> You want to process all invoices to count them
> and to sum up the amounts on a per month/area/type
> basis. The initial data size is in GB, but the
> size of the expected result is in KB (namely 2 data
> for each 100 areas * 12 months * 4 types).
The key to handling large datasets for data mining is pre-aggregation based on the smallest time frame needed for details.
I'd suggest running these large queries and storing the results in other tables, and then writing a set of functions to work with those aggregate tables.
No sense in summing up the same set of static data more than once if you can help it.
Darren