postgres-errors-deadlocks

Quick Reference :

A deadlock is two or more transactions each holding a lock the other one needs : neither can proceed. PostgreSQL detects the cycle automatically and aborts one transaction (the victim) so the rest complete. The victim's session sees :

ERROR:  deadlock detected           -- SQLSTATE 40P01
DETAIL: Process 123 waits for ShareLock on transaction 456; blocked by process 789.
        Process 789 waits for ShareLock on transaction 999; blocked by process 123.
HINT:   See server log for query details.

A deadlock is NEVER a PostgreSQL bug. It is always the application acquiring locks in an inconsistent order across code paths. The two fixes :