Postgres 11

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

3 rows affected

dt	total
2019-07-01 11:00:00	161
2019-07-01 11:15:00	163
2019-07-01 12:00:00	170

select d.dt, seqnum,
min(t.total) over (order by d.dt desc), 
                  max(t.total) over (order by d.dt asc),
       max(seqnum) filter (where t.total is null) over (order by d.dt asc),
                         min(seqnum) filter (where t.total is null) over (order by d.dt desc),
                        
       coalesce(t.total,
                (max(t.total) over (order by d.dt asc) +
                 (min(t.total) over (order by d.dt desc) - 
                  max(t.total) over (order by d.dt asc)
                 ) *
                 (seqnum - max(seqnum) filter (where t.total is not null) over (order by d.dt asc)) /
                  nullif(min(seqnum) filter (where t.total is not null) over (order by d.dt desc) -
                         max(seqnum) filter (where t.total is not null) over (order by d.dt asc),
                         0
                        )
                 )
                )
from (select dt, count(*) over (order by dt) as seqnum
      from (select generate_series(min(dt), max(dt), interval '15 minute') as dt
            from t
            ) d
     ) d left join
     t
     on t.dt = d.dt;

dt	seqnum	min	max	max	min	coalesce
2019-07-01 11:00:00	1	161	161	null	3	161
2019-07-01 11:15:00	2	163	163	null	3	163
2019-07-01 11:30:00	3	170	163	3	3	165
2019-07-01 11:45:00	4	170	163	4	4	167
2019-07-01 12:00:00	5	170	170	4	null	170