Postgres query that implements the "last non-null value"

gistfile1.txt

steelwheels=# select a1_h0_l0
steelwheels-#         , a1_h0_l1
steelwheels-#         , a1_h0_l2
steelwheels-#         , a0_h0_l0
steelwheels-#         , a0_h0_l1
steelwheels-#         , m0
steelwheels-#         , first_value(m0)
steelwheels-#             over (
steelwheels(#                 partition by p0
steelwheels(#                 order by a1_h0_l0
steelwheels(#                     , a1_h0_l1
steelwheels(#                     , a1_h0_l2
steelwheels(#                     , a0_h0_l0
steelwheels(#                     , a0_h0_l1)
steelwheels-#             as m1
steelwheels-#         , p0
steelwheels-# from (
steelwheels(#     select a1_h0_l0
steelwheels(#             , a1_h0_l1
steelwheels(#             , a1_h0_l2
steelwheels(#             , a0_h0_l0
steelwheels(#             , a0_h0_l1
steelwheels(#             , m0
steelwheels(#             , sum(case when m0 is null then 0 else 1 end)
steelwheels(#                 over ( order by a1_h0_l0 , a1_h0_l1 , a1_h0_l2 , a0_h0_l0 , a0_h0_l1 )
steelwheels(#                 as p0
steelwheels(#     from (
steelwheels(#         select d0.year_id as a1_h0_l0
steelwheels(#                 , d0.qtr_name as a1_h0_l1
steelwheels(#                 , d0.month_name as a1_h0_l2
steelwheels(#                 , d1.territory as a0_h0_l0
steelwheels(#                 , d1.country as a0_h0_l1
steelwheels(#                 , sum(case when d1.country is null then null else orderfact.quantityordered end) as m0
steelwheels(#         from orderfact
steelwheels(#         join (select time_id, year_id, qtr_id, qtr_name, month_id, month_name
steelwheels(#               from dim_time where year_id = 2003
steelwheels(#             ) d0 on orderfact.time_id = d0.time_id
steelwheels(#         full outer join (
steelwheels(#             select customernumber, territory, country
steelwheels(#             from customer_w_ter where territory = 'APAC' and country = 'Australia'
steelwheels(#         ) d1 on orderfact.customernumber = d1.customernumber
steelwheels(#         group by a1_h0_l0
steelwheels(#                 , d0.qtr_id
steelwheels(#                 , a1_h0_l1
steelwheels(#                 , d0.month_id
steelwheels(#                 , a1_h0_l2
steelwheels(#                 , a0_h0_l0
steelwheels(#                 , a0_h0_l1
steelwheels(#         order by d0.year_id
steelwheels(#                 , d0.qtr_id
steelwheels(#                 , d0.month_id
steelwheels(#                 , a0_h0_l0
steelwheels(#                 , a0_h0_l1
steelwheels(#     ) as meas
steelwheels(# ) part;
 a1_h0_l0 | a1_h0_l1 | a1_h0_l2 | a0_h0_l0 | a0_h0_l1  |  m0  |  m1  | p0 
----------+----------+----------+----------+-----------+------+------+----
     2003 | QTR1     | Feb      |          |           |      |      |  0
     2003 | QTR1     | Jan      |          |           |      |      |  0
     2003 | QTR1     | Mar      |          |           |      |      |  0
     2003 | QTR2     | Apr      | APAC     | Australia |  525 |  525 |  1
     2003 | QTR2     | Apr      |          |           |      |  525 |  1
     2003 | QTR2     | Jun      |          |           |      |  525 |  1
     2003 | QTR2     | May      | APAC     | Australia |   66 |   66 |  2
     2003 | QTR2     | May      |          |           |      |   66 |  2
     2003 | QTR3     | Aug      |          |           |      |   66 |  2
     2003 | QTR3     | Jul      | APAC     | Australia |  266 |  266 |  3
     2003 | QTR3     | Jul      |          |           |      |  266 |  3
     2003 | QTR3     | Sep      | APAC     | Australia |  546 |  546 |  4
     2003 | QTR3     | Sep      |          |           |      |  546 |  4
     2003 | QTR4     | Dec      |          |           |      |  546 |  4
     2003 | QTR4     | Nov      | APAC     | Australia | 1111 | 1111 |  5
     2003 | QTR4     | Nov      |          |           |      | 1111 |  5
     2003 | QTR4     | Oct      |          |           |      | 1111 |  5