Read committed behavior in PostgreSQL - Time & Space Complexity

Time Complexity: Read committed behavior

O(n)

Understanding Time Complexity

When using read committed behavior in PostgreSQL, we want to understand how the time to read data changes as the amount of data grows.

We ask: How does the database handle reading data when other transactions are changing it?

Scenario Under Consideration

Analyze the time complexity of the following SQL query under read committed isolation.

BEGIN;
SELECT * FROM orders WHERE status = 'pending';
-- Other transactions may update orders concurrently
COMMIT;

This code reads all orders with status 'pending' while other transactions might be updating the table.

Identify Repeating Operations

Look for repeated work done during the query execution.

• Primary operation: Scanning rows in the orders table to find those with status 'pending'.
• How many times: Once per query execution, scanning all relevant rows.

How Execution Grows With Input

As the number of orders grows, the time to scan and read pending orders grows roughly in proportion.

Input Size (n)	Approx. Operations
10	About 10 row checks
100	About 100 row checks
1000	About 1000 row checks

Pattern observation: The work grows linearly with the number of rows scanned.

Final Time Complexity

Time Complexity: O(n)

This means the time to read pending orders grows roughly in direct proportion to the number of rows checked.

Common Mistake

[X] Wrong: "Read committed means the query always reads a fixed small number of rows quickly regardless of table size."

[OK] Correct: The query still scans rows to find matching data, so more rows mean more work, even if it sees only committed data.

Interview Connect

Understanding how read committed isolation affects query time helps you explain database behavior clearly and shows you grasp how transactions impact performance.

Self-Check

What if we added an index on the status column? How would the time complexity change?

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