Unable to reset partition groups (Window functions and PostgreSQL specifically)

Question

I have a simple dataset like this:

SELECT UNNEST(ARRAY['A', 'A', 'A', 'B', 'B', 'A', 'C', 'B']) AS customer_name, generate_series(8, 1, -1) AS order_time;

+-------------+------------+
| customer_id | order_time |
+-------------+------------+
| "A"         | 8          |
+-------------+------------+
| "A"         | 7          |
+-------------+------------+
| "A"         | 6          |
+-------------+------------+
| "B"         | 5          |
+-------------+------------+
| "B"         | 4          |
+-------------+------------+
| "A"         | 3          |
+-------------+------------+
| "C"         | 2          |
+-------------+------------+
| "B"         | 1          |
+-------------+------------+

I am looking for one row:

+-------------+------------+
| customer_id | order_time |
+-------------+------------+
| "A"         | 6          |
+-------------+------------+

Which is to say, I want to get the first order_time of the latest (consecutive) customer_id. With the following SQL, I only ever obtain "3" as order_time from customer_id A. I can't seem to "reset" the partition.

SELECT customer_name, LAST_VALUE(order_time) OVER W
FROM
(
  SELECT UNNEST(ARRAY['A', 'A', 'A', 'B', 'B', 'A', 'C', 'B']) AS customer_name, generate_series(8, 1, -1) AS order_time
) X
WINDOW W AS (PARTITION BY customer_name ORDER BY order_time DESC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)
ORDER BY order_time DESC
LIMIT 1;

Using PostgreSQL 11.5

Answer 1

You can use difference between

ROW_NUMBER() OVER (ORDER BY order_time DESC) and

ROW_NUMBER() OVER (PARTITION BY customer_name ORDER BY order_time DESC) to provide grouping for gaps-and-islands structure :

SELECT XX.customer_name, LAST_VALUE(order_time) OVER W FROM
(
 SELECT X.*, ROW_NUMBER() OVER (ORDER BY order_time DESC)-
             ROW_NUMBER() OVER (PARTITION BY customer_name ORDER BY order_time DESC) 
             AS rn

   FROM
   (
     SELECT UNNEST(ARRAY['A', 'A', 'A', 'B', 'B', 'A', 'C', 'B']) AS customer_name, 
            generate_series(8, 1, -1) AS order_time
    ) X
 ) XX
WINDOW W AS (PARTITION BY rn ORDER BY order_time DESC 
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)
LIMIT 1; 

Demo