Re: Need LIMIT and ORDER BY for UPDATE

"D. Dante Lorenso" <dante(at)lorenso(dot)com> wrote:
>
> All,
>
> I'd really like to have ORDER BY and LIMIT for UPDATE and DELETE
> commands. Is this possible?
>
> UPDATE invoice i
> SET reserve_ts = NOW() + '1 hour'::timestamp
> FROM account a
> WHERE a.acct_id = i.acct_id
> AND i.reserve_ts < NOW()
> AND a.status = 'A'
> AND i.is_paid IS FALSE
> ORDER BY i.create_ts ASC
> LIMIT 1
> RETURNING invoice_id;
>
> This query would find JUST ONE invoice record which is not paid and
> reserve the right to operate on the row using the 'reserve_ts' column
> for all active accounts. The one row would be the oldest invoice
> matching the criteria. Only that one row would be updated and the
> invoice_id of the updated row (if any) would be returned.
>
> Running a query like this over and over would pop just one record off
> the queue and would guarantee an atomic reservation.

While I'm not going to argue as to whether your suggestion would be
a good idea or not, I will suggest you look at SELECT FOR UPDATE, which
will allow you to do what you desire.

--
Bill Moran
http://www.potentialtech.com