CREATE TABLE users (username text, email text);
CREATE TABLE simple_users () INHERITS (users);
CREATE TABLE users_with_password (password text) INHERITS (users);

Our three tables look like this:

users

Column	Type
username	text
email	text

simple_users

Column	Type
username	text
email	text

users_with_password

Column	Type
username	text
email	text
password	text

Let's create two simple tables:

CREATE TABLE users (username text, email text);
CREATE TABLE simple_users () INHERITS (users);

Adding columns

ALTER TABLE simple_users ADD COLUMN password text;

simple_users

Column	Type
username	text
email	text
password	text

Adding the same column to the parent table will merge the definition of both columns:

ALTER TABLE users ADD COLUMN password text;

NOTICE: merging definition of column "password" for child "simple_users"

Dropping columns

Using our altered tables:

ALTER TABLE users DROP COLUMN password;

users

Column	Type
username	text
email	text

simple_users

Column	Type
username	text
email	text
password	text

Since we first added the column to simple_users, PostgreSQL makes sure this column isn't dropped.

Now if we had another child table, its password column would, of course, have been dropped.