Row-level locking (FOR UPDATE, FOR SHARE) in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What rows are locked by FOR UPDATE?

Consider a table accounts with columns id and balance. Which rows will be locked by this query?

SELECT * FROM accounts WHERE balance > 1000 FOR UPDATE;

PostgreSQL

CREATE TABLE accounts (id SERIAL PRIMARY KEY, balance INT);
INSERT INTO accounts (balance) VALUES (500), (1500), (2000);

A Only rows with balance > 1000 are locked for update.

B All rows in the table are locked for update.

C No rows are locked; FOR UPDATE does not lock rows.

D Only rows with balance <= 1000 are locked for update.

💡 Hint

FOR UPDATE locks rows returned by the query to prevent concurrent updates.

The FOR UPDATE clause locks only the rows returned by the query. Here, only rows with balance > 1000 are locked.

❓ query_result

intermediate

What is the effect of FOR SHARE?

Given the table orders with columns order_id and status, what does this query do?

SELECT * FROM orders WHERE status = 'pending' FOR SHARE;

PostgreSQL

CREATE TABLE orders (order_id SERIAL PRIMARY KEY, status TEXT);
INSERT INTO orders (status) VALUES ('pending'), ('shipped'), ('pending');

A Locks rows with status 'pending' to prevent other transactions from updating or deleting them.

B Locks rows with status 'pending' to prevent other transactions from reading them.

C Locks all rows in the table regardless of status.

D Does not lock any rows; FOR SHARE is ignored.

💡 Hint

FOR SHARE locks rows to prevent updates or deletes but allows reads.

The FOR SHARE clause locks the selected rows to prevent other transactions from modifying or deleting them, but it allows concurrent reads.

📝 Syntax

advanced

Identify the syntax error in this locking query

Which option contains a syntax error when trying to lock rows for update?

SELECT * FROM products WHERE price > 100 FOR UPDATE NOWAIT;

A SELECT * FROM products WHERE price > 100 FOR UPDATE NOWAIT;

B SELECT * FROM products WHERE price > 100 FOR UPDATE OF products NOWAIT;

C SELECT * FROM products WHERE price > 100 FOR SHARE NOWAIT;

D SELECT * FROM products WHERE price > 100 FOR UPDATE WAIT;

💡 Hint

Check the valid locking options and keywords in PostgreSQL.

PostgreSQL supports NOWAIT but not WAIT as a locking option. Option D uses an invalid keyword WAIT, causing a syntax error.

❓ optimization

advanced

Which query minimizes locking duration?

You want to lock only necessary rows for update to reduce lock contention. Which query is best?

PostgreSQL

CREATE TABLE inventory (item_id SERIAL PRIMARY KEY, quantity INT);
INSERT INTO inventory (quantity) VALUES (10), (0), (5);

A SELECT * FROM inventory FOR SHARE;

B SELECT * FROM inventory WHERE quantity > 0 FOR UPDATE;

C SELECT * FROM inventory WHERE quantity <= 0 FOR SHARE;

D SELECT * FROM inventory FOR UPDATE;

💡 Hint

Lock only rows you intend to update.

Query A locks only rows with quantity > 0 for update, minimizing locked rows and reducing contention.

🧠 Conceptual

expert

What happens if two transactions lock the same row with FOR UPDATE?

Transaction A and Transaction B both execute SELECT * FROM employees WHERE id = 1 FOR UPDATE; at the same time. What happens?

PostgreSQL

CREATE TABLE employees (id SERIAL PRIMARY KEY, name TEXT);
INSERT INTO employees (name) VALUES ('Alice');

A Both transactions acquire the lock simultaneously without waiting.

B Transaction B immediately fails with a deadlock error.

C Transaction B waits until Transaction A commits or rolls back before acquiring the lock.

D Transaction A releases the lock immediately after selecting the row.

💡 Hint

Consider how row-level locks serialize concurrent updates.

When two transactions try to lock the same row with FOR UPDATE, the second waits until the first finishes (commits or rolls back) to maintain data consistency.

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