SELECT AND AGG huge tables

Dear all,
We have a DB containing transactional data.
There are about *50* to *100 x 10^6* rows in one *huge* table.
We are using postgres 9.1.6 on linux with a *SSD card on PCIex* providing us
a constant seeking time.

A typical select (see below) takes about 200 secs. As the database is the
backend for a web-based reporting facility 200 to 500 or even more secs
response times are not acceptable for the customer.

Is there any way to speed up select statements like this:

SELECT
SUM(T.x),
SUM(T.y),
SUM(T.z),
AVG(T.a),
AVG(T.b)
FROM T
GROUP BY
T.c
WHERE
T.creation_date=$SOME_DATE;

There is an Index on T.c. But would it help to partition the table by T.c?
It should be mentioned, that T.c is actually a foreign key to a Table
containing a
tiny number of rows (15 rows representing different companies).
my postgres.conf is actually the default one, despite the fact that we
increased the value for work_mem=128MB

Thanks in advance
Houman

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