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):
-- 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):
-- 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:
-- 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:
-- 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