How to enumerate groups of partitions in my Postgres table with window functions?

Question

Suppose I have a table like this:

id  | part  | value
----+-------+-------
 1  | 0     | 8
 2  | 0     | 3
 3  | 0     | 4
 4  | 1     | 6
 5  | 0     | 13
 6  | 0     | 4
 7  | 1     | 2
 8  | 0     | 11
 9  | 0     | 15
 10 | 0     | 3
 11 | 0     | 2

I would like to enumerate groups between rows that have part atribute 1.

So I would like to get this:

id  | part  | value | number
----+-------+-----------------
 1  | 0     | 8     |   1
 2  | 0     | 3     |   1
 3  | 0     | 4     |   1
 4  | 1     | 6     |   0
 5  | 0     | 13    |   2
 6  | 0     | 4     |   2
 7  | 1     | 2     |   0
 8  | 0     | 11    |   3
 9  | 0     | 15    |   3
 10 | 0     | 3     |   3
 11 | 0     | 2     |   3

Is it possible to achieve this with Postgres window functions or is there any other way?

Answer 1

You seem to want something like 1 more than the cumulative sum of the parts. The simplest method is:

select t.*,
       (case when part = 1 then 0  -- the easy case
             else 1 + sum(part) over (order by id)
        end) as number
from t;

If part can take on values other than 0 and 1:

select t.*,
       (case when part = 1 then 0  -- the easy case
             else 1 + sum( (part = 1)::int ) over (order by id)
        end) as number
from t;