I think what lucian1900 may be thinking is that instead of create table purchase_order ( id int primary key, ordered_on timestamptz not nul

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.