Database deadlocks

Question

One of the classical reasons we have a database deadlock is when two transactions are inserting and updating tables in a different order.

For example, transaction A

Answer 1

Deadlocks are no biggie. Just be prepared to retry your transactions on failure.

And keep them short. Short transactions consisting of queries that touch very few records (via the magic of indexing) are ideal to minimize deadlocks - fewer rows are locked, and for a shorter period of time.

You need to know that modern database engines don't lock tables; they lock rows; so deadlocks are a bit less likely.

You can also avoid locking by using MVCC and the CONSISTENT READ transaction isolation level: instead of locking, some threads will just see stale data.