Re: Disable unique constraint in Postgres

On Fri, Nov 25, 2022 at 08:46:22AM +0100, Laurenz Albe wrote:
> On Fri, 2022-11-25 at 12:48 +0530, Nikhil Ingale wrote:
> > We have an alter command to disable any constraints in the oracle db. Similarly do we have any
> > command to disable the UNIQUE constraint in postgres. Most of the postgres db forum suggests
> > dropping the UNIQUE constraint. But, I don't want to drop the constraint here instead just
> > disable the unique constraint.
> >
> > Need your inputs on the same.
>
> The db forums were right: you cannot disable a unique constraint in PostgreSQL.

Hi,

hope, that I've understood the problem correctly...
At least since v13.x dropping a unique constraint seems to work while
dropping the related index:

-- two constraints added:
np=# create table a (i integer unique constraint hu check(i<100));
CREATE TABLE
np=# \d+ a
Table "public.a"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
--------+---------+-----------+----------+---------+---------+--------------+-------------
i | integer | | | | plain | |
Indexes:
"a_i_key" UNIQUE CONSTRAINT, btree (i)
Check constraints:
"hu" CHECK (i < 100)
Access method: heap

np=# insert into a values(1);
INSERT 0 1
np=# insert into a values(1);
ERROR: duplicate key value violates unique constraint "a_i_key"
DETAIL: Key (i)=(1) already exists.
np=# insert into a values(2);
INSERT 0 1
np=# insert into a values(101);
ERROR: new row for relation "a" violates check constraint "hu"
DETAIL: Failing row contains (101).

-- So far, so good. It works as expected.
-- Now:

np=# alter table a drop constraint a_i_key;
ALTER TABLE
np=# \d+ a
Table "public.a"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
--------+---------+-----------+----------+---------+---------+--------------+-------------
i | integer | | | | plain | |
Check constraints:
"hu" CHECK (i < 100)
Access method: heap

-- only constraint i<100 left:

np=# insert into a values(1);
INSERT 0 1
np=# insert into a values(101);
ERROR: new row for relation "a" violates check constraint "hu"
DETAIL: Failing row contains (101).

np=# select * from a;
i
---
1
2
1

-- have to check that against different postgresql versions and docs...

cheers

/np

>
> Yours,
> Laurenz Albe
>