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

Yes, it helps, thank you.

But I have many tables with many indexes and many columns!

Tons of combinations… ☹

 

From: David G. Johnston <david.g.johnston@gmail.com>
Sent: Friday, March 4, 2022 4:32 PM
To: Voillequin, Jean-Marc <Jean-Marc.Voillequin@moodys.com>
Cc: pgsql-sql@lists.postgresql.org
Subject: Re: unique index with several columns

 

 

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On Fri, Mar 4, 2022 at 8:07 AM Voillequin, Jean-Marc <Jean-Marc.Voillequin@moodys.com> wrote:

 

When one of the field is null, PG considers that the tuple is not the same:

('a',null) is not equal to ('a',null)

So, the unique constraint is not violated in PG.

 

But is there a way to have the same feature than Oracle?

 

I already tried with:

create unique index idx on t(coalesce(c1,''),coalesce(c2,''))

 

But in this case, I cannot insert several (null,null) without raising a duplicate key error.

 

 

You just said you wanted nulls to be considered equal to each other...

 

Not tested but these should work in conjunction with each other:

 

CREATE UNIQUE INDEX idx ON t (c1, c2) WHERE c1 IS NOT NULL AND c2 IS NOT NULL;

CREATE UNIQUE INDEX idx2 ON t (c1) WHERE c2 IS NULL AND c1 IS NOT NULL;

CREATE UNIQUE INDEX idx3 ON t (c2) WHERE c1 IS NULL AND c2 IS NOT NULL;

--CREATE INDEX idx4 ON t (c1, c2) WHERE c1 IS NULL AND c2 IS NULL; (pointless...but symmetric with the others)

 

Hopefully this helps.

 

David J.

 

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