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+---
+title: Prevent Deadlocks with Consistent Lock Ordering
+impact: MEDIUM-HIGH
+impactDescription: Eliminate deadlock errors, improve reliability
+tags: deadlocks, locking, transactions, ordering
+---
+
+## Prevent Deadlocks with Consistent Lock Ordering
+
+Deadlocks occur when transactions lock resources in different orders. Always
+acquire locks in a consistent order.
+
+**Incorrect (inconsistent lock ordering):**
+
+```sql
+-- Transaction A                    -- Transaction B
+begin;                              begin;
+update accounts                     update accounts
+set balance = balance - 100         set balance = balance - 50
+where id = 1;                       where id = 2;  -- B locks row 2
+
+update accounts                     update accounts
+set balance = balance + 100         set balance = balance + 50
+where id = 2;  -- A waits for B     where id = 1;  -- B waits for A
+
+-- DEADLOCK! Both waiting for each other
+```
+
+**Correct (lock rows in consistent order first):**
+
+```sql
+-- Explicitly acquire locks in ID order before updating
+begin;
+select * from accounts where id in (1, 2) order by id for update;
+
+-- Now perform updates in any order - locks already held
+update accounts set balance = balance - 100 where id = 1;
+update accounts set balance = balance + 100 where id = 2;
+commit;
+```
+
+Alternative: use a single statement to update atomically:
+
+```sql
+-- Single statement acquires all locks atomically
+begin;
+update accounts
+set balance = balance + case id
+  when 1 then -100
+  when 2 then 100
+end
+where id in (1, 2);
+commit;
+```
+
+Detect deadlocks in logs:
+
+```sql
+-- Check for recent deadlocks
+select * from pg_stat_database where deadlocks > 0;
+
+-- Enable deadlock logging
+set log_lock_waits = on;
+set deadlock_timeout = '1s';
+```
+
+Reference:
+[Deadlocks](https://www.postgresql.org/docs/current/explicit-locking.html#LOCKING-DEADLOCKS)