Get summary field to not calculate everything every time
I have a sales report which uses a summary field to add up sales totals. I sort in monthly trailing groups and I can compare sales from month to month. What I like most about this feature is that I can see sales update the current monthly total and I can project if we are on target. I can produce charts from this too and it looks great on the iPhone. I've only got six months data on my database and it takes nearly a minute to summarise the totals. I am soon going to migrate the sales records from the last ten years into the database. Imagine how long that will take to summarise in our hosted database over 3G. The only month in which new sales occur is the current month.
So I was wondering is it possible to make my sales report summarise only the current month and the last month's (sometimes sales are entered late) data and at same time see the data from all previous months?