Isolation levels in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Read Phenomena in Isolation Levels

Which isolation level in MySQL prevents dirty reads but allows non-repeatable reads and phantom reads?

A READ UNCOMMITTED

B SERIALIZABLE

C REPEATABLE READ

D READ COMMITTED

💡 Hint

Think about which isolation level only blocks reading uncommitted changes but does not lock rows for repeatable reads.

READ COMMITTED prevents dirty reads by only reading committed data. However, it does not guarantee repeatable reads or prevent phantom reads.

❓ query_result

intermediate

Effect of REPEATABLE READ on Query Results

Given two transactions running under REPEATABLE READ isolation, if Transaction 1 reads rows matching a condition, and Transaction 2 inserts a new matching row before Transaction 1 commits, what will Transaction 1 see if it re-issues the same query?

A It will see the new row inserted by Transaction 2.

B It will cause a deadlock error.

C It will see the original rows only, excluding the new row.

D It will see partial data causing inconsistent results.

💡 Hint

REPEATABLE READ guarantees the same rows for repeated reads within the same transaction.

Under REPEATABLE READ, a transaction sees a consistent snapshot of data as of the start of the transaction, so new rows inserted by others are not visible until commit.

📝 Syntax

advanced

Setting Isolation Level Syntax

Which of the following MySQL commands correctly sets the transaction isolation level to SERIALIZABLE for the current session?

A SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;

B SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

C SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;

D SET ISOLATION LEVEL SERIALIZABLE;

💡 Hint

Consider the scope of the setting and correct syntax for session level.

To set the isolation level for the current session, use SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;. Option A sets it for the next transaction only, C sets globally, and D is invalid syntax.

❓ optimization

advanced

Choosing Isolation Level for High Concurrency

You have a high-traffic MySQL database with many concurrent read and write transactions. Which isolation level is generally best to balance data consistency and performance?

A REPEATABLE READ

B READ COMMITTED

C READ UNCOMMITTED

D SERIALIZABLE

💡 Hint

Consider the default MySQL isolation level and its trade-offs.

REPEATABLE READ is MySQL's default and balances consistency with performance by preventing dirty and non-repeatable reads without the heavy locking of SERIALIZABLE.

🔧 Debug

expert

Diagnosing Phantom Reads in SERIALIZABLE Isolation

A developer claims that phantom reads occur even when using SERIALIZABLE isolation level in MySQL. Which of the following is the most likely cause?

A The transaction is using READ COMMITTED instead of SERIALIZABLE.

B The storage engine does not support gap locking.

C The SELECT queries are not using FOR UPDATE or LOCK IN SHARE MODE.

D The transaction is using REPEATABLE READ, not SERIALIZABLE.

💡 Hint

Phantom reads are prevented by gap locks which depend on storage engine support.

Phantom reads can still occur if the storage engine (like MyISAM) does not support gap locking, even under SERIALIZABLE isolation. InnoDB supports gap locking which prevents phantoms.

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