between strictness and speed.

Practice

1. Which isolation level allows a transaction to read data that has been modified by other transactions but not yet committed?

easy

A. REPEATABLE READ

B. READ COMMITTED

C. READ UNCOMMITTED

D. SERIALIZABLE

Solution

1. Step 1: Understand isolation levels and dirty reads

   READ UNCOMMITTED allows reading uncommitted changes, causing dirty reads.

2. Step 2: Compare with other levels

   Other levels prevent dirty reads by requiring commits before reading.

3. Final Answer:

   READ UNCOMMITTED -> Option C

4. Quick Check:

   Dirty reads allowed = READ UNCOMMITTED [OK]

Hint: Dirty reads happen only in READ UNCOMMITTED [OK]

Common Mistakes:

• Confusing READ COMMITTED with allowing dirty reads
• Thinking SERIALIZABLE allows dirty reads
• Assuming REPEATABLE READ reads uncommitted data

2. Which of the following is the correct syntax to set the transaction isolation level to SERIALIZABLE in MySQL?

easy

A. SET ISOLATION SERIALIZABLE;

B. SET ISOLATION LEVEL = SERIALIZABLE;

C. SET TRANSACTION LEVEL SERIALIZABLE;

D. SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Solution

1. Step 1: Recall correct syntax for setting isolation level

   The correct syntax is: SET TRANSACTION ISOLATION LEVEL followed by the level.

2. Step 2: Check each option

   Only SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; matches the correct syntax exactly.

3. Final Answer:

   SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; -> Option D

4. Quick Check:

   Correct syntax starts with SET TRANSACTION ISOLATION LEVEL [OK]

Hint: Use full phrase: SET TRANSACTION ISOLATION LEVEL [OK]

Common Mistakes:

• Omitting TRANSACTION keyword
• Using = sign incorrectly
• Shortening syntax incorrectly

3. Consider two transactions T1 and T2 running concurrently. T1 reads a row, then T2 updates and commits that row. If the isolation level is REPEATABLE READ, what will T1 see when it reads the same row again?

medium

A. The updated value from T2

B. The original value before T2's update

C. An error due to conflict

D. A partially updated value

Solution

1. Step 1: Understand REPEATABLE READ behavior

   REPEATABLE READ ensures that repeated reads in the same transaction see the same data snapshot.

2. Step 2: Apply to scenario

   T1 will see the original value it read first, ignoring T2's committed update during T1's transaction.

3. Final Answer:

   The original value before T2's update -> Option B

4. Quick Check:

   REPEATABLE READ = consistent snapshot reads [OK]

Hint: REPEATABLE READ keeps data snapshot stable during transaction [OK]

Common Mistakes:

• Assuming T1 sees T2's committed update immediately
• Confusing with READ COMMITTED behavior
• Thinking it causes errors on concurrent updates

4. You set the isolation level to READ COMMITTED but notice phantom reads occurring. What is the likely cause?

medium

A. READ COMMITTED does not prevent phantom reads

B. The isolation level was not set correctly

C. Phantom reads only occur in READ UNCOMMITTED

D. The database engine does not support transactions

Solution

1. Step 1: Recall what phantom reads are

   Phantom reads happen when new rows appear in repeated queries within a transaction.

2. Step 2: Understand READ COMMITTED limits

   READ COMMITTED prevents dirty reads but does not prevent phantom reads.

3. Final Answer:

   READ COMMITTED does not prevent phantom reads -> Option A

4. Quick Check:

   Phantom reads allowed in READ COMMITTED [OK]

Hint: Phantom reads happen above READ COMMITTED level [OK]

Common Mistakes:

• Assuming READ COMMITTED blocks phantom reads
• Blaming incorrect isolation level setting
• Ignoring database engine capabilities

5. You want to ensure no dirty reads, no non-repeatable reads, and no phantom reads in a high-concurrency environment. Which isolation level should you choose and why?

hard

A. SERIALIZABLE, because it prevents all concurrency anomalies

B. READ UNCOMMITTED, because it allows fastest access

C. READ COMMITTED, because it balances speed and accuracy

D. REPEATABLE READ, because it prevents dirty and phantom reads

Solution

1. Step 1: Identify isolation level requirements

   No dirty reads, no non-repeatable reads, and no phantom reads means highest isolation.

2. Step 2: Match requirements to isolation levels

   SERIALIZABLE is the strictest level preventing all these anomalies by serializing transactions.

3. Step 3: Compare with other levels

   READ COMMITTED and REPEATABLE READ allow some anomalies; READ UNCOMMITTED allows dirty reads.

4. Final Answer:

   SERIALIZABLE, because it prevents all concurrency anomalies -> Option A

5. Quick Check:

   SERIALIZABLE = strictest isolation, no anomalies [OK]

Hint: Choose SERIALIZABLE for full isolation and no anomalies [OK]

Common Mistakes:

• Choosing REPEATABLE READ and ignoring phantom reads
• Picking READ COMMITTED for full isolation
• Confusing speed with isolation guarantees