MVCC mental model in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all rows from the table 'employees'.

PostgreSQL

SELECT * FROM [1];

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

Aemployees

Bdepartments

Csalaries

Dprojects

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong table name like 'departments' or 'projects'.

The table name to select from is 'employees'.

2fill in blank

medium

Complete the code to begin a transaction in PostgreSQL.

PostgreSQL

[1] TRANSACTION;

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

ASTOP

BBEGIN

CCOMMIT

DROLLBACK

Attempts:

3 left

💡 Hint

Common Mistakes

Using COMMIT or ROLLBACK instead of BEGIN.

To start a transaction, use the command BEGIN TRANSACTION.

3fill in blank

hard

Fix the error in the SQL to see the current transaction's snapshot.

PostgreSQL

SELECT txid_snapshot_[1]();

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

Acurrent

Blast

Cactive

Dfirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using txid_snapshot_last() or txid_snapshot_active() which do not exist.

The correct function to get the current transaction snapshot is txid_snapshot_current().

4fill in blank

hard

Fill both blanks to write a query that shows all visible rows for the current transaction using MVCC.

PostgreSQL

SELECT * FROM employees WHERE xmin [1] [2];

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

A<

B>

C1000

Dtxid_current()

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' or a fixed number instead of txid_current().

In MVCC, rows with xmin less than the current transaction ID (txid_current()) are visible.

5fill in blank

hard

Fill all three blanks to write a query that deletes rows visible to the current transaction and committed before it.

PostgreSQL

DELETE FROM employees WHERE ctid IN (SELECT ctid FROM employees WHERE xmin [1] [2] AND xmax = [3]);

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

A<

B>

C0

Dtxid_current()

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators or wrong values for xmax.

This query deletes rows where xmin is less than the current transaction ID and xmax is 0 (not deleted).

Practice

1. What does MVCC in PostgreSQL primarily allow multiple users to do?

easy

A. Delete data instantly without backups

B. Run only one transaction at a time

C. Work with data simultaneously without waiting for locks

D. Automatically create database indexes

Solution

1. Step 1: Understand MVCC purpose

   MVCC stands for Multi-Version Concurrency Control, which allows multiple users to access data concurrently.

2. Step 2: Identify MVCC effect in PostgreSQL

   It lets users work without waiting for locks by providing each transaction a snapshot of data.

3. Final Answer:

   Work with data simultaneously without waiting for locks -> Option C

4. Quick Check:

   MVCC = concurrent access without waiting [OK]

Hint: MVCC means no waiting for others' data changes [OK]

Common Mistakes:

• Thinking MVCC locks data exclusively
• Believing MVCC deletes old data immediately
• Assuming only one transaction runs at a time

2. Which SQL statement correctly starts a transaction in PostgreSQL to use MVCC?

easy

A. BEGIN;

B. START;

C. BEGINNING TRANSACTION;

D. OPEN TRANSACTION;

Solution

1. Step 1: Recall PostgreSQL transaction syntax

   PostgreSQL uses BEGIN; to start a transaction.

2. Step 2: Compare options

   Only A is valid syntax. B and C use incorrect keywords, D is invalid.

3. Final Answer:

   BEGIN; -> Option A

4. Quick Check:

   PostgreSQL transaction start = BEGIN; [OK]

Hint: Use BEGIN; to start transactions in PostgreSQL [OK]

Common Mistakes:

• Using START; which is invalid syntax
• Typing OPEN TRANSACTION; which is invalid
• Confusing transaction start with commit or rollback

3. Consider this sequence in PostgreSQL:
BEGIN;
SELECT * FROM products WHERE id = 1;
UPDATE products SET price = 20 WHERE id = 1;
COMMIT;
What does the SELECT see if another transaction updated the same row before this transaction started?

medium

A. An error due to concurrent update

B. The new price updated by the other transaction

C. No rows returned because of the update

D. The old price before the other transaction's update

Solution

1. Step 1: Understand snapshot isolation in MVCC

   The SELECT sees data as it was at transaction start, ignoring later committed changes.

2. Step 2: Apply to given scenario

   Since another transaction updated before this one started, the snapshot at start excludes that committed update, so SELECT sees the old price.

3. Final Answer:

   The old price before the other transaction's update -> Option D

4. Quick Check:

   MVCC snapshot = data at tx start [OK]

Hint: SELECT sees snapshot at transaction start, not later changes [OK]

Common Mistakes:

• Expecting an error due to concurrent update
• Thinking SELECT sees the old price before the other transaction's update
• Thinking no rows returned because of the update

4. You run this code:
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
SELECT balance FROM accounts WHERE id = 1;
ROLLBACK;
Why might the SELECT show the updated balance even though the transaction is not committed?

medium

A. Because the transaction sees its own changes inside the transaction

B. Because ROLLBACK commits the changes automatically

C. Because SELECT ignores transaction boundaries

D. Because balance is cached outside the database

Solution

1. Step 1: Understand visibility of changes inside a transaction

   Within a transaction, you see your own uncommitted changes.

2. Step 2: Explain why SELECT shows updated balance

   Even before commit, SELECT sees the updated balance because it's in the same transaction.

3. Final Answer:

   Because the transaction sees its own changes inside the transaction -> Option A

4. Quick Check:

   Transaction sees own changes before commit [OK]

Hint: Inside transaction, you see your own updates [OK]

Common Mistakes:

• Thinking ROLLBACK commits changes
• Believing SELECT ignores transaction state
• Assuming external cache affects SELECT results

5. In PostgreSQL, if two transactions try to update the same row simultaneously, what happens to maintain MVCC consistency?

hard

A. Both transactions update the row and overwrite each other

B. One transaction waits or fails due to a lock conflict

C. PostgreSQL merges both updates automatically

D. The second transaction reads old data but commits anyway

Solution

1. Step 1: Understand MVCC row update behavior

   PostgreSQL uses row-level locks to prevent conflicting updates.

2. Step 2: Explain conflict resolution

   When two transactions update the same row, one waits or fails to keep data consistent.

3. Final Answer:

   One transaction waits or fails due to a lock conflict -> Option B

4. Quick Check:

   Concurrent updates cause lock wait or failure [OK]

Hint: Concurrent updates cause lock wait or error [OK]

Common Mistakes:

• Assuming updates merge automatically
• Believing both updates overwrite without conflict
• Thinking second transaction commits ignoring locks