Re: pgsql: doc: add examples of creative use of unique expressionindexes

On Fri, Apr 10, 2020 at 08:17:09PM -0400, Bruce Momjian wrote:
> On Fri, Apr 10, 2020 at 07:21:29PM -0400, Tom Lane wrote:
> > Bruce Momjian <bruce@momjian.us> writes:
> > > I agree with your analysis.  I still want to have some mention that
> > > partial indexes can be used to create single-NULL columns, which might
> > > be required for compatibility with other databases.  Attached is an
> > > updated patch which removes the previous commit but adds a mention of
> > > this.
> > 
> > The single-null thing is probably a useful example, but please make
> > it an actual separate example, or at least its own para outside the
> > existing <example> sections.
> > 
> > Also, the existing example demonstrating that seems overcomplicated;
> > why not just
> > 
> >     create unique index ... (1) where (foo is null);
> 
> I ended up using "true" since that is ony one byte;  patch attached.

[ thread moved to docs]

I now remember that I wrote the first IS NULL in:

  CREATE UNIQUE INDEX tests_target_one_null ON tests ((target IS NULL)) WHERE target IS NULL;

in hope that if someone is looking for the null value in the column, the
IS NULL would allow the index to be used to find it, while 1 or true
would not.

Also, I think the most popular use for this ability would be for
multi-column indexes where you want only one NULL value for a
combination of columns, e.g.:

  CREATE UNIQUE INDEX tests_target_one_null ON test (x, (y IS NULL)) WHERE y IS NULL;

I have added that.  It also hows the use of columns and expressions in
the same index.  Proposed patch attached.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

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