redshift: how to find row_number after grouping and aggregating?

Suppose I have a table of customer purchases ("my_table") like this:

--------------------------------------
customerid | date_of_purchase | price
-----------|------------------|-------
     1     |  2019-09-20      | 20.23
     2     |  2019-09-21      | 1.99
     1     |  2019-09-21      | 123.34
 ...

I'd like to be able to find the nth highest spending customer in this table (say n = 5). So I tried this:

with cte as (
  select customerid, sum(price) as total_pay, 
    row_number() over (partition by customerid order by total_pay desc) as rn 
 from my_table group by customerid order by total_pay desc)
select * from cte where rn = 5;

But this gives me nonsense results. For some reason rn doesn't seem to be unique (for example there are a bunch of customers with rn = 1). I don't understand why. Isn't rn supposed to be just a row number?

Solution

Remove the partition by in the definition of row_number():

with cte as (
      select customerid, sum(price) as total_pay, 
            row_number() over (order by total_pay desc) as rn 
      from my_table
      group by customerid
     )
select *
from cte
where rn = 5;

You are already aggregating by customerid, so each customer has only one row. So the value of rn will always be 1.