Subject: Re: Immutable attributes?
From: Troels Arvin <troels@arvin.dk>
To: pgsql-sql@postgresql.org
In-Reply-To: <1057076907.11365.411.camel@camel>
References: <pan.2003.07.01.09.59.23.580853@arvin.dk>
	<1057076907.11365.411.camel@camel>
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Message-Id: <1057087030.15874.38.camel@localhost>
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Date: 01 Jul 2003 21:17:10 +0200

Hello,

On Tue, 2003-07-01 at 18:28, Robert Treat <xzilla@users.sourceforge.net>
wrote:
> > want to make sure that the "crated" attribut for a tuple is 
> > not changed once it has been set.
> > 
> > I'm thinking about implementing it through a trigger, but is there a
> > better way to create such "immutable" attributes? 
> 
> I don't know if it's "better", but this is one of the things people find
> the RULE system really handy for.

I thought about using the rule system for that. However:
 - I would like to be able to throw an exception if an immutable
   attribute is changed; it seems that can't be done with
   the rule system(?)
 - it seems that RULEs are a PostgreSQL-only phenomenon; I
   try to keep my SQL more portable than that

> Check the docs, I believe there are examples of this.

I haven't been able to find any related examples.

Anyways, I have now found a way to implement my immutable timestamp
fields using a stored procedure and a trigger:

create function create_time_unchanged() returns trigger as '
  begin
    if
       old.time_created <> new.time_created
    then
       raise exception
          ''time_created may not be changed: % <> %'',
          old.time_created,
          new.time_created
       ;
    end if;
    return new;
  end;'
language 'plpgsql';

create trigger ensure_create_time_unchanged before update on
transaction_pbs for each row execute procedure create_time_unchanged();

Now, let's say that the "transaction" relation has a field
"time_created" of type timestamp with time zone and that a record with
time_created=2003-07-01 20:56:11.393664+02 :

=> update transaction
=> set time_created='2003-07-01 20:56:11.393664+02'::timestamptz
=> where order_id=1000; -- NOTE: No change.
UPDATE 1
=> update transaction
=> set time_created='2003-07-01 20:56:00+02'::timestamptz
=> where order_id=1000; -- NOTE: Changed.
ERROR: time_created may not be changed: 2003-07-01 20:56:11.393664+02 <>
2003-07-01 20:56:00+02

So things work.

-- 
Troels Arvin <troels@arvin.dk>