Re: concurrent updates problem

On Mon, Mar 19, 2001 at 11:12:01AM -0500, Tom Lane wrote:
> >> I have on a web application
> >> update threads set views = views + 1 where forum_id = 1 and thread_id = 1
>
> It should work to do
>
> begin;
> select * from threads where forum_id = 1 and thread_id = 1 FOR UPDATE;
> update threads set views = views + 1 where forum_id = 1 and thread_id = 1;
> end;
>
> Note the FOR UPDATE to lock the row and the transaction wrapping to
> define the scope of the lock.  Without this I'd expect you to lose
> some counter increments as a result of two processes doing the UPDATE
> at about the same time (both will read the old value of "views" and
> increment it by one).

But the one-line version without select for update is equivalent, right?
That is, a single UPDATE statement is atomic? According to my test
(with 3 Java threads making a total of 15000 updates that way), it is:
no single update lost in the process.

-JPL