Re: changing multiple pk's in one update

On 2009-04-08, Stuart McGraw <smcg2297(at)frii(dot)com> wrote:
> Hello all,
>
> I have a table with a primary key column
> that contains sequential numbers.
>
> Sometimes I need to shift them all up or down
> by a fixed amount. For example, if I have
> four rows with primary keys, 2, 3, 4, 5, I
> might want to shift them down by 1 by doing:
>
> UPDATE mytable SET id=id-1
>
> (where "id" is the pk column) so that the pk's
> are now 1, 2, 3, 4.
>
> When I try to shift them up by using +1 in the
> above update statement, I get (not surprisingly)
> a duplicate key error. I also realize that the
> -1 case above works only by luck.
>
> So my question:
> Is there some way, perhaps with ORDER BY, that
> I can achieve the change I want with a single
> update statement? (If I have an unused key
> range large enough, I suppose I could update
> all the keys to that range, and then back to
> my target range but the requires two updates
> (there are a lot of foreign keys referencing
> these primary keys) and requires that I have
> an available range, so a single update statement
> would be preferable.)
>
> Thanks for any enlightenment.

begin a transaction
suspend the constraint (use SET CONSTRAINTS ... DEFERRED)
drop the index
do the update(s)
recreate the index
commit the transaction.

I see no reason to keep the index (and its associated UNIQUE
constraint) during the update, AFAICT all it does is slow the process
down.