Re: Window function for get the last value to extend missing rows

>>>>> "Durumdara" == Durumdara <durumdara(at)gmail(dot)com> writes:

Durumdara> I have to make a virtual table which is minute based.

Durumdara> I thought I would make a generated temp table
Durumdara> (generate_series) and then join these values based on minue.

Durumdara> 03:00 10
Durumdara> 03:01 NULL
Durumdara> 03:02 12
Durumdara> 03:03 11
Durumdara> 03:04 NULL
Durumdara> 03:05 13

Durumdara> I need a code to replace the value to the last value on
Durumdara> NULL.

It is possible to do this, even without the IGNORE NULLS option for
window functions, but it's somewhat awkward.

Someone else already mentioned using generate_series in lateral position
to fill in values; in most cases this is probably the best approach,
though it requires a little care.

There is also a window function approach based on using the non-null
values to delimit partitions:

create table tmp_test_table(mmin,val)
as select o, v
from unnest(array[1,5,NULL,3,NULL,NULL,10,7,NULL,NULL,NULL,4])
with ordinality as u(v,o);
select * from tmp_test_table order by mmin;
mmin | val
------+-----
1 | 1
2 | 5
3 |
4 | 3
5 |
6 |
7 | 10
8 | 7
9 |
10 |
11 |
12 | 4
(12 rows)

First we take advantage of the fact that mmin is increasing to generate
a distinguishing value for each block of nulls:

select *,
max(case when val is not null then mmin end)
over (order by mmin) as grp
from tmp_test_table
order by mmin;
mmin | val | grp
------+-----+-----
1 | 1 | 1
2 | 5 | 2
3 | | 2
4 | 3 | 4
5 | | 4
6 | | 4
7 | 10 | 7
8 | 7 | 8
9 | | 8
10 | | 8
11 | | 8
12 | 4 | 12
(12 rows)

Then we can fill in the missing vals by using the fact that there is
at most one non-null val in each group:

select *,
max(val) over (partition by grp) as val2
from (select *,
max(case when val is not null then mmin end)
over (order by mmin) as grp
from tmp_test_table) s
order by mmin;
mmin | val | grp | val2
------+-----+-----+------
1 | 1 | 1 | 1
2 | 5 | 2 | 5
3 | | 2 | 5
4 | 3 | 4 | 3
5 | | 4 | 3
6 | | 4 | 3
7 | 10 | 7 | 10
8 | 7 | 8 | 7
9 | | 8 | 7
10 | | 8 | 7
11 | | 8 | 7
12 | 4 | 12 | 4
(12 rows)

This _really_ isn't efficient, though; you end up with typically three
sorts of the data. For a one-off operation or for generating a
materialized view it might be acceptable.

--
Andrew (irc:RhodiumToad)