MySQL to Postgres migration

mysql to postgres.sql

In MYSQL I can do something like this:
  SELECT activities.*, @num := if(@item = CONCAT(item_type, item_id), @num + 1, 1) AS row_number, @item := CONCAT(item_type, item_id) AS item FROM activities

And that gives me an output like this: 
  item    |   row_number
  --------|-------------
  Idea1   |   1
  Idea1   |   2
  Idea1   |   3
  Idea2   |   1
  Idea2   |   2
  Idea3   |   1
  ...     |   ...
          
Now how should I do something like this in postgres?
I Have tried stuff like this:

CREATE TEMP TABLE item AS SELECT concat(activities.item_type, activities.item_id) as item FROM activities ORDER BY item DESC;
SELECT *, row_number() over () from item
Which returns: 

Idea5	  1
Idea5	  2
Idea5	  3
Idea5	  4
Idea43	5 <--! I want to start counting from 1 here :)  
Idea43	6
Idea43	7

I want to count the number of occurrences that I have already seen:
My input looks something like this:
Idea5 |
Idea5 |
Idea5 |
Idea5 |
Idea43 |
Idea43 |
Idea43 |
Idea43 |
Idea43 |
Idea42 |
Idea42 |
Idea42 |
Idea42 |
Idea41 |
Idea41 |
Idea41 |

I want to transform it to this:
Idea5 | 1
Idea5 | 2
Idea5 | 3
Idea5 | 4
Idea43 | 1
Idea43 | 2
Idea43 | 3
Idea43 | 4
Idea43 | 5
Idea42 | 1
Idea42 | 2
Idea42 | 3
Idea42 | 4
Idea41 | 1
Idea41 | 2
Idea41 | 3