I have a table which is with following kind of information

activity     cost    order   date  other information
10            1       100     --
20            2       100
10            1       100
30            4       100
40            4       100
20            2       100
40            4       100
20            2       100
10            1       101
10            1       101
20            1       101

My requirement is to get sum of all activities over a work order ex: for order 100

1+2+4+4=11

1(for activity 10) 2(for activity 20) 4 (for activity 30) etc.

i tried with group by, its taking lot time for calculation. There are 1lakh plus records in warehouse. is there any possibility in efficient way.

SELECT SUM(MIN(cost))
FROM COST_WAREHOUSE a
WHERE   order = 100
GROUP BY (order, ACTIVITY)