Window functions and more “local” aggregation

Question

Suppose I have this table:

select * from window_test;

 k | v
---+---
 a | 1
 a | 2
 b | 3
 a | 4

Ultimately I want to get:

Answer 1

EDIT: I've came up with the following query — without window functions at all:

WITH RECURSIVE tree AS (
  SELECT k, v, ''::text as next_k, 0 as next_v, 0 AS level FROM window_test
  UNION ALL
  SELECT c.k, c.v, t.k, t.v + level, t.level + 1
    FROM tree t JOIN window_test c ON c.k = t.k AND c.v + 1 = t.v),
partitions AS (
  SELECT t.k, t.v, t.next_k,
         coalesce(nullif(t.next_v, 0), t.v) AS next_v, t.level
    FROM tree t
   WHERE NOT EXISTS (SELECT 1 FROM tree WHERE next_k = t.k AND next_v = t.v))
SELECT min(k) AS k, v AS min_v, max(next_v) AS max_v
  FROM partitions p
 GROUP BY v
 ORDER BY 2;

I've provided 2 working queries now, I hope one of them will suite you.

SQL Fiddle for this variant.

---

Another way how to achieve this is to use a support sequence.

1. Create a support sequence:

   CREATE SEQUENCE wt_rank START WITH 1;
   

2. The query:

   WITH source AS (
     SELECT k, v,
            coalesce(lag(k) OVER (ORDER BY v), k) AS prev_k
       FROM window_test
       CROSS JOIN (SELECT setval('wt_rank', 1)) AS ri),
   ranking AS (
     SELECT k, v, prev_k,
            CASE WHEN k = prev_k THEN currval('wt_rank')
                 ELSE nextval('wt_rank') END AS rank
       FROM source)
   SELECT r.k, min(s.v) AS min_v, max(s.v) AS max_v
       FROM ranking r
       JOIN source s ON r.v = s.v
      GROUP BY r.rank, r.k
      ORDER BY 2;