Subject: Re: unique index with several columns
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: "Voillequin, Jean-Marc" <Jean-Marc.Voillequin@moodys.com>,
 "David G. Johnston" <david.g.johnston@gmail.com>,
 "pgsql-sql@lists.postgresql.org" <pgsql-sql@lists.postgresql.org>
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 <6699c0e9-e358-1fc2-7ce4-7a24d4bc419e@yandex.ru>
 <3966274.1646418725@sss.pgh.pa.us>
From: Alexey M Boltenkov <padrebolt@yandex.ru>
Message-ID: <8b4b7a41-5181-60db-2bd9-a6fabcd6226f@yandex.ru>
Date: Fri, 4 Mar 2022 23:47:12 +0300
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On 03/04/22 21:32, Tom Lane wrote:
> Alexey M Boltenkov <padrebolt@yandex.ru> writes:
>> You need the new v15 feature:
>> NULLS [NOT] DISTINCT
> That won't replicate the behavior shown by the OP though.
> In particular, not the weird inconsistency for all-null rows.
>
> 			regards, tom lane
>

But why?

# create table t(c1 char, c2 char);
CREATE TABLE
# create unique index idx on t(c1,c2) nulls not distinct where c1 is not 
null or c2 is not null;
CREATE INDEX
# insert into t(c1,c2) values (null,null);
INSERT 0 1
# insert into t(c1,c2) values (null,null);
INSERT 0 1
# insert into t(c1,c2) values ('a',null);
INSERT 0 1
# insert into t(c1,c2) values ('a',null);
ERROR:  23505: duplicate key value violates unique constraint "idx"
DETAIL:  Key (c1, c2)=(a, null) already exists.
SCHEMA NAME:  public
TABLE NAME:  t
CONSTRAINT NAME:  idx
LOCATION:  _bt_check_unique, nbtinsert.c:664
# \d+ t
                                                Table "public.t"
  Column │     Type     │ Collation │ Nullable │ Default │ Storage │ 
Compression │ Stats target │ Description
════════╪══════════════╪═══════════╪══════════╪═════════╪══════════╪═════════════╪══════════════╪═════════════
  c1     │ character(1) │           │          │         │ extended 
│             │              │
  c2     │ character(1) │           │          │         │ extended 
│             │              │
Indexes:
     "idx" UNIQUE, btree (c1, c2) NULLS NOT DISTINCT WHERE c1 IS NOT 
NULL OR c2 IS NOT NULL
Access method: heap
# table t;
  c1 │ c2
════╪════
  ¤  │ ¤
  ¤  │ ¤
  a  │ ¤
(3 rows)