Read committed behavior in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a transaction with read committed isolation level in PostgreSQL.

PostgreSQL

BEGIN TRANSACTION ISOLATION LEVEL [1];

Drag options to blanks, or click blank then click option'

Aread committed

Bserializable

Crepeatable read

Dread uncommitted

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'read uncommitted' which is not supported in PostgreSQL.

Confusing 'repeatable read' with 'read committed'.

The read committed isolation level ensures that any data read is committed at the moment it is read. This is the default in PostgreSQL.

2fill in blank

medium

Complete the SQL query to select data inside a read committed transaction.

PostgreSQL

BEGIN TRANSACTION ISOLATION LEVEL read committed; SELECT * FROM [1]; COMMIT;

Drag options to blanks, or click blank then click option'

Ausers

Btransactions

Corders

Dproducts

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a table name that does not exist in the database.

Forgetting to commit the transaction.

The users table is selected here as an example to show data reading inside a read committed transaction.

3fill in blank

hard

Fix the error in the transaction isolation level syntax.

PostgreSQL

BEGIN TRANSACTION ISOLATION [1] read committed;

Drag options to blanks, or click blank then click option'

AAT

BLEVEL

CWITH

DON

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'WITH' or 'ON' which are invalid here.

Omitting the keyword entirely.

The correct syntax in PostgreSQL is BEGIN TRANSACTION ISOLATION LEVEL read committed;. The keyword between ISOLATION and read committed is LEVEL.

4fill in blank

hard

Fill both blanks to create a query that reads only committed data and avoids dirty reads.

PostgreSQL

SET TRANSACTION ISOLATION LEVEL [1]; SELECT * FROM [2] WHERE status = 'active';

Drag options to blanks, or click blank then click option'

ALEVEL

Bread committed

Cusers

Dorders

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the blanks or using wrong keywords.

Using a table name that does not exist.

The command SET TRANSACTION ISOLATION LEVEL read committed; sets the isolation level correctly. The users table is used here to select active users.

5fill in blank

hard

Fill all three blanks to write a transaction that sets read committed isolation, updates a table, and commits.

PostgreSQL

BEGIN TRANSACTION ISOLATION LEVEL [1]; UPDATE [2] SET status = 'completed' WHERE id = [3]; COMMIT;

Drag options to blanks, or click blank then click option'

Aread committed

Btasks

C42

Dserializable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'serializable' instead of 'read committed' for this example.

Forgetting to commit the transaction.

Using a non-numeric id value.

This transaction sets the isolation level to read committed, updates the tasks table setting the status to 'completed' for the task with id 42, and then commits the changes.

Practice

1. What does the Read Committed isolation level guarantee in PostgreSQL?

easy

A. It prevents phantom reads by locking the entire table.

B. It allows reading uncommitted (dirty) data from other transactions.

C. It only reads data that has been committed by other transactions.

D. It locks all rows read until the transaction ends.

Solution

1. Step 1: Understand Read Committed isolation

   Read Committed isolation level ensures that a transaction sees only data committed before the query began, avoiding dirty reads.

2. Step 2: Compare options with definition

   It only reads data that has been committed by other transactions. matches this definition exactly. The other options describe behaviors of other isolation levels or incorrect behaviors.

3. Final Answer:

   It only reads data that has been committed by other transactions. -> Option C

4. Quick Check:

   Read Committed = no dirty reads [OK]

Hint: Read Committed means no dirty reads, only committed data [OK]

Common Mistakes:

• Confusing Read Committed with Read Uncommitted
• Thinking it locks rows until transaction ends
• Assuming it prevents phantom reads

2. Which of the following is the correct way to set the transaction isolation level to Read Committed in PostgreSQL?

easy

A. SET TRANSACTION LEVEL READ COMMITTED;

B. SET ISOLATION LEVEL READ COMMITTED TRANSACTION;

C. BEGIN TRANSACTION ISOLATION READ COMMITTED;

D. SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

Solution

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

   The correct syntax in PostgreSQL is: SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

2. Step 2: Check each option

   SET TRANSACTION ISOLATION LEVEL READ COMMITTED; matches the correct syntax exactly. The other options have incorrect word order or missing keywords.

