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From: Yushu Chen <[email protected]>
To: David G. Johnston <[email protected]>
To: [email protected]
To: [email protected]
Subject: Re: Example. Foreign Keys Constraints. Wrong Columns
Date: Fri, 17 Apr 2026 23:06:28 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAKFQuwbm4BD49A8ZqjZnxHKW8MsEURa6hrEvAWKxw0_nu=FZwg@mail.gmail.com>
References: <[email protected]>
	<CAKFQuwbm4BD49A8ZqjZnxHKW8MsEURa6hrEvAWKxw0_nu=FZwg@mail.gmail.com>

David G. Johnston wrote:
> On Wed, Apr 15, 2026 at 7:51 AM PG Doc comments form <[email protected]>
> wrote:
> 
>> The following documentation comment has been logged on the website:
>>
>> Page: https://www.postgresql.org/docs/18/ddl-constraints.html
>> Description:
>>
>>
>> https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-FK
>>
>>
> Given that users has:
> 
>>     PRIMARY KEY (tenant_id, user_id)
>>
>>
> This:
> 
> 
>>     FOREIGN KEY (tenant_id, author_id) REFERENCES users ON DELETE SET NULL
>> (author_id)
>>
>>
> And this:
> 
> 
>>     FOREIGN KEY (tenant_id, author_id) REFERENCES users (tenant_id,
>> user_id)
>> ON DELETE SET NULL (author_id)
>>
> 
> Produce an identical outcome.
> 
> The absence of a column list on the former causes the system to look at the
> primary key for the named table and use its column list - which is
> (tenant_id, user_id), same as the later explicit version.
> 
> David J.
> 

Thanks for explanation.

I think "columns mapping" (just how I call it in this example) makes
this example slightly non-intuitive, and reflects a less-common use case.
Would it help to change `author_id` to `user_id` as a more
straightforward case?

Yushu Chen

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