How to diagnose and fix the 40P01 deadlock_detected error code in Postgres. 

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The 40P01 deadlock_detected error in PostgreSQL occurs when two or more transactions are waiting for each other to release locks, and none of them can proceed. PostgreSQL will detect this situation, abort one of the transactions to break the deadlock, and return this error to the client application.

Here are some common scenarios that can lead to a deadlock, along with examples and sample code to diagnose and fix the issue:

1. Conflicting Row Locks with UPDATE Statements

Two transactions update rows in a different order, leading to a deadlock.

Example:
Transaction 1:

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
-- Waits for Transaction 2 to release the lock on id = 2
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

Transaction 2:

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 2;
-- Waits for Transaction 1 to release the lock on id = 1
UPDATE accounts SET balance = balance + 100 WHERE id = 1;
COMMIT;

Fix:
Ensure that all transactions update rows in the same order.

-- Use a consistent update order in both transactions
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

2. Conflicting FOREIGN KEY Locks

Two transactions are trying to update rows that have FOREIGN KEY constraints in a way that causes a deadlock.

Example:
Transaction 1:

BEGIN;
UPDATE orders SET order_status = 'shipped' WHERE order_id = 123;
-- Waits for Transaction 2 to release the lock on the corresponding customer
UPDATE customers SET last_order_status = 'shipped' WHERE customer_id = 456;
COMMIT;

Transaction 2:

BEGIN;
UPDATE customers SET last_order_status = 'processing' WHERE customer_id = 456;
-- Waits for Transaction 1 to release the lock on the corresponding order
UPDATE orders SET order_status = 'processing' WHERE order_id = 123;
COMMIT;

Fix:
Like with row locks, ensure that transactions that involve FOREIGN KEY constraints update the affected tables in the same order.

3. Deadlocks Involving Explicit Locks

Transactions explicitly acquire locks in a conflicting order.

Example:
Transaction 1:

BEGIN;
LOCK TABLE accounts IN SHARE ROW EXCLUSIVE MODE;
-- Waits for Transaction 2 to release the lock on table customers
LOCK TABLE customers IN SHARE ROW EXCLUSIVE MODE;
COMMIT;

Transaction 2:

BEGIN;
LOCK TABLE customers IN SHARE ROW EXCLUSIVE MODE;
-- Waits for Transaction 1 to release the lock on table accounts
LOCK TABLE accounts IN SHARE ROW EXCLUSIVE MODE;
COMMIT;

Fix:
Acquire explicit locks in the same order across different transactions.

Diagnosing and Fixing Deadlocks

To diagnose deadlocks:

  • Review the PostgreSQL error logs, which provide details about the deadlock, including the queries involved and the lock types.
  • Analyze the application code to identify transactions that might be acquiring locks in different orders.

To fix deadlocks:

  • Ensure that all transactions access tables and rows in a consistent order.
  • Keep transactions as short as possible to reduce the time locks are held.
  • Use the least restrictive lock necessary for the operation.
  • Consider using SELECT FOR UPDATE or SELECT FOR SHARE in a consistent order to lock rows.

By following these guidelines and reviewing the application logic related to database transactions, you can prevent deadlocks and resolve the 40P01 deadlock_detected error in PostgreSQL. It’s important to identify patterns in your application’s transactions that could lead to deadlocks and refactor the code to access resources in a consistent and deadlock-free manner.

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