Read phenomena (dirty reads, phantom reads) in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a dirty read in database transactions?

A dirty read happens when a transaction reads data that has been modified by another transaction but not yet committed. If the other transaction rolls back, the read data becomes invalid.

beginner

Explain phantom reads in simple terms.

Phantom reads occur when a transaction reads a set of rows matching a condition, but another transaction inserts or deletes rows that change the result set if the first transaction reads again.

intermediate

How does read committed isolation level handle dirty reads?

The read committed isolation level prevents dirty reads by ensuring a transaction only reads data that has been committed by other transactions.

intermediate

What is the difference between non-repeatable reads and phantom reads?

Non-repeatable reads happen when a row's data changes between reads in the same transaction. Phantom reads happen when new rows appear or disappear between reads due to inserts or deletes.

advanced

Which isolation level can prevent phantom reads?

The serializable isolation level prevents phantom reads by locking the range of rows so no new rows can be inserted or deleted during the transaction.

What does a dirty read allow a transaction to do?

A Prevent other transactions from reading data

B Read uncommitted changes from another transaction

C Read only committed data

D Lock rows to prevent changes

Dirty reads allow reading data that another transaction has changed but not yet committed.

Which isolation level prevents dirty reads?

A Read committed

B Read uncommitted

C Repeatable read

D Serializable

Read committed isolation level ensures only committed data is read, preventing dirty reads.

Phantom reads occur when:

A New rows appear or disappear between reads

B A row is updated twice in the same transaction

C Uncommitted data is read

D A transaction is rolled back

Phantom reads happen when new rows are inserted or deleted, changing the result set between reads.

Which isolation level is the strictest and prevents phantom reads?

A Read uncommitted

B Read committed

C Repeatable read

D Serializable

Serializable isolation level prevents phantom reads by locking ranges of rows.

Non-repeatable reads differ from phantom reads because non-repeatable reads involve:

A Reading uncommitted data

B Transaction rollbacks

C Changes to existing rows between reads

D New rows appearing or disappearing

Non-repeatable reads happen when existing rows change between reads, unlike phantom reads which involve new rows.

Describe what dirty reads and phantom reads are, and give a simple example of each.

Think about reading data that might change or appear/disappear during a transaction.

Explain how different isolation levels affect the occurrence of dirty reads and phantom reads.

Consider how strict the rules are for reading data in each isolation level.

Practice

1. Which of the following best describes a dirty read in database transactions?

easy

A. Reading data that has been modified but not yet committed by another transaction.

B. Seeing new rows appear in a query result after re-running the same query.

C. Reading only committed data and never uncommitted changes.

D. Locking rows to prevent other transactions from reading them.

Solution

1. Step 1: Understand dirty read definition

   A dirty read happens when a transaction reads data that another transaction has changed but not committed yet.

2. Step 2: Compare options to definition

   Reading data that has been modified but not yet committed by another transaction. matches this exactly. Seeing new rows appear in a query result after re-running the same query. describes phantom reads, C describes a safe read, and D is about locking, not dirty reads.

3. Final Answer:

   Reading data that has been modified but not yet committed by another transaction. -> Option A

4. Quick Check:

   Dirty read = reading uncommitted data [OK]

Hint: Dirty read = reading uncommitted changes [OK]

Common Mistakes:

• Confusing dirty read with phantom read
• Thinking dirty read only reads committed data
• Mixing locking with read phenomena

2. Which SQL transaction isolation level allows dirty reads?

easy

A. SERIALIZABLE

B. REPEATABLE READ

C. READ COMMITTED

D. READ UNCOMMITTED

Solution

1. Step 1: Recall isolation levels and dirty reads

   Only the READ UNCOMMITTED isolation level allows dirty reads, meaning it can read uncommitted changes.

2. Step 2: Match options to isolation levels

   READ COMMITTED and higher levels prevent dirty reads. SERIALIZABLE is the strictest. So, READ UNCOMMITTED is correct.

3. Final Answer:

   READ UNCOMMITTED -> Option D

4. Quick Check:

   Dirty reads allowed only in READ UNCOMMITTED [OK]

Hint: Dirty reads only happen at READ UNCOMMITTED level [OK]

Common Mistakes:

• Choosing READ COMMITTED as allowing dirty reads
• Confusing phantom reads with dirty reads
• Not knowing isolation level hierarchy

3. Consider two transactions running concurrently on a table Orders. Transaction 1 reads all rows where status = 'pending'. Transaction 2 inserts a new row with status = 'pending' and commits. If Transaction 1 repeats the read, what phenomenon might it observe?

medium

A. Dirty read

B. Non-repeatable read

C. Phantom read

D. Lost update

Solution

1. Step 1: Analyze the scenario with inserted rows

   Transaction 1 reads rows with status 'pending'. Transaction 2 inserts a new 'pending' row and commits.

2. Step 2: Identify the phenomenon on repeated read

   When Transaction 1 reads again, it sees the new row that wasn't there before. This is a phantom read, where new rows appear between reads.

3. Final Answer:

   Phantom read -> Option C

4. Quick Check:

   New rows appearing = Phantom read [OK]

Hint: New rows between reads = phantom read [OK]

Common Mistakes:

• Confusing phantom read with dirty read
• Thinking non-repeatable read applies to new rows
• Mixing lost update with read phenomena

4. A developer wrote this SQL transaction code but observes inconsistent reads. Identify the error related to read phenomena:

BEGIN TRANSACTION;
SELECT * FROM Products WHERE price > 100;
-- Another transaction inserts a new product with price 150 and commits
SELECT * FROM Products WHERE price > 100;
COMMIT;

What is the likely cause?

medium

A. Using READ UNCOMMITTED isolation level causing dirty reads

B. Using READ COMMITTED isolation level causing phantom reads

C. Using SERIALIZABLE isolation level causing lost updates

D. Using REPEATABLE READ isolation level causing non-repeatable reads

Solution

1. Step 1: Understand the scenario with repeated SELECTs

   The transaction reads products with price > 100 twice. Between reads, another transaction inserts a new matching row and commits.

2. Step 2: Identify the read phenomenon

   Because the isolation level is likely READ COMMITTED, the second read sees the new row, causing a phantom read.

3. Final Answer:

   Using READ COMMITTED isolation level causing phantom reads -> Option B

4. Quick Check:

   Phantom reads occur at READ COMMITTED [OK]

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

Common Mistakes:

• Assuming dirty reads cause this behavior
• Confusing lost updates with read phenomena
• Thinking REPEATABLE READ allows phantom reads

5. You want to prevent both dirty reads and phantom reads in your application. Which transaction isolation level should you choose?

hard

A. SERIALIZABLE

B. READ COMMITTED

C. REPEATABLE READ

D. READ UNCOMMITTED

Solution

1. Step 1: Review isolation levels and phenomena prevented

   READ UNCOMMITTED allows dirty reads. READ COMMITTED prevents dirty reads but allows phantom reads. REPEATABLE READ prevents dirty and non-repeatable reads but may allow phantom reads.

2. Step 2: Identify level preventing both dirty and phantom reads

   SERIALIZABLE is the strictest level and prevents dirty reads, non-repeatable reads, and phantom reads by fully isolating transactions.

3. Final Answer:

   SERIALIZABLE -> Option A

4. Quick Check:

   SERIALIZABLE prevents dirty and phantom reads [OK]

Hint: Use SERIALIZABLE to prevent dirty and phantom reads [OK]

Common Mistakes:

• Choosing REPEATABLE READ thinking it blocks phantom reads
• Confusing READ COMMITTED with preventing all read anomalies
• Ignoring performance impact of SERIALIZABLE