Advisory locks in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to acquire a session-level advisory lock with key 12345.

PostgreSQL

SELECT pg_advisory_lock([1]);

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

Alock_key

B'12345'

C12345

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the key making it a string.

Passing NULL instead of a key.

The function pg_advisory_lock takes an integer key to acquire a session-level advisory lock. The key should be a number, not a string.

2fill in blank

medium

Complete the code to release the advisory lock with key 12345.

PostgreSQL

SELECT pg_advisory_unlock([1]);

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

A12345

B'12345'

Clock_id

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a number for the key.

Trying to release a lock with NULL.

The pg_advisory_unlock function releases the advisory lock identified by the integer key. The key must be a number without quotes.

3fill in blank

hard

Fix the error in the code to check if the advisory lock with key 12345 is held by the current session.

PostgreSQL

SELECT pg_try_advisory_lock([1]);

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

A'12345'

BNULL

Clock_key

D12345

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the key causing a type mismatch.

Passing NULL instead of a key.

The pg_try_advisory_lock function attempts to acquire the lock and returns true if successful. The key must be an integer without quotes.

4fill in blank

hard

Fill both blanks to acquire a transaction-level advisory lock with two 32-bit keys 123 and 456.

PostgreSQL

SELECT pg_advisory_xact_lock([1], [2]);

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

A123

B456

C'123'

D'456'

Attempts:

3 left

💡 Hint

Common Mistakes

Using string keys instead of integers.

Swapping the order of keys.

The pg_advisory_xact_lock function takes two integer keys to acquire a transaction-level advisory lock. Both keys must be integers without quotes.

5fill in blank

hard

Fill all three blanks to check if the advisory lock with keys 789 and 1011 is currently held by any session.

PostgreSQL

SELECT pg_try_advisory_lock_shared([1], [2]) AS acquired, pg_advisory_unlock_shared([3], [2]);

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

A789

B1011

C'789'

D'1011'

Attempts:

3 left

💡 Hint

Common Mistakes

Using string keys instead of integers.

Mixing keys between blanks incorrectly.

The functions pg_try_advisory_lock_shared and pg_advisory_unlock_shared use two integer keys to manage shared advisory locks. Keys must be integers without quotes. The first and third blanks use the same key 789.

Practice

1. What is the main purpose of advisory locks in PostgreSQL?

easy

A. To control access to resources using user-defined keys

B. To automatically manage table-level locks during transactions

C. To speed up query execution by caching results

D. To backup the database safely without downtime

Solution

1. Step 1: Understand advisory locks concept

   Advisory locks allow applications to coordinate access to resources by using custom keys, not automatic locks on tables or rows.

2. Step 2: Compare options

   The other options describe other database features unrelated to advisory locks.

3. Final Answer:

   To control access to resources using user-defined keys -> Option A

4. Quick Check:

   Advisory locks = user-defined resource control [OK]

Hint: Advisory locks use keys to manage resource access [OK]

Common Mistakes:

• Confusing advisory locks with automatic table locks
• Thinking advisory locks speed up queries
• Assuming advisory locks handle backups

2. Which of the following is the correct syntax to acquire a session-level advisory lock with key 12345?

easy

A. SELECT pg_advisory_lock(12345);

B. LOCK TABLE pg_advisory_lock(12345);

C. SELECT acquire_lock(12345);

D. BEGIN LOCK 12345;

Solution

1. Step 1: Recall advisory lock syntax

   PostgreSQL uses the function pg_advisory_lock(key) to acquire a session-level advisory lock.

2. Step 2: Evaluate options

   SELECT pg_advisory_lock(12345); is the correct function call. The other options use invalid syntax or non-existent functions.

3. Final Answer:

   SELECT pg_advisory_lock(12345); -> Option A

4. Quick Check:

   pg_advisory_lock(key) = correct syntax [OK]

Hint: Use SELECT pg_advisory_lock(key) to lock [OK]

Common Mistakes:

• Using LOCK TABLE instead of function call
• Calling non-existent functions like acquire_lock
• Trying to lock with BEGIN LOCK syntax

3. What will be the result of this query if the advisory lock with key 999 is already held by another session?

SELECT pg_try_advisory_lock(999);

medium

A. true

B. false

C. null

D. error

Solution

1. Step 1: Understand pg_try_advisory_lock behavior

   This function tries to acquire the lock immediately and returns true if successful, false if the lock is held by someone else.

2. Step 2: Analyze the scenario

   Since the lock with key 999 is already held, the function returns false without waiting.

3. Final Answer:

   false -> Option B

4. Quick Check:

   pg_try_advisory_lock returns false if lock busy [OK]

Hint: pg_try_advisory_lock returns false if lock busy [OK]

Common Mistakes:

• Expecting true even if lock is held
• Thinking it returns null or error
• Confusing pg_try_advisory_lock with pg_advisory_lock

4. You wrote this code:

SELECT pg_advisory_unlock(123);

But the lock was never acquired before. What will happen?

medium

A. The function returns true and releases the lock

B. The function blocks until the lock is acquired

C. The function throws an error

D. The function returns false because no lock was held

Solution

1. Step 1: Understand pg_advisory_unlock behavior

   This function releases a lock if held and returns true; if no lock was held, it returns false.

2. Step 2: Analyze the scenario

   Since the lock was never acquired, the function returns false without error or blocking.

3. Final Answer:

   The function returns false because no lock was held -> Option D

4. Quick Check:

   Unlock returns false if lock not held [OK]

Hint: Unlock returns false if no lock held [OK]

Common Mistakes:

• Expecting an error when unlocking unheld lock
• Thinking unlock blocks or waits
• Assuming unlock always returns true

5. You want to ensure two different sessions do not run a critical section simultaneously using advisory locks. Which approach is best?

-- Session 1 and 2 run this code:
SELECT pg_try_advisory_lock(42);
-- If true, run critical section, then
SELECT pg_advisory_unlock(42);

hard

A. Use pg_advisory_unlock before acquiring lock to clear old locks

B. Use pg_try_advisory_lock to attempt lock and skip if busy

C. Use pg_advisory_lock to wait until lock is available before running

D. Use random keys each time to avoid conflicts

Solution

1. Step 1: Understand locking strategies

   pg_try_advisory_lock returns immediately and may skip critical section if lock busy; pg_advisory_lock waits until lock is free.

2. Step 2: Choose best approach for critical section

   To ensure only one session runs critical section at a time, waiting for the lock is safer than skipping it.

3. Step 3: Evaluate other options

   Unlocking before acquiring is unsafe and random keys defeat locking purpose.

4. Final Answer:

   Use pg_advisory_lock to wait until lock is available before running -> Option C

5. Quick Check:

   Waiting lock ensures exclusive access [OK]

Hint: Use pg_advisory_lock to wait for exclusive access [OK]

Common Mistakes:

• Using try lock and skipping critical section silently
• Unlocking before locking without owning lock
• Using random keys causing no real locking