Large Query Question. (Slow Select while using 'IN')

Hi all, I hope someone can help me out.

I'm doing single-table select statements on a large table and I could use
some help in speeding it up.

My query is of the form:
SELECT col, count(col) FROM tab WHERE id IN (3,
4,7,2, ...) GROUP BY COL ORDER BY count

for a very large number of rows.

I have an index on id, so the explain looks like:

Aggregate  (cost=12.12..12.14 rows=1 width=5) ->  Group  (cost=12.12..12.13 rows=4 width=5)       ->  Sort
(cost=12.12..12.12rows=4 width=5)
 
col_id_idx2 on tab  (cost=0.00..12.08 rows=4 width=5)

So, it does a separate index scan for each row in the IN statement, which
takes forever.

How do I force the query parser to emulate the behaviour displayed by this
query:

SELECT col, count(col) FROM tab WHERE (0 = id % 5) GROUP BY COL ORDER BY
count

Aggregate  (cost=3.75..3.86 rows=2 width=5) ->  Group  (cost=3.75..3.81 rows=21 width=5)       ->  Sort
(cost=3.75..3.75rows=21 width=5)             ->  Index Scan using col_id_idx2 on tab
 
(cost=0.00..3.29 rows=21 width=5)

Which only does one index scan for an equivelant number of records.

Thanks for any help.  Please cc to my e-mail.