Marko Tiikkaja <> wrote:

> On 5/21/10 11:47 PM +0300, Jeff Davis wrote:
> > It also allows you to enforce the constraint that only one tuple exists
> > in a table by doing something like:
> >
> >    create table a
> >    (
> >      i int,
> >      exclude using gist (i with<>),
> >      unique (i)
> >    );

+1.  I've not read the code, but it might be considerable that we can
abort index scans if we find a first index entry for "i". While we must
scan all candidates for "WHERE i <> ?", but we can abort for the constraint
case because we know existing values are all the same.

> FWIW, this is achievable a lot more easily:
> CREATE UNIQUE INDEX "a_single_row" ON a ((1));

The former exclusion constraint means "one same value for all rows",
but your alternative means "a_single_row", right?

Takahiro Itagaki
NTT Open Source Software Center

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