how to number distinct values while respecting their original ordering? - postgresql

Question

Here's my input data:
CREATE TEMP TABLE test AS SELECT * FROM (VALUES
(1, 12),
(2, 7),
(3, 8),
(4, 8),
(5, 7)
) AS rows (position, value);
I want to, in a single query (no subqueries or CTEs), assign a unique number for each distinct value. However, I also want those numbers to ascend according to the associated position -- i.e., a distinct value's number should be assigned according to its lowest position.
Assumptions:
each row will always have a unique position
value is not guaranteed unique per row
the number of a distinct value is only for ordinal purposes, e.g. it doesn't matter whether distinct_values goes 1-2-3 or 3-8-14
The desired output is:
position | value | distinct_value
----------+-------+----------------
1 | 12 | 1
2 | 7 | 2
3 | 8 | 3
4 | 8 | 3
5 | 7 | 2
I can get close using DENSE_RANK to number distinct values:
SELECT
position,
value,
DENSE_RANK() OVER (ORDER BY value) AS distinct_value
FROM test ORDER BY position;
The result obviously ignores position:
position | value | distinct_value
----------+-------+----------------
1 | 12 | 3
2 | 7 | 1
3 | 8 | 2
4 | 8 | 2
5 | 7 | 1
Is there a better window function for this?

Answer 1

with
t(x,y) as (values
(1, 12),
(2, 7),
(3, 8),
(4, 8),
(5, 7)),
pos(i,y) as (select min(x), y from t group by y),
ind(i,y) as (select row_number() over(order by i), y from pos)
select * from ind join t using(y) order by x;