Re: pgsql: doc: add examples of creative use of unique expression indexes

On Mon, Apr 20, 2020 at 04:21:32PM -0400, Tom Lane wrote:
> [ sorry, I'd lost track of this thread ]
>
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > 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.
>
> Well, that's not the case:
>
> regression=# create index tenk1_null_index on tenk1((1)) where ten is null;
> CREATE INDEX
> regression=# explain select * from tenk1 where ten is null;
> QUERY PLAN
> --------------------------------------------------------------------------------
> Index Scan using tenk1_null_index on tenk1 (cost=0.12..8.14 rows=1 width=244)
> (1 row)
>
> (Maybe it was true at some time in the past, but not any more.)
>
> Also, it complicates the example, and since you didn't explain the
> reason for the complication, I think it's pretty confusing.
>
> But really I still don't see the need for these additional examples
> at all. It's especially weird that what you want to do is have
> some examples on that page have <example> markup and others not.

OK, seems like only you and I care about this issue, which I take to
mean that we should minimize what we are adding here. What the attached
patch does is to remove the previous commit, and just add a sentence to
the last example to mention the ability restrict a column to a single
NULL.

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

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