I have nothing else to say than thanks for the writeup. And you are most likely correct. I think what I (and OP) wanted to say was that there should be a stag

I have nothing else to say than thanks for the writeup. And you are most likely correct. I think what I (and OP) wanted to say was that there should be a stage where you store the intermediate results of the aggregation as well. Unless you have that in an index already. And that also costs. But you may be right in that it is possibly in the scale of a million so not a huge deal.