PostgreSQL Foreign Keys – ON DELETE SET NULL Still Deletes with TRUNCATE CASCADE

postgresqltruncate

Maybe I am missing something here:

CREATE TABLE public.example_table (
    id integer UNIQUE
);

CREATE TABLE public.foreign_table (
    id integer,
    example_table_id integer,
    CONSTRAINT fk_example_table_id
    FOREIGN KEY (example_table_id)
    REFERENCES public.example_table (id)
    ON DELETE SET NULL
);

INSERT INTO public.example_table (id) VALUES
    (1);

INSERT INTO public.foreign_table (id, example_table_id) VALUES
    (1, 1),
    (2, null);

If I run TRUNCATE CASCADE, both tables are wiped which is not what I expected would happen.

TRUNCATE example_table CASCADE;

SELECT COUNT(*) FROM public.foreign_table;

0

What I would expect to happen would be that foreign_table would alter to:

(1, null)
(2, null)

Am I not understanding what SET NULL is supposed to accomplish?

Is there a way to use TRUNCATE CASCADE without having it delete from the other table? I use Laravel where I can call Model::truncate(); and it will automatically truncate the table and reset my indexes, I was hoping I could call this on the example_table and have it reset all the rows in foreign_table to null instead of just deleting the whole table.

Thanks for your help.

Best Answer

If I understand the documentation properly:

https://www.postgresql.org/docs/current/sql-truncate.html

TRUNCATE CASCADE truncates every table that has a foreign key relationship to the table in common, regardless of what action is specified for the foreign key. Example:

create table parent 
( x int not null primary key);

create table child 
( y int not null primary key
, x int not null references parent(x) 
                 on delete restrict 
                 on update restrict
);

insert into parent (x) values (1),(2),(3);
insert into child (y,x) values (1,1),(3,2);

truncate parent cascade;

select * from child;

There are no results to be displayed.

Is there something in particular that prevents you from:

delete from parent;

?

Related Question