Improving performance on multicolumn query

Hi, all!

I've been using postgresql for a long time now, but today I had some
problem I couldn't solve properly - hope here some more experienced
users have some hint's for me.

First, I'm using postgresql 7.4.7 on a 2GHz machine having 1.5GByte RAM
and I have a table with about 220 columns and 20000 rows - and the first
five columns build a primary key (and a unique index).

Now my problem: I need really many queries of rows using it's primary
key and fetching about five different columns but these are quite slow
(about 10 queries per second and as I have some other databases which
can have about 300 queries per second I think this is slow):

transfer=> explain analyse SELECT * FROM test WHERE test_a=9091150001
AND test_b=1 AND test_c=2 AND test_d=0 AND test_e=0;

Index Scan using test_idx on test (cost=0.00..50.27 rows=1 width=1891)
(actual time=0.161..0.167 rows=1 loops=1)
Index Cond: (test_a = 9091150001::bigint)
Filter: ((test_b = 1) AND (test_c = 2) AND (test_d = 0) AND (test_e 0))

So, what to do to speed things up? If I understand correctly this
output, the planner uses my index (test_idx is the same as test_pkey
created along with the table), but only for the first column.

Accidently I can't refactor these tables as they were given to me.

Thanks for any hint!
Jan