Observing deadlocks and similar issues

Deadlocks are an important issue and can happen in every database. Basically, a deadlock will happen if two transactions have to wait on each other.

In this section, you will see how this can happen. Let’s suppose we have a table containing two rows:

CREATE TABLE t_deadlock (id int);
INSERT INTO t_deadlock VALUES (1), (2);

The following example shows what can happen:

...

Transaction 1	Transaction 2
BEGIN;	BEGIN;
UPDATE t_deadlock SET id = id * 10 WHERE id = 1;	UPDATE t_deadlock SET id = id * 10 WHERE id = 2;
UPDATE t_deadlock SET id = id * 10 WHERE id = 2;
Waiting on transaction 2	UPDATE t_deadlock SET id = id * 10 WHERE id = 1;