Deadlock detection and prevention in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a deadlock in a database system?

A deadlock happens when two or more transactions wait forever for each other to release locks, so none can proceed.

intermediate

How does PostgreSQL detect deadlocks?

PostgreSQL periodically checks for cycles in the waiting transactions graph to find deadlocks and then aborts one transaction to break the cycle.

beginner

Name one common method to prevent deadlocks.

One method is to always acquire locks in a consistent order across transactions to avoid circular waiting.

intermediate

What happens when PostgreSQL detects a deadlock?

PostgreSQL cancels one of the transactions involved in the deadlock and returns an error to that transaction, allowing others to continue.

advanced

Why is deadlock prevention better than detection in some cases?

Prevention avoids deadlocks before they happen, improving performance and user experience by not having to roll back transactions.

What causes a deadlock in PostgreSQL?

A Two transactions waiting on each other's locks

B A transaction running too long

C A missing index

D A syntax error in SQL

Deadlocks occur when transactions wait on each other's locks, creating a cycle with no progress.

How does PostgreSQL resolve a detected deadlock?

A It cancels one transaction involved in the deadlock

B It restarts the database server

C It ignores the deadlock and waits

D It locks all tables

PostgreSQL cancels one transaction to break the deadlock cycle.

Which practice helps prevent deadlocks?

A Acquiring locks in random order

B Acquiring locks in a consistent order

C Using only read locks

D Disabling transactions

Consistent lock order prevents circular waiting and deadlocks.

What is a sign of a deadlock in PostgreSQL logs?

A NOTICE: query completed

B WARNING: slow query

C ERROR: deadlock detected

D INFO: connection established

PostgreSQL logs 'ERROR: deadlock detected' when it finds a deadlock.

Why might deadlock prevention be preferred over detection?

A It uses more CPU

B It disables locking

C It allows deadlocks to happen

D It avoids transaction rollbacks

Prevention avoids deadlocks and the need to rollback transactions, improving performance.

Explain what a deadlock is and how PostgreSQL detects it.

Think about transactions waiting on each other's locks.

Describe two ways to prevent deadlocks in database transactions.

Focus on how to avoid circular waiting and long lock holding.

Practice

1. What is a deadlock in PostgreSQL?

easy

A. A performance optimization technique for faster queries.

B. A syntax error in SQL statements causing query failure.

C. A backup process that locks tables during data export.

D. A situation where two or more transactions wait indefinitely for each other to release locks.

Solution

1. Step 1: Understand transaction locking

   Transactions acquire locks on resources to maintain data integrity.

2. Step 2: Define deadlock

   A deadlock occurs when transactions wait on each other in a cycle, causing indefinite waiting.

3. Final Answer:

   A situation where two or more transactions wait indefinitely for each other to release locks. -> Option D

4. Quick Check:

   Deadlock = cyclic waiting [OK]

Hint: Deadlock means transactions wait forever on each other [OK]

Common Mistakes:

• Confusing deadlock with syntax errors
• Thinking deadlock improves performance
• Mixing deadlock with backup locking

2. Which of the following is the correct way to acquire locks to prevent deadlocks in PostgreSQL?

easy

A. Acquire locks on resources in random order.

B. Acquire locks on resources in the same order in all transactions.

C. Never acquire any locks in transactions.

D. Acquire locks only after committing the transaction.

Solution

1. Step 1: Understand lock acquisition order

   Acquiring locks in a consistent order prevents circular waiting.

2. Step 2: Identify correct practice

   All transactions should acquire locks on resources in the same order to avoid deadlocks.

3. Final Answer:

   Acquire locks on resources in the same order in all transactions. -> Option B

4. Quick Check:

   Consistent lock order = no deadlock [OK]

Hint: Always lock resources in the same order [OK]

Common Mistakes:

• Locking resources randomly
• Not locking resources at all
• Locking after commit

3. Consider two transactions in PostgreSQL:
-- Transaction 1
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 50 WHERE id = 2;
-- waits here

-- Transaction 2
BEGIN;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
UPDATE accounts SET balance = balance - 50 WHERE id = 1;
-- waits here
What will PostgreSQL do when both transactions wait for each other?

medium

A. Both transactions will wait forever causing a deadlock.

B. Both transactions will succeed without any issue.

C. PostgreSQL will detect the deadlock and abort one transaction automatically.

D. PostgreSQL will merge both transactions into one.

Solution

1. Step 1: Identify deadlock scenario

   Both transactions hold locks and wait for the other's lock, creating a cycle.

2. Step 2: PostgreSQL deadlock detection

   PostgreSQL automatically detects deadlocks and aborts one transaction to break the cycle.

3. Final Answer:

   PostgreSQL will detect the deadlock and abort one transaction automatically. -> Option C

4. Quick Check:

   Deadlock detected = abort one transaction [OK]

Hint: PostgreSQL aborts one transaction on deadlock detection [OK]

Common Mistakes:

• Assuming infinite waiting without abort
• Thinking transactions merge automatically
• Believing both succeed without conflict

4. You have the following PostgreSQL code causing a deadlock:

BEGIN;
LOCK TABLE orders IN ACCESS EXCLUSIVE MODE;
UPDATE customers SET name = 'Alice' WHERE id = 1;
-- Transaction 2 starts here
BEGIN;
LOCK TABLE customers IN ACCESS EXCLUSIVE MODE;
UPDATE orders SET status = 'shipped' WHERE id = 10;

What is the main issue causing the deadlock?

medium

A. Transactions lock tables in different orders causing circular wait.

B. Using SHARE MODE lock instead of EXCLUSIVE MODE.

C. Updating different tables in the same transaction.

D. Missing COMMIT statements after updates.

Solution

1. Step 1: Analyze lock order

   Transaction 1 locks orders first, then updates customers; Transaction 2 locks customers first, then updates orders.

2. Step 2: Identify circular wait

   Each transaction waits for the other's locked table, causing deadlock due to different lock order.

3. Final Answer:

   Transactions lock tables in different orders causing circular wait. -> Option A

4. Quick Check:

   Different lock order = deadlock risk [OK]

Hint: Lock tables in same order to avoid deadlock [OK]

Common Mistakes:

• Blaming lock mode instead of order
• Thinking updating different tables causes deadlock
• Ignoring missing COMMIT as cause

5. You want to prevent deadlocks in a multi-user PostgreSQL system updating inventory and sales tables. Which strategy is best?

hard

A. Keep transactions short and acquire locks on inventory then sales in all transactions.

B. Acquire locks on sales first, then inventory, but only in some transactions.

C. Avoid using transactions to prevent locking.

D. Use long transactions to batch updates and reduce lock frequency.

Solution

1. Step 1: Understand deadlock prevention

   Keeping transactions short reduces lock time; consistent lock order prevents cycles.

2. Step 2: Apply best practice

   Always lock inventory first, then sales, in all transactions to avoid deadlocks.

3. Final Answer:

   Keep transactions short and acquire locks on inventory then sales in all transactions. -> Option A

4. Quick Check:

   Short transactions + consistent lock order = deadlock prevention [OK]

Hint: Short transactions + consistent lock order prevent deadlocks [OK]

Common Mistakes:

• Locking in inconsistent order
• Avoiding transactions entirely
• Using long transactions increasing lock time