How to reference output rows with window functions? - postgresql

Question

Suppose I have a table with quantity column.
CREATE TABLE transfers (
user_id integer,
quantity integer,
created timestamp default now()
);
I'd like to iteratively go thru a partition using window functions, but access the output rows, not the input table rows.
To access the input table rows I could do something like this:
SELECT LAG(quantity, 1, 0)
OVER (PARTITION BY user_id ORDER BY created)
FROM transfers;
I need to access the previous output row to calculate the next output row. How can i access the lag row in the output? Something like:
CREATE VIEW balance AS
SELECT LAG(balance.total, 1, 0) + quantity AS total
OVER (PARTITION BY user_id ORDER BY created)
FROM transfers;
Edit
This is a minimal example to support the question of how to access the previous output row within a window partition. I don't actually want a sum.

Answer 1

It seems you attempt to calculate a running sum. Luckily that's just what Sum() window function does:
WITH transfers AS(
SELECT i, random()-0.3 AS quantity FROM generate_series(1,100) as i
)
SELECT i, quantity, sum(quantity) OVER (ORDER BY i) from transfers;

Answer 2

I guess, looking at the question, that the only you need is to calculate a cumulative sum.
To calculate a cumulative summ use this query:
SELECT *,
SUM( CASE WHEN quantity IS NULL THEN 0 ELSE quantity END)
OVER ( PARTITION BY user_id ORDER BY created
ROWS BETWEEN unbounded preceding AND current row
) As cumulative_sum
FROM transfers
ORDER BY user_id, created
;
But if you want more complex calculations, especially containing some conditions (decisions) that depend on a result from prevoius row, then you need a recursive approach.