Aggregate query for multiple records

Hello, I am new to the list, my apology if this question is beyond the
scope or charter of this list.

My questions is:
What is the best method to perform an aggregate query to calculate
sum() values for each distinct wid as in the example below, but except
for all wid's (not just WHERE wid='01/1-6-1-30w1/0').

Also, performance wise, would it be better to build a function for this
query.  The table has 9 million records and these aggregate queries
take hours.


SELECT
   SUM(oil) as sumoil, SUM(hours) as sumhours,
FROM
   (SELECT oil, hours prd_data WHERE wid='01/1-6-1-30w1/0'
    ORDER BY date LIMIT 6) subtable
;


Table description:
               Table "prd_data"
  Column |         Type          | Modifiers
--------+-----------------------+-----------
  date   | integer               |
  hours  | real                  |
  oil    | real                  |
  gas    | real                  |
  water  | real                  |
  pwid   | integer               |
  wid    | character varying(20) |
  year   | smallint              |
Indexes: wid_index6


Actual table (prd_data), 9 million records:

   date  | hours |  oil  | gas  | water | pwid |       wid       | year
--------+-------+-------+------+-------+------+-----------------+------
  196507 |   360 | 159.4 | 11.3 |  40.9 |  413 | 01/1-1-1-31w1/0 | 1965
  196508 |   744 |   280 |   20 |  27.2 |  413 | 01/1-1-1-31w1/0 | 1965
  196509 |   360 | 171.1 | 11.4 |  50.4 |  413 | 01/1-1-1-31w1/0 | 1965
  196510 |   744 | 202.1 |   25 |  89.8 |  413 | 01/1-1-1-31w1/0 | 1965
  196512 |   744 | 201.3 | 23.8 |  71.9 |  413 | 01/1-1-1-31w1/0 | 1965
  196511 |   720 |   184 | 17.6 |  78.9 |  413 | 01/1-1-1-31w1/0 | 1965
  196610 |   744 |  99.8 | 15.4 |  53.7 |  413 | 01/1-1-1-31w1/0 | 1966
  196612 |   744 |    86 | 12.8 |  36.1 |  413 | 01/1-1-1-31w1/0 | 1966
  196611 |   720 |    86 | 12.6 |  41.7 |  413 | 01/1-1-1-31w1/0 | 1966
  196601 |   744 | 191.6 | 22.6 |  50.7 |  413 | 01/1-1-1-31w1/0 | 1966
  200301 |   461 |  68.8 |   0 | 186.3 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200310 |   740 | 446.3 |   0 | 563.1 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200306 |   667 |  92.1 |   0 | 968.8 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200304 |     0 |     0 |   0 |     0 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200308 |   457 | 100.7 |   0 |  82.8 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200307 |   574 |    78 |   0 |   752 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200312 |   582 | 360.9 |   0 |   569 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200311 |   681 | 260.8 |   0 | 563.9 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200305 |   452 |     0 |   0 |     0 | 47899 | 9G/6-1-50-24w3/0 | 2003
  200309 |   637 | 244.6 |   0 | 193.8 | 47899 | 9G/6-1-50-24w3/0 | 2003
(20 rows)



Thanks,

--
Scott A. Gerhardt, P.Geo.
Gerhardt Information Technologies