Keep the sum even on days without revenue in cumulative sum when using window function in Presto

So my problem is that I have sales data from, say for the sake of clarity, 3 different products. I will be selling 10 of these a week and I want to visualize them in a cumulative sum. I have been using the following little snippet to get the cumulative sum of the revenue.

SUM(revenue) OVER (PARTITION BY purchase_date, product ORDER BY day) AS cumulative_revenue

However, this is not sufficient since I have the product in the window function. It works only as long as I have sales data on every single day of the week for each product. If I sell product_1 four pieces a week, the result from the query will show cumulative revenue only from those days. So this means that if I have all sales of that product on Monday to Wednesday, the rest of the week won’t have them listed in the output. This causes problems if I try to visualize the data by stacking the results as the rest of the week will have lower cumulative revenue that the beginning of the week.

So what I want is to get it to show 0 on revenue for all products on all days of the week. I can of course do this with some cross join magic, but it is sloooow since I have quite a lot of rows, so is there a way to do it with a window function?

My data looks a bit like this

purchase_date|product|buyer|revenue
-----------------------------------
12/12/2020   | pr_1  |  a  | 100.0
12/12/2020   | pr_2  |  b  | 200.0
13/12/2020   | pr_1  |  d  | 100.0
14/12/2020   | pr_1  |  t  | 100.0
...

Answer

You can generate a row for all product/date combinations by using a cross join and then a left join. I suspect you want something like this:

select p.product, d.purchase_date,
       sum(t.revenue) as revenue_on_date,
       sum(sum(t.revenue)) over (partition by product order by d.purchase_date) as cumulative_revenue
from (select distinct product from t) p cross join
     (select distinct purchase_date from t) d left join
     t
     on p.product = t.product and
        d.purchase_date = t.purchase_date
group by p.product, p.purchase_date;

Note: This assumes that there is at least one purchase on each day. Otherwise, you might need another source for all dates in the range you care about.