Re: uniqueness constraint with NULLs

In response to Leo Mannhart :
> 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:

Maybe, depends on the query.

>
> 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));

bad idea if 1 is a valid value for this column.

Andreas
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Andreas Kretschmer
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