Row-level locking (FOR UPDATE, FOR SHARE) in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to lock selected rows for update in PostgreSQL.

PostgreSQL

SELECT * FROM orders WHERE status = 'pending' [1];

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

ALOCK IN SHARE MODE

BWITH LOCK

CFOR UPDATE

DFOR SHARE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FOR SHARE instead of FOR UPDATE when intending to lock for writing.

Using LOCK IN SHARE MODE which is MySQL syntax, not PostgreSQL.

The FOR UPDATE clause locks the selected rows so no other transaction can modify them until the current transaction ends.

2fill in blank

medium

Complete the code to lock rows for shared access in PostgreSQL.

PostgreSQL

SELECT * FROM products WHERE stock > 0 [1];

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

AFOR UPDATE

BFOR NO KEY UPDATE

CFOR KEY SHARE

DFOR SHARE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FOR UPDATE which locks rows exclusively.

Confusing FOR SHARE with FOR KEY SHARE.

The FOR SHARE clause locks rows for shared access, allowing concurrent reads but preventing updates.

3fill in blank

hard

Fix the error in the code to lock rows for update correctly.

PostgreSQL

SELECT * FROM customers WHERE active = true [1];

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

AFOR UPDATE SKIP LOCKED

BFOR UPDATE NOWAIT

CFOR UPDATE WAIT

DFOR SHARE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FOR UPDATE WAIT which is not valid syntax.

Using FOR SHARE which does not lock rows for update.

FOR UPDATE SKIP LOCKED locks rows for update but skips rows already locked by other transactions, avoiding waiting.

4fill in blank

hard

Fill both blanks to lock rows for update but avoid waiting if locked.

PostgreSQL

SELECT * FROM invoices WHERE paid = false [1] [2];

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

AFOR UPDATE

BNOWAIT

CSKIP LOCKED

DFOR SHARE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FOR SHARE instead of FOR UPDATE.

Using NOWAIT instead of SKIP LOCKED when wanting to skip locked rows.

Use FOR UPDATE SKIP LOCKED to lock rows for update and skip those already locked by others.

5fill in blank

hard

Fill all three blanks to lock rows for key share and avoid waiting in PostgreSQL.

PostgreSQL

SELECT * FROM sessions WHERE active = true [1] [2] [3];

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

AFOR KEY SHARE

BNOWAIT

CSKIP LOCKED

DFOR UPDATE

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing FOR UPDATE with key share locking.

Using NOWAIT and SKIP LOCKED together incorrectly.

FOR KEY SHARE NOWAIT SKIP LOCKED locks rows with key share, avoids waiting, and skips locked rows.

Practice

1. What does the FOR UPDATE clause do in a PostgreSQL query?

easy

A. Locks selected rows to prevent other transactions from modifying them until the current transaction ends.

B. Locks the entire table to prevent any access by other transactions.

C. Allows other transactions to read but not modify the selected rows.

D. Unlocks rows that were previously locked by another transaction.

Solution

1. Step 1: Understand the purpose of FOR UPDATE

   The FOR UPDATE clause locks the rows returned by the query to prevent other transactions from modifying them.

2. Step 2: Compare with other locking types

   Unlike FOR SHARE, which allows reading but prevents writing, FOR UPDATE blocks other transactions from modifying the locked rows until the current transaction ends.

3. Final Answer:

   Locks selected rows to prevent other transactions from modifying them until the current transaction ends. -> Option A

4. Quick Check:

   FOR UPDATE = exclusive row lock [OK]

Hint: FOR UPDATE locks rows for writing, blocking others [OK]

Common Mistakes:

• Confusing FOR UPDATE with table-level locks
• Thinking FOR UPDATE allows other writes
• Mixing FOR UPDATE with FOR SHARE behavior

2. Which of the following is the correct syntax to lock rows for reading using row-level locking in PostgreSQL?

easy

A. SELECT * FROM orders FOR UPDATE;

B. SELECT * FROM orders FOR SHARE;

C. SELECT * FROM orders LOCK ROW;

D. SELECT * FROM orders WITH LOCK;

Solution

1. Step 1: Identify the clause for read locks

   In PostgreSQL, FOR SHARE is used to lock rows for reading, allowing other transactions to read but not modify.

2. Step 2: Check syntax correctness

   Options B and C are invalid syntax. SELECT * FROM orders FOR UPDATE; locks rows for writing, not reading.

