Multi update position based on ROW_NUMBER()

Question

it might be a silly question but I am struggling with postgres update. I have following table:

id | tableX_id| position |
---+----------+---------+
 1 |    10    |         |    
 2 |    10    |         |    
 3 |    10    |         |    
 4 |    10    |         |    
 5 |    10    |         |    
 6 |    11    |         |    
 7 |    11    |         |    
 8 |    12    |         |    

I need to update position like this:

    id | tableX_id| position |
    ---+----------+---------+
     1 |    10    |    1    |    
     2 |    10    |    2    |    
     3 |    10    |    3    |    
     4 |    10    |    4    |    
     5 |    10    |    5    |    
     6 |    11    |    1    |    
     7 |    11    |    2    |    
     8 |    12    |    1    |  

I have following update that doesnt work(update all position to 1):

UPDATE tableY y 
SET position = subquery.pos 
FROM (
  SELECT ROW_NUMBER() OVER() as pos 
  FROM tableY y2 
  JOIN tableX x on x.id = y2.tableX_id
) as subquery

Answer 1

add where subquery.id = tableY.id , as below:

t=# update x set position = pos
from (select *,ROW_NUMBER() OVER(partition by x order by id) as pos FROM x) sub
where x.id = sub.id;
UPDATE 8
Time: 10.015 ms
t=# select * from x;
 id | x  | position
----+----+----------
  1 | 10 |        1
  2 | 10 |        2
  3 | 10 |        3
  4 | 10 |        4
  5 | 10 |        5
  6 | 11 |        1
  7 | 11 |        2
  8 | 12 |        1
(8 rows)