Thread: UPDATE .. RETURNING OLD.*

Hi everyone,

Today I needed a feature like $subject. The use case was: UPDATE foo SET
bar = bar + 1 WHERE id=$1, but I wanted to only do it when bar was 0. In
order to give the user an informative error message, I also needed to
distinguish the two cases: a row with id = $1 doesn't exist, and bar was
0, so I couldn't put bar != 0 into the WHERE clause. This time I got
around it by using RETURNING bar and checking that it was 1 on the
client side, but I can come up with other cases where you can't do that.

Comments?

Regards,
Marko Tiikkaja

On Fri, Aug 28, 2009 at 06:12:30PM +0300, Marko Tiikkaja wrote:
> Hi everyone,
>
> Today I needed a feature like $subject.  The use case was: UPDATE
> foo SET bar = bar + 1 WHERE id=$1, but I wanted to only do it when
> bar was 0.  In order to give the user an informative error message,
> I also needed to distinguish the two cases: a row with id = $1
> doesn't exist, and bar was 0, so I couldn't put bar != 0 into the
> WHERE clause.  This time I got around it by using RETURNING bar and
> checking that it was 1 on the client side, but I can come up with
> other cases where you can't do that.
>
> Comments?

We talked about this briefly in IRC last night, and since that's not
recorded, I'd like to mention a few things here:

* OLD is already a reserved word.  We could use it without fear of a badly named database object.

* Having access to both the old and new row could make debugging complex UPDATE queries much easier.

* There's some interesting use cases if the UPDATE...RETURNING can also be used as a subquery.  Auditing would be one.

Cheers,
David.
-- 
David Fetter <david@fetter.org> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david.fetter@gmail.com

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