Re: transactions

On Mon, Jun 06, 2005 at 09:21:15AM -0400, Douglas McNaught wrote:
> "Surabhi Ahuja " <surabhi(dot)ahuja(at)iiitb(dot)ac(dot)in> writes:
>
> > say i have two transactions going on simultaneoulsy.
> > in one transaction i do an insertion
> > and the other transaction i do a delete.
> > what will happen if in both the transactions, the insert(in one) and
> > delete(in the other) are issued at exactly the same time? will bothe get
> > blocked and enter into a deadlock, if yes then do both the transactions
> > get aborted after some time of waiting.
>
> The transactions will be executed completely independently, unless you
> do explicit locking (via SELECT FOR UPDATE or LOCK TABLE).

Deadlock can still happen if Transaction A's INSERT blocks because
of something Transaction B did previously, and Transaction B's
DELETE blocks because of something Transaction A did previously.
Foreign key relationships are a typical cause, but even UPDATEs on
simple tables can cause later operations to deadlock if the application
isn't careful with its order of operations.

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/