Postgres - Using window function in grouped rows - postgresql

Question

According to the Postgres Doc at https://www.postgresql.org/docs/9.4/queries-table-expressions.html#QUERIES-WINDOW it states
If the query contains any window functions (...), these functions are evaluated after any grouping, aggregation, and HAVING filtering is performed. That is, if the query uses any aggregates, GROUP BY, or HAVING, then the rows seen by the window functions are the group rows instead of the original table rows from FROM/WHERE.
I didn't get the concept of " then the rows seen by the window functions are the group rows instead of the original table rows from FROM/WHERE". Allow me to use an example to explain my doubt:
Using this ready to run example below
with cte as (
select 1 as primary_id, 1 as foreign_id, 10 as begins
union
select 2 as primary_id, 1 as foreign_id, 20 as begins
union
select 3 as primary_id, 1 as foreign_id, 30 as begins
union
select 4 as primary_id, 2 as foreign_id, 40 as begins
)
select foreign_id, count(*) over () as window_rows_count, count(*) as grouped_rows_count
from cte
group by foreign_id
You may notice that the result is
So if "the rows seen by the window functions are the group rows".. then ¿why window_rows_count is returning a different value from grouped_rows_count?

Answer 1

If you remove the window function from the query:
select foreign_id, count(*) as grouped_rows_count
from cte
group by foreign_id
the result, as expected is this:
> foreign_id | grouped_rows_count
> ---------: | -----------------:
> 1 | 3
> 2 | 1
and on this result, which is 2 rows, if you also apply the window function count(*) over(), it will return 2, because it counts all the rows of the resultset since the over clause is empty, without any partition.

Answer 2

You should follow the last comment on your post.
And for more analysis, you may process the following query :
with cte as (
select 1 as primary_id, 1 as foreign_id, 10 as begins
union
select 2 as primary_id, 1 as foreign_id, 20 as begins
union
select 3 as primary_id, 1 as foreign_id, 30 as begins
union
select 4 as primary_id, 2 as foreign_id, 40 as begins
)
select foreign_id, count(*) over (PARTITION BY foreign_id) as window_rows_count, count(*) as grouped_rows_count
from cte
group by foreign_id ;
You'll see this time that you are getting 1 row for each foreign id.
Checkout the documentation on postgres at this url :
https://www.postgresql.org/docs/13/tutorial-window.html
The window function is applied to the whole set obtained by the former query.