无法重置分区组（特别是 Window 函数和 PostgreSQL）

Question

我有一个像这样的简单数据集：

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          |
+-------------+------------+

我正在寻找一排：

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

也就是说，我想获得最新（连续）customer_id 中的第一个order_time。使用以下 SQL，我只能从 customer_id A 获得“3”作为 order_time。我似乎无法“重置”分区。

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;

使用 PostgreSQL 11.5

Answer 1

你可以使用区别

ROW_NUMBER() OVER (ORDER BY order_time DESC) 和

ROW_NUMBER() OVER (PARTITION BY customer_name ORDER BY order_time DESC) 为gaps-and-islands 结构提供分组：

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