Necesita usar CTE recursivo:
with recursive
stock_temp as (
select
*,
row_number() over(partition by product_id order by row_num) as rn
from
stock_table
)
,cte as (
select
document_type, document_date,
product_id, qty_out, qty_in, price,
row_num, stock_balance, rn,
price as wac
from
stock_temp where document_type = 'SI'
union all
select
sub.document_type, sub.document_date,
sub.product_id, sub.qty_out, sub.qty_in, sub.price,
sub.row_num, sub.stock_balance, sub.rn,
case when sub.qty_in = 0 then main.wac else
((sub.stock_balance - sub.qty_in) * main.wac + sub.qty_in * sub.price)
/ ((sub.stock_balance - sub.qty_in) + sub.qty_in) end as wac
from
cte as main
join stock_temp as sub
on (main.product_id = sub.product_id and main.rn + 1 = sub.rn)
)
select * from cte
