RE: unique index with several columns - Mailing list pgsql-sql

Thanks a lot guys.

I'm waiting for the 15 release to implement Alexey solution.

But for now:
create unique index idx on t((array[c1,c2])) where not((c1,c2) is null);
will make the job

not((c1,c2) is null) is the tricky condition to check that not all fields are null...

Proof:
RRT=> create table t(c1 char,c2 char);
CREATE TABLE
RRT=> create unique index idx on t((array[c1,c2])) where not((c1,c2) is null);
CREATE INDEX
RRT=> insert into t values (null,null);
INSERT 0 1
RRT=> insert into t values (null,null);
INSERT 0 1
RRT=> insert into t values (null,'a');
INSERT 0 1
RRT=> insert into t values (null,'a');
ERROR:  duplicate key value violates unique constraint "idx"
DETAIL:  Key ((ARRAY[c1, c2]))=({NULL,a}) already exists.

Regards

-----Original Message-----
From: Alexey M Boltenkov <padrebolt@yandex.ru> 
Sent: Friday, March 4, 2022 9:47 PM
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
Subject: Re: unique index with several columns

 

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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
intot(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)

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