Row-level locking (FOR UPDATE, FOR SHARE) in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does FOR UPDATE do in a PostgreSQL SELECT query?

FOR UPDATE locks the selected rows so that other transactions cannot modify or delete them until the current transaction ends. It ensures exclusive access to those rows.

intermediate

What is the difference between FOR UPDATE and FOR SHARE locks?

FOR UPDATE locks rows exclusively, blocking other transactions from modifying or locking them for update.
FOR SHARE locks rows in a shared mode, allowing other transactions to also acquire shared locks but preventing updates or deletes.

beginner

Why is row-level locking useful in a multi-user database environment?

It prevents conflicts and data corruption by controlling concurrent access to the same rows, allowing safe updates without blocking the entire table.

intermediate

How does FOR UPDATE affect other transactions trying to update the same rows?

Other transactions trying to update those rows will wait until the lock is released by the first transaction, ensuring serialized access.

advanced

Can FOR SHARE locks block FOR UPDATE locks on the same rows?

Yes, FOR SHARE locks block FOR UPDATE locks because shared locks prevent exclusive locks until all shared locks are released.

What does SELECT ... FOR UPDATE do in PostgreSQL?

A Locks selected rows exclusively to prevent concurrent updates

B Allows other transactions to update the rows simultaneously

C Locks the entire table for reading

D Deletes the selected rows immediately

FOR UPDATE locks only the selected rows exclusively, blocking other transactions from updating them until the lock is released.

Which lock allows multiple transactions to read the same rows but prevents updates?

A FOR UPDATE

B FOR DELETE

C FOR SHARE

D FOR INSERT

FOR SHARE allows shared locks so multiple transactions can read but not update or delete the rows.

If a transaction holds a FOR UPDATE lock on a row, what happens to another transaction trying to update the same row?

A It updates immediately

B It deletes the row

C It ignores the lock and proceeds

D It waits until the lock is released

The second transaction must wait until the first transaction releases the FOR UPDATE lock.

Which of the following is true about FOR SHARE locks?

A They block other shared locks

B They allow concurrent shared locks but block exclusive locks

C They allow exclusive locks simultaneously

D They delete rows after locking

FOR SHARE allows multiple shared locks but blocks exclusive locks like FOR UPDATE.

Why might you use FOR UPDATE in a banking app when transferring money?

A To lock account rows and prevent concurrent conflicting updates

B To speed up the query by skipping locks

C To delete the account after transfer

D To allow multiple transfers at the same time without waiting

Locking account rows prevents race conditions and ensures money is transferred correctly without conflicts.

Explain how FOR UPDATE and FOR SHARE locks work in PostgreSQL and when you would use each.

Think about how to prevent conflicts when multiple users access the same data.

Describe a real-life situation where row-level locking with FOR UPDATE is important.

Imagine two people trying to withdraw money from the same bank account at the same time.

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