Re: multi-row check constraints? - Mailing list pgsql-general

Greetings

Select COLUMN FROM TABLE WHERE (some condition) for UPDATE OF COLUMN
is not supported?

what would happen in a Table Deadlock scenario???

M-
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----- Original Message ----- 
From: "Jeff Davis" <pgsql@j-davis.com>
To: "Angva" <angvaw@gmail.com>
Cc: <pgsql-general@postgresql.org>
Sent: Thursday, March 22, 2007 8:09 PM
Subject: Re: [GENERAL] multi-row check constraints?


> On Tue, 2007-03-20 at 13:21 -0700, Angva wrote:
>> Dear Postgres fans,
>> 
>> Hi, I was wondering what is the best way to achieve a multi-row check
>> constraint. For example, you have a table with two columns: ID and
>> percent, no primary key. The goal is to enforce that all values of
>> percent, per ID, add up to exactly 100%. I come from an Oracle
>> background, and what you would probably do on Oracle is create a
>> materialized view with the sum(percent) grouped by ID, then put a
>> constraint on the sum column. This problem is also solvable using
>> triggers, but it's messy and imposes a lot of serialization. Not to
>> mention easy to get wrong.
>> 
>> So, I've come across this problem in Postgres and was hoping someone
>> could steer me in the right direction.
>> 
> 
> Your Oracle solution is interesting, and can indeed be implemented in
> PostgreSQL in exactly the same way. Look at materialized views here:
> 
> http://www.varlena.com/varlena/GeneralBits/Tidbits/matviews.html
> 
> Another way to do it without using an entire materialized view is to
> obtain a row level lock on the ID using SELECT ... WHERE id=123 FOR
> UPDATE. To do this you need to have a table that contains all the IDs
> and where id has a unique index to prevent race conditions when adding
> new IDs.
> 
> What are you trying to do exactly? Why does the table have no primary
> key?
> 
> Regards,
> Jeff Davis
> 
> 
> 
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