For my future reference:)
A Summed area table(SAT) can be used to query the sum of values over a rectangular region.
From this you can also derive the average value, by dividing by the # of pixels in the rectangle.
It can be used as an alternative to mip mapping.
One advantage over mip mapping is that the query region can be an arbitrary rectangle, unlike mip mapping which is square.
A disadvantage is that that it requires more and more precision as you approach the lower right(the final value is the sum of all previous values).
Thus SAT generally requires increased memory.