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Re: Multitable uniqueness ?

From: Jan Wieck <JanWieck(at)Yahoo(dot)com>
To: Bruno Wolff III <bruno(at)wolff(dot)to>
Cc: Andreas <maps(dot)on(at)gmx(dot)net>, pgsql-sql(at)postgresql(dot)org
Subject: Re: Multitable uniqueness ?
Date: 2004-05-26 13:56:17
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Lists: pgsql-sql
Bruno Wolff III wrote:

> On Wed, May 26, 2004 at 05:13:14 +0200,
>   Andreas <maps(dot)on(at)gmx(dot)net> wrote:
>> Is there a way to have something like this :  UNIQUE (, 
> Postgres doesn't support database constraints at this time which is
> what you would need to do this simply.
> You can enforce this constraint by creating a third table that has the
> ids, id type and a separate id type field for each possible type that
> will be null except for the type field that matches the actual type.
> You also need to add a type field to the original two tables. Then you
> set up a composite foreign key from the new table to each of the per type
> tables using the id and the matching id type fields. Each per type table
> should have a foriegn key refernce for id to the combined table.
> This will enforce a 1-1 relationship between the combined table and the
> union of the per type tables.

I think a third table with just the unique id plus custom triggers on 
both tables that insert/update/delete the id into/from the third table 
are totally enough.


> It is probably possible to get rid of the redundant copies of the field type
> using triggers, but I don't know that there is much of a benefit to doing so.
> The redundant values will all be kept in sync with constraints, so there isn't
> a data consistancy problem with doing it that way. This method is going to be
> more portable than using triggers. This method will probably be within a
> constant factor as efficient as anything you do with triggers.
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# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
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