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From: KES <[email protected]>
To: Bruce Momjian <[email protected]>
To: PostgreSQL-development <[email protected]>
Subject: Re: Typo in doc or wrong EXCLUDE implementation
Date: Wed, 08 Aug 2018 13:55:53 +0300
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

I do not know many internals and maybe wrong.

But from my point of view with my current knowledge. 
If such exclusion constraint would be marked as UNIQUE we can use it for FK while implementing temporal/bi-temporal tables.

And this will be simplify relationing while implementing them.

07.08.2018, 20:37, "Bruce Momjian" <[email protected]>:
> This email was sent to docs, but I think it is a hackers issue. The
> person is asking why exclusion constraints aren't marked as UNIQUE
> indexes that can be used for referential integrity. I think the reason
> is that non-equality exclusion constraints, like preventing overlap, but
> don't uniquely identify a specific value, and I don't think we want to
> auto-UNIQUE just for equality exclusion constraints.
>
> ---------------------------------------------------------------------------
>
> On Tue, Jul 10, 2018 at 09:34:36AM +0000, PG Doc comments form wrote:
>>  The following documentation comment has been logged on the website:
>>
>>  Page: https://www.postgresql.org/docs/10/static/sql-createtable.html
>>  Description:
>>
>>  Hi.
>>
>>  https://www.postgresql.org/docs/current/static/sql-createtable.html#sql-createtable-exclude
>>  If all of the specified operators test for equality, this is equivalent to a
>>  UNIQUE constraint
>>
>>  Exclusion constraints are implemented using an index
>>
>>  ALTER TABLE person
>>    add constraint person_udx_person_id2
>>    EXCLUDE USING gist (
>>      person_id WITH =
>>    )
>>  ;
>>
>>  tucha=> ALTER TABLE "person_x_person" ADD CONSTRAINT
>>  "person_x_person_fk_parent_person_id"
>>  tucha-> FOREIGN KEY ("parent_person_id")
>>  tucha-> REFERENCES "person" ("person_id")
>>  tucha-> ON DELETE CASCADE ON UPDATE NO ACTION DEFERRABLE;
>>  ERROR: there is no unique constraint matching given keys for referenced
>>  table "person"
>>
>>  because gist does not support unique indexes, I try with 'btree'
>>
>>  ALTER TABLE person
>>    add constraint person_udx_person_id2
>>    EXCLUDE USING btree (
>>      person_id WITH =
>>    )
>>  ;
>>
>>  \d person
>>  ...
>>  "person_udx_person_id2" EXCLUDE USING btree (person_id WITH =)
>>
>>  tucha=> ALTER TABLE "person_x_person" ADD CONSTRAINT
>>  "person_x_person_fk_parent_person_id"
>>  tucha-> FOREIGN KEY ("parent_person_id")
>>  tucha-> REFERENCES "person" ("person_id")
>>  tucha-> ON DELETE CASCADE ON UPDATE NO ACTION DEFERRABLE;
>>  ERROR: there is no unique constraint matching given keys for referenced
>>  table "person"
>>
>>  Why postgres does not add unique flag. Despite on: "this is equivalent to a
>>  UNIQUE constraint"
>>  I thought it should be:
>>  "person_udx_person_id2" UNIQUE EXCLUDE USING btree (person_id WITH =)
>>
>>  PS.
>>  > For example, you can specify a constraint that no two rows in the table
>>  contain overlapping circles (see Section 8.8) by using the && operator.
>>
>>  Also I expect that this:
>>  ALTER TABLE person
>>    add constraint person_udx_person_id
>>    EXCLUDE USING gist (
>>      person_id WITH =,
>>      tstzrange(valid_from, valid_till, '[)' ) WITH &&
>>    )
>>
>>  also should raise UNIQUE flag for exclusion thus we can use it in FK
>
> --
>   Bruce Momjian <[email protected]> http://momjian.us
>   EnterpriseDB http://enterprisedb.com
>
> + As you are, so once was I. As I am, so you will be. +
> + Ancient Roman grave inscription +

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