Deadlocks
A deadlock is an event that happens when different transactions depend on each other in a circular way. Deadlocks are, to some extent, normal events in a concurrent database environment and nothing an administrator should worry about, unless they become extremely frequent, meaning there is some dependency error in the applications and the transactions.
When a deadlock happens, there is no choice but to terminate the locked transactions. PostgreSQL has a very powerful deadlock detection engine that does exactly this job: it finds stalled transactions and, in the case of a deadlock, terminates them (producing a ROLLBACK).
In order to produce a deadlock, imagine two concurrent transactions applying changes to the very same tuples in a conflicting way. For example, the first transaction could do something like the following:
-- session 1
forumdb=> BEGIN;
BEGIN
forumdb=> SELECT txid_current();
txid_current
--------------
4875
(1 row)
forumdb=> UPDATE...