Re: uniqueness and null could benefit from a hint for dba

On Tue, Nov 5, 2019 at 12:13:06PM -0500, Bruce Momjian wrote:
> On Tue, Oct 29, 2019 at 02:00:38PM +0200, Tuomas Leikola wrote:
> > On Thu, Oct 24, 2019 at 5:31 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> >
> > Uh, I am wondering if it is just too details for our docs.  Can you
> > think of some text and its location?
> >
> >
> >
> > "Unique indexes on functions can be used to create special types of
> > constraints, like considering unique values equal (coalesce) or only allowing a
> > single unique integer value of a float column (floor). A filtered unique index
> > only enforces uniqueness on the subset of rows that match the filter."
> >
> > I guess this would append to the paragraph "When an index is declared unique,
> > multiple table rows with equal indexed values are not allowed. Null values are
> > not considered equal. A multicolumn unique index will only reject cases where
> > all indexed columns are equal in multiple rows.". Links to the mentioned
> > keywords and concepts would of course be helpful, but not strictly necessary.
> >
> > I also thought of mentioning the caveat of (un)scannability of function or
> > filtered indexes but maybe that is a bit much.
>
> I have reviewed our documentation and found approrpiate places to
> mention your floor() example, and my IS NULL example. Patch attached.

Patch applied back through 9.4. Thanks.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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