Filtered Joins

I am having some difficulty in constructing outer joins. I have
simplified what I need to do and have included sample SQL statements:

create table tab_a (id int, descr varchar(10), qty int)
insert into tab_a values (1, 'item one', 10)
insert into tab_a values (2, 'item two', 20)
insert into tab_a values (3, 'item three', 30)
insert into tab_a values (4, 'item four', 40)

create table tab_b (id2 int, descr2 varchar(10), qty2 int)
insert into tab_b values (1, 'item one', 10)
insert into tab_b values (2, 'item two', 20)
insert into tab_b values (3, 'item three', 30)
insert into tab_b values (4, 'item four', 40)

Here is the statement that I have:

SELECT tab_a.id,
tab_a.descr,
tab_a.qty,
tab_b.id2,
tab_b.descr2,
tab_b.qty2
FROM tab_a LEFT OUTER JOIN tab_b
ON (tab_a.id = tab_b.id2 )
WHERE tab_a.qty <= 30 AND
tab_b.qty2 > 20

What I am trying to do is left outer join between tab_a and tab_b after
they have been filtered based on the qty column. (for tab_a: qty <=
30; and for tab_b: qty > 20).

How would I go about that? I would like to do this efficiently since
the two tables have about a million records and several other columns
each.