Should I add a Index Key in this case ?

Hi Experts,

I have a use case as follows :

add constraints to the database so that no two reservations for the same viewing may refer to the same seat.

So, say I have a primary key like this in the table A:

SEAT_id integer NOT NULL GENERATED ALWAYS AS IDENTITY ( INCREMENT 1 START 1 MINVALUE 1 MAXVALUE 2147483647 CACHE 1 ),
CONSTRAINT A_pkey PRIMARY KEY (SEAT_id))
WITH (
OIDS = FALSE
)
TABLESPACE pg_default;

So, basically I would like to create a SEAT_Viewing_Id.

In this case, do I create a Index Key or ?

How should I construct or rather alter my table A to accomodate this SEAT_Viewing_Id ?

Hope someone can tell me how.

Furthermore, whenever an insertion is done via WebApp, I would have to insert the Index key as well or does PostgreSQL will have a way to increment the Index key which is the SEAT_Viewing_Id at the same time?

Thanks & regards,
Karen