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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bruce Momjian <bruce@momjian.us>
cc: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: Re: pgsql: doc: add examples of creative use of unique expression
 indexes
In-reply-to: <20200411122532.GE24988@momjian.us>
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Comments: In-reply-to Bruce Momjian <bruce@momjian.us>
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[ sorry, I'd lost track of this thread ]

Bruce Momjian <bruce@momjian.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=3D# create index tenk1_null_index on tenk1((1)) where ten is nu=
ll;
CREATE INDEX
regression=3D# explain select * from tenk1 where ten is null;
                                   QUERY PLAN                             =
      =

--------------------------------------------------------------------------=
------
 Index Scan using tenk1_null_index on tenk1  (cost=3D0.12..8.14 rows=3D1 w=
idth=3D244)
(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.

			regards, tom lane