Read committed behavior in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does Read Committed isolation level mean in PostgreSQL?

It means a transaction only sees data committed before the query started or committed during its execution, but it never sees uncommitted changes from other transactions.

intermediate

In Read Committed mode, can a query inside a transaction see changes made by other transactions that commit after the query started?

No, each query sees only data committed before that query began, not changes committed after the query started.

beginner

How does Read Committed behavior prevent dirty reads?

It prevents dirty reads by never showing data from transactions that have not yet committed.

intermediate

Can Read Committed isolation level cause non-repeatable reads?

Yes, because each query sees the latest committed data, data can change between queries in the same transaction.

beginner

What is a real-life example of Read Committed behavior?

Imagine reading a shared notebook: you only see notes that others have finished writing and put away, but if they add new notes later, your next read will see those new notes.

Which of the following best describes Read Committed isolation level?

A Queries see only committed data as of the start of each query

B Queries see all data including uncommitted changes

C Queries see a snapshot of data as of the start of the transaction

D Queries lock all rows they read

Read Committed means each query sees only data committed before that query started.

Can a transaction in Read Committed isolation see changes committed by another transaction after it started but before its next query?

A No, it only sees data committed before the transaction started

B Yes, each query sees the latest committed data at its start

C No, it sees a fixed snapshot throughout the transaction

D Yes, but only if it explicitly refreshes

In Read Committed, each query sees the latest committed data at the time the query starts.

Which problem is prevented by Read Committed isolation?

A Lost updates

B Phantom reads

C Non-repeatable reads

D Dirty reads

Read Committed prevents dirty reads by not showing uncommitted data.

Which problem can still occur under Read Committed isolation?

A Dirty reads

B Write skew

C Non-repeatable reads

D Reading uncommitted data

Non-repeatable reads can happen because data can change between queries in the same transaction.

In PostgreSQL, what is the default transaction isolation level?

A Read Committed

B Repeatable Read

C Serializable

D Read Uncommitted

PostgreSQL uses Read Committed as the default isolation level.

Explain how Read Committed isolation level controls visibility of data changes during a transaction.

Think about when data becomes visible to each query.

Describe a real-life analogy that helps understand Read Committed behavior.

Imagine reading notes others write and finish at different times.

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