[fauigerzigerk]

I think what lucian1900 may be thinking is that instead of

    create table purchase_order (
        id int primary key,
        ordered_on timestamptz not null,
        customer_id int not null references customer,
        canceled_on timestamptz
    );

you could have

    create table purchase_order (
        id int primary key,
        ordered_on timestamptz not null,
        customer_id int not null references customer
    );

    create table order_cancelation (
        order_id int primary key references purchase_order,
        canceled_on timestamptz not null
    );

This is indeed a better normalised schema and it allows you to index order_cancelation.canceled_on without worrying about nulls.

[crazygringo]

Oh then, absolutely. I was assuming a constraint that columns couldn't be removed from the original table. But if you can, then your example is an even better solution.

[lucian1900]

Exactly, that’s what I thought was being described.