Message-ID: <49E36E1B.6020107@frii.com>
Date: Mon, 13 Apr 2009 10:53:47 -0600
From: Stuart McGraw <smcg2297@frii.com>
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To: pgsql-sql@postgresql.org
Subject: Re: changing multiple pk's in one update
References: <49DC1034.1060705@frii.com> <grnl9e$49a$1@reversiblemaps.ath.cx>
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Jasen Betts wrote:
> On 2009-04-08, Stuart McGraw <smcg2297@frii.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.

Thanks for the suggestion.

Unfortunately I am doing this key renumbering in
an interactive gui app and as there are several million 
rows involved, rebuilding indexes take too long.

I have continued searching for other ways to do this
but most of my google results are floating in a sea
of "update if insert fails, like mysql" results so 
I still have a little hope it is possible.
I thought I remember seeing, a year or two ago, an 
update statement with an ordered subquery that avoided 
duplicate key errors but I am probably misrembering.