Identity columns should own only one sequence

Identity columns don't work if they own more than one sequence.

So if one tries to convert a "serial" column to an identity column,
the following can happen:

test=> CREATE TABLE ser(id serial);
CREATE TABLE
test=> ALTER TABLE ser ALTER id ADD GENERATED ALWAYS AS IDENTITY;
ERROR: column "id" of relation "ser" already has a default value

Hm, ok, let's drop the column default value.

test=> ALTER TABLE ser ALTER id DROP DEFAULT;
ALTER TABLE

Now it works:

test=> ALTER TABLE ser ALTER id ADD GENERATED ALWAYS AS IDENTITY;
ALTER TABLE

But not very much:

test=> INSERT INTO ser (id) VALUES (DEFAULT);
ERROR: more than one owned sequence found

I propose that we check if there already is a dependent sequence
before adding an identity column.

The attached patch does that, and also forbids setting the ownership
of a sequence to an identity column.

I think this should be backpatched.

Yours,
Laurenz Albe