create table tovar
(
    id        serial primary key,
    name    char( 16 ),
    caption    char( 64 ),
    price    numeric( 6 , 2 ),
    parent    int,
    del        bool,
    grp        bool
);

create table postavshik
(
    id        serial primary key,
    name    char( 16 ),
    comment    char( 64 )
);
create table tovary
(
    id            serial primary key,
    tovar        int references tovar(id),
    count        int,
    sklad        int references sklad(id),
    n            bool default false, -- пришлось добавить
    unique(tovar,sklad)
);

create or replace rule tovary_add_tovar as
on insert to tovary
where (exists ( select  1  from tovary where tovary.tovar=new.tovar and tovary.sklad=new.sklad))
do instead-- nothing;
update tovary set "count"=(tovary."count"*tovary.n::int)+new."count",n=true where tovar=new.tovar and sklad=new.sklad;

... insert before ...
... update ...
if found then
return NULL;
else
return new;
...