Example. Foreign Keys Constraints. Wrong Columns
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
In the last example of this section it seems the `users` table is referenced
wrong.
```sql
CREATE TABLE users (
tenant_id integer REFERENCES tenants ON DELETE CASCADE,
user_id integer NOT NULL,
PRIMARY KEY (tenant_id, user_id)
);
CREATE TABLE posts (
tenant_id integer REFERENCES tenants ON DELETE CASCADE,
post_id integer NOT NULL,
author_id integer,
PRIMARY KEY (tenant_id, post_id),
FOREIGN KEY (tenant_id, author_id) REFERENCES users ON DELETE SET NULL
(author_id)
);
```
In this example `FOREIGN KEY (tenant_id, author_id) REFERENCES users ON
DELETE SET NULL (author_id)` implies that `users` table columns are named
`(tenant_id, author_id)` but in fact `users` table does not have a
`author_id` column.
That line should be probably like this because `users` tables has a
`user_id` column instead of `author_id`
```sql
CREATE TABLE posts (
# ...
FOREIGN KEY (tenant_id, author_id) REFERENCES users (tenant_id, user_id)
ON DELETE SET NULL (author_id)
);
```
On Wed, Apr 15, 2026 at 7:51 AM PG Doc comments form <noreply@postgresql.org>
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.
David G. Johnston wrote:
On Wed, Apr 15, 2026 at 7:51 AM PG Doc comments form <noreply@postgresql.org>
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