Re: Table update problem works on MySQL but not Postgres

On 8/31/2010 8:17 PM, Merlin Moncure wrote:
> On Tue, Aug 31, 2010 at 7:56 PM, Raymond C. Rodgers<sinful622(at)gmail(dot)com> wrote:
>> Let me stress that this is not a bug in PostgreSQL; if anything at all,
>> it's only a lack of a stupid feature.
>>
>> I'm working on a project for a client where I have a table for arbitrary
>> categories to be applied to their data, and they need to be able to set the
>> order in which the categories appear. A simplified version of the table as I
>> created is as follows:
>>
>> create table mydemo (cat_id int not null, cat_name varchar(25) not null,
>> cat_order int not null, primary key(cat_id,cat_order));
>>
>> During my coding, I unwittingly backed myself into a corner, fully expecting
>> to issue queries such as:
>>
>> update mydemo set cat_order = cat_order + 1 where client_id = 1 and
>> cat_order>= 0
>>
>> in order to insert categories at the top of the sorted list for example.
>> As you can probably guess, this query doesn't work very well. On both MySQL
>> and PostgreSQL I get a constraint violation. That makes sense; I screwed up.
>>
>> But out of pure curiosity to see if I could circumvent this issue, I added
>> an order clause, making that query this instead:
>>
>> update mydemo set cat_order = cat_order + 1 where client_id = 1 and
>> cat_order>= 0 order by cat_order desc
>>
>> This is where the interesting thing happens: On MySQL the query actually
>> works as intended, but it doesn't on PostgreSQL. As I said, I'm sure this is
>> not a bug in PostgreSQL, but the lack of a stupid user trick. While my
>> project is on MySQL, and I could theoretically leave my code as is to take
>> advantage of this trick, I'm sure I'd be a complete idiot to leave it
>> instead of fixing it.
> You have it backwards, mysql is broken, postgresql is not.
>
> Anyways, you can do it in postgres like this:
> alter table mydemo alter cat_order type int using cat_order + 1;
>
> merlin
Like I said and stressed twice, it's not a problem with PostgreSQL.
David's solution is actually better than that, but I accidentally sent
just a private reply to him acknowledging that it's good to know that
PostgreSQL can also save a stupid programmer's butt... :-)

Raymond