Re: Window function for get the last value to extend missing rows - Mailing list pgsql-general

>>>>> "Durumdara" == Durumdara  <durumdara@gmail.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)