PostgreSQL 10 High Performance

Using Window functions for cumulatives

As a second example, consider the case where you're computing sales for each month orders were placed:

    SELECT EXTRACT(year FROM orderdate) AS year, EXTRACT(month FROM orderdate) AS month,sum(netamount) as sales FROM orders GROUP BY year,month ORDER BY year,month;
     year | month |   sales   
    ------+-------+-----------
     2004 |     1 | 199444.50
     2004 |     2 | 200269.86
     2004 |     3 | 194516.68
     2004 |     4 | 200131.18
     2004 |     5 | 197131.77
     2004 |     6 | 199919.21
     2004 |     7 | 190953.70
     2004 |     8 | 194102.55
     2004 |     9 | 196798.83
     2004 |    10 | 195798.80
     2004 |    11 | 199882.86
     2004 |    12 | 202769.80
  

What if you wanted a cumulative total at each point during the year? As that problem also requires referencing things that are ...