3. Final Answer:

   SET TRANSACTION ISOLATION LEVEL READ COMMITTED; -> Option D

4. Quick Check:

   SET TRANSACTION ISOLATION LEVEL READ COMMITTED; [OK]

Hint: Remember: SET TRANSACTION ISOLATION LEVEL ... [OK]

Common Mistakes:

• Mixing order of keywords
• Omitting 'ISOLATION' or 'LEVEL'
• Using BEGIN with isolation level incorrectly

3. Consider two transactions running concurrently under Read Committed isolation. Transaction 1 updates a row but has not committed yet. Transaction 2 tries to read that same row. What will Transaction 2 see?

medium

A. The original data before Transaction 1's update.

B. The updated but uncommitted data from Transaction 1.

C. An error due to concurrent update conflict.

D. No data, the row is locked and invisible.

Solution

1. Step 1: Understand Read Committed behavior on concurrent reads

   Under Read Committed, a transaction sees only committed data. Uncommitted changes from other transactions are invisible.

2. Step 2: Apply to scenario

   Transaction 1's update is uncommitted, so Transaction 2 reads the original committed data before the update.

3. Final Answer:

   The original data before Transaction 1's update. -> Option A

4. Quick Check:

   Read Committed hides uncommitted changes [OK]

Hint: Uncommitted changes are invisible under Read Committed [OK]

Common Mistakes:

• Assuming dirty reads are allowed
• Thinking a read error occurs
• Believing the row is locked and unreadable

4. You wrote this code in PostgreSQL:

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
SELECT balance FROM accounts WHERE id = 1;
COMMIT;

But you notice the SELECT shows the updated balance even before COMMIT. Why?

medium

A. Because the transaction reads its own uncommitted changes under Read Committed.

B. Because the isolation level is set to Read Uncommitted.

C. Because SELECT statements ignore transaction boundaries.

D. Because the UPDATE was not executed properly.

Solution

1. Step 1: Understand transaction visibility in Read Committed

   In Read Committed, a transaction sees its own changes immediately, even if not committed yet.

2. Step 2: Apply to given code

   The SELECT inside the same transaction sees the updated balance from the UPDATE before COMMIT.

3. Final Answer:

   Because the transaction reads its own uncommitted changes under Read Committed. -> Option A

4. Quick Check:

   Transaction sees own changes before commit [OK]

Hint: A transaction always sees its own changes immediately [OK]

Common Mistakes:

• Confusing Read Committed with Read Uncommitted
• Thinking SELECT ignores transaction boundaries
• Assuming UPDATE failed without checking

5. You want to avoid phantom reads in a banking app using PostgreSQL. You currently use Read Committed isolation. Which approach best prevents phantom reads while keeping most benefits of Read Committed?

hard

A. Switch to Repeatable Read isolation level for the transaction.

B. Use explicit row-level locks with SELECT FOR UPDATE.

C. Increase the transaction timeout to avoid conflicts.

D. Use Read Committed but commit after every statement.

Solution

1. Step 1: Understand phantom reads and Read Committed

   Read Committed does not prevent phantom reads (new rows appearing during a transaction).

2. Step 2: Evaluate options to prevent phantom reads

   Switching to Repeatable Read prevents phantom reads but changes the isolation level and may reduce concurrency. Increasing transaction timeout or committing after every statement does not prevent phantom reads. Using explicit row-level locks with SELECT FOR UPDATE keeps Read Committed while preventing changes to selected rows, mitigating phantom reads.

3. Final Answer:

   Use explicit row-level locks with SELECT FOR UPDATE. -> Option B

4. Quick Check:

   Row-level locks prevent phantom reads under Read Committed [OK]

Hint: Use SELECT FOR UPDATE to lock rows and avoid phantoms [OK]

Common Mistakes:

• Assuming Repeatable Read is always best
• Ignoring phantom reads in Read Committed
• Thinking timeout affects phantom reads