How I was introduced to SQL window function

I was called at my firm for having to create a analytics dashboard for a multiTenet CRM dashboard where the magnitude of rows were in thousands and the efficiency i could not blame postgress or servers I had to figure out how to index, query and cache the data. The business questions were something like this Give the sales agents aggregated sales data from the month of January, till the current month in waterfall structure so that the numbers are aggregated in that order. Ranking the sales agent