optimal insert

Hello experts,

I have a database that contains three tables:

create table a ( id serial primary key, ... -- some more fields not relevant for my question
);
create table b ( id serial primary key, ... -- some more fields not relevant for my question
);
create table a_b ( a int not null references a, b int not null references b
);

Tables a and b have already been filled with lots of rows. Now my
application needs to insert the relationship of a to b into table a_b
and is currently doing it with inserts like the following:

insert into a_b(a,b) values(1,100);
insert into a_b(a,b) values(1,200);
insert into a_b(a,b) values(1,54);
insert into a_b(a,b) values(1,4577);

So for a batch of inserts the value of a stays the same, while for by
arbitrary values are inserted. Now I have wondered if PostreSQL offers
a smarter way to insert those values? A solution can contains usage of
some plpgsql code.

-- 
---> Dirk Jagdmann
----> http://cubic.org/~doj
-----> http://llg.cubic.org