Bulk Inserts

Hi,

I'm trying to measure the performance of the following: Multiple txns
inserting tuples into a table concurrently vs single txn doing the whole
insertion.

*new table created as:*
create table tab2 (
id serial,
attr1 integer not null,
attr2 integer not null,
primary key(id)
);

*EXP 1: inserts with multiple txn:*
insert into tab2 (attr1, attr2) (select attr1, attr2 from tab1 where attr2
= 10);
insert into tab2 (attr1, attr2) (select attr1, attr2 from tab1 where attr2
= 20);

note: attr2 has only two values 10 and 20

*EXP 2: inserts with a single txn:*
insert into tab2 (attr1, attr2) (select attr1, attr2 from tab1);

I also performed another experiment as follows:
*EXP 3:* select attr1, attr2 into tab2 from tab1;

The observation here is EXP 3 is much faster than EXP 2 probably due to
bulk inserts used by Postgres. However I could not find a way to insert id
values in tab2 using EXP 3. Also select .. into .. from .. throws an error
if we create a table first and then populate the tuples using the command.

I have the following questions:
1. Is it possible to have an id column in tab2 and perform a bulk insert
using select .. into .. from .. or using some other means?
2. If a table is already created, is it possible to do bulk inserts via
multiple txns inserting into the same table (EXP 3)?

Best,
-SB