From: Leo Mannhart <Leo.Mannhart@beecom.ch>
To: pgsql-sql@postgresql.org
Subject: Re: uniqueness constraint with NULLs
Date: Mon, 29 Jun 2009 09:23:51 +0200
Message-ID: <1246260231.7296.2.camel@mars.lemaco.ch> (raw)
In-Reply-To: <4A486872.5060906@cs.anu.edu.au>
References: <4A485431.3080707@cs.anu.edu.au>
<20090629062602.GB24737@a-kretschmer.de>
<4A486872.5060906@cs.anu.edu.au>
On Mon, 2009-06-29 at 17:08 +1000, Robert Edwards wrote:
> A. Kretschmer wrote:
> > In response to Robert Edwards :
> >> Can anyone suggest a way that I can impose uniqueness on a and b when
> >> c is NULL?
> >
> > Sure, use a functional index:
> >
> > test=# create table bobtest (a int, b int, c int);
> > CREATE TABLE
> > test=*# create unique index idx_bobtest on
> > bobtest(a,b,coalesce(c::text,'NULL'));
> > CREATE INDEX
> > test=*# insert into bobtest (a, b) values (1, 4);
> > INSERT 0 1
> > test=*# insert into bobtest (a, b, c) values (1, 4, NULL);
> > ERROR: duplicate key value violates unique constraint "idx_bobtest"
> > test=!#
> >
Why changing the datatype of coulumn "c"? Will this not probably disable
the use of the index? Why not only:
lem=# create table bobtest (a int, b int, c int);
CREATE TABLE
lem=# create unique index idc_bobtest on bobtest (a, b, coalesce(c, 1));
CREATE INDEX
lem=# insert into bobtest (a, b) values (1, 4);
INSERT 0 1
lem=# insert into bobtest (a, b) values (1, 4);
ERROR: duplicate key value violates unique constraint "idc_bobtest"
lem=# insert into bobtest (a, b, c) values (1, 4, null);
ERROR: duplicate key value violates unique constraint "idc_bobtest"
lem=#
> >
> > Regards, Andreas
>
> Beautiful!
>
> Many thanks,
>
> Bob Edwards.
>
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To: pgsql-sql@postgresql.org
Cc: Leo.Mannhart@beecom.ch
Subject: Re: uniqueness constraint with NULLs
In-Reply-To: <1246260231.7296.2.camel@mars.lemaco.ch>
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