Date: Wed, 11 Jul 2012 10:16:07 +0200
From: Andreas Kretschmer <akretschmer@spamfence.net>
To: pgsql-sql@postgresql.org
Subject: Re: Prevent double entries ... no simple unique index
Message-ID: <20120711081607.GA17798@tux>
References: <4FFD3050.3010509@gmx.net>
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Andreas <maps.on@gmx.net> wrote:

> Hi,
>
> I've got a log-table that records events regarding other objects.
> Those events have a state that shows the progress of further work on  
> this event.
> They can be  open, accepted or rejected.
>
> I don't want to be able to insert addition events regarding an object X  
> as long there is an open or accepted event.
> On the other hand as soon as the current event gets rejected a new event  
> should be possible.
>
> So there may be several rejected events at any time but no more than 1  
> open or accepted entry.
>
> Can I do this within the DB so I don't have to trust the client app?
>
> The layout looks like this
> Table : objects ( id serial, .... )
>
> Table : event_log ( id serial, oject_id integer references objects.id,  
> state integer, date_created timestamp, ... )
> where state is   0 = open, -1 = reject, 1 = accept

test=# create table log (state int not null, check (state in (-1,0,1)));
CREATE TABLE
Time: 37,527 ms
test=*# commit;
COMMIT
Time: 0,556 ms
test=# create unique index on log((case when state in (0,1) then 1 else
null end));
CREATE INDEX
Time: 18,558 ms
test=*# insert into log values (-1);
INSERT 0 1
Time: 0,611 ms
test=*# insert into log values (-1);
INSERT 0 1
Time: 0,274 ms
test=*# insert into log values (-1);
INSERT 0 1
Time: 0,248 ms
test=*# insert into log values (1);
INSERT 0 1
Time: 0,294 ms
test=*# insert into log values (0);
ERROR:  duplicate key value violates unique constraint "log_case_idx"
DETAIL:  Key ((
CASE
    WHEN state = ANY (ARRAY[0, 1]) THEN 1
    ELSE NULL::integer
END))=(1) already exists.
test=!#

HTH.


Andreas
-- 
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