postgres doesn't drop not null

Question

I drop not null on primary key column, and it executes, after I check schema of table and there is not null

table:

-- auto-generated definition
create table warehouses
(
    id_warehouse       serial      not null
        constraint warehouse_pkey
            primary key,
    responsible_person varchar(30) not null
);

script to drop not null:

alter table warehouses
    drop constraint warehouse_pkey;

alter table warehouses
    alter id_warehouse drop not null;

alter table warehouses
    add constraint warehouse_pkey
        primary key (id_warehouse);

Answer 1

Per information here:

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

PRIMARY KEY (column constraint)

The PRIMARY KEY constraint specifies that a column or columns of a table can contain only unique (non-duplicate), nonnull values. Only one primary key can be specified for a table, whether as a column constraint or a table constraint.

A column defined as PRIMARY KEY will also have the NOT NULL constraint set.