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Date: Fri, 6 Jun 2003 07:20:01 -0500
From: Bruno Wolff III <bruno@wolff.to>
To: mallah@trade-india.com
Cc: pgsql-sql@postgresql.org
Subject: Re: simulating partial fkeys..
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On Thu, Jun 05, 2003 at 20:25:32 +0530,
  mallah@trade-india.com wrote:
> 
> 
> hi ,
> 
> Is there any way to enforce fkeys only on subset of
> the table something on the lines of unique partial indexes
> 
> or any work around ? (on update or insert trigger is the only thing i can think of)

If the foreign column is null it isn't checked against the other table.
You can probably use this to do what you want.