3. Final Answer:

   SELECT * FROM orders FOR SHARE; -> Option B

4. Quick Check:

   Read lock = FOR SHARE [OK]

Hint: FOR SHARE locks rows for reading, syntax: SELECT ... FOR SHARE [OK]

Common Mistakes:

• Using FOR UPDATE instead of FOR SHARE for read locks
• Using non-existent LOCK ROW or WITH LOCK syntax
• Confusing table-level locks with row-level locks

3. Consider the following two transactions running concurrently:

-- Transaction 1
BEGIN;
SELECT * FROM products WHERE id = 10 FOR UPDATE;
-- Transaction 2
BEGIN;
SELECT * FROM products WHERE id = 10 FOR SHARE;

What will happen when Transaction 2 tries to execute its SELECT statement?

medium

A. Transaction 2 will immediately acquire the FOR SHARE lock and proceed.

B. Transaction 2 will cause a deadlock error immediately.

C. Transaction 2 will wait until Transaction 1 commits or rolls back before proceeding.

D. Transaction 2 will ignore the lock and read the row without waiting.

Solution

1. Step 1: Understand locking conflict between FOR UPDATE and FOR SHARE

   A FOR UPDATE lock is exclusive and blocks other transactions from acquiring conflicting locks, including FOR SHARE.

2. Step 2: Analyze Transaction 2 behavior

   Transaction 2's FOR SHARE lock request conflicts with Transaction 1's FOR UPDATE lock, so it must wait until Transaction 1 finishes.

3. Final Answer:

   Transaction 2 will wait until Transaction 1 commits or rolls back before proceeding. -> Option C

4. Quick Check:

   FOR UPDATE blocks FOR SHARE until commit [OK]

Hint: FOR UPDATE blocks FOR SHARE until transaction ends [OK]

Common Mistakes:

• Assuming FOR SHARE can proceed during FOR UPDATE lock
• Expecting deadlock without waiting
• Ignoring lock conflicts between FOR UPDATE and FOR SHARE

4. You wrote this query to lock rows for update:

SELECT * FROM customers FOR SHARE;

But you want to prevent other transactions from modifying these rows. What is wrong and how to fix it?

medium

A. FOR SHARE locks the entire table; use WHERE clause to limit rows.

B. FOR SHARE is deprecated; use LOCK TABLE instead.

C. FOR SHARE requires an explicit NOWAIT clause to lock rows.

D. FOR SHARE locks rows only for reading; replace it with FOR UPDATE to lock for writing.

Solution

1. Step 1: Identify the locking behavior of FOR SHARE

   FOR SHARE locks rows to allow reading but does not prevent other transactions from modifying them.

2. Step 2: Choose correct clause to prevent modifications

   To block other transactions from modifying rows, use FOR UPDATE instead of FOR SHARE.

3. Final Answer:

   FOR SHARE locks rows only for reading; replace it with FOR UPDATE to lock for writing. -> Option D

4. Quick Check:

   Prevent writes = use FOR UPDATE [OK]

Hint: Use FOR UPDATE to block writes, not FOR SHARE [OK]

Common Mistakes:

• Thinking FOR SHARE blocks writes
• Using LOCK TABLE unnecessarily
• Missing WHERE clause but unrelated to locking type

5. You want to update multiple rows in a table but ensure no other transaction can read or modify these rows until your transaction finishes. Which locking clause should you use in your SELECT statement before updating?

hard

A. SELECT * FROM table_name FOR UPDATE;

B. SELECT * FROM table_name FOR SHARE;

C. SELECT * FROM table_name FOR NO KEY UPDATE;

D. SELECT * FROM table_name FOR KEY SHARE;

Solution

1. Step 1: Understand the locking levels

   FOR UPDATE locks rows exclusively, blocking writes by others until commit, but does not block reads.

2. Step 2: Compare with other lock types

   FOR SHARE and FOR KEY SHARE allow reads; FOR NO KEY UPDATE is less restrictive and allows some concurrent updates.

3. Step 3: Choose the strictest lock to block writes

   Only FOR UPDATE fully blocks other transactions from modifying the rows, but it does not block reads.

4. Final Answer:

   SELECT * FROM table_name FOR UPDATE; -> Option A

5. Quick Check:

   Block writes = FOR UPDATE [OK]

Hint: FOR UPDATE blocks writes until commit [OK]

Common Mistakes:

• Choosing FOR SHARE or KEY SHARE which allow writes
• Using NO KEY UPDATE which is less strict
• Not understanding lock levels and their effects