Postgres 12

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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?.

create table my_table
(
  loval INTEGER ,
  hival INTEGER ,
  valcount INTEGER ,
  vals INTEGER[]
);
insert into my_table(loval, hival, valcount)
values
(1,10,9),
(3,22,6),
(2,42,8);
3 rows affected

update my_table
  set vals = array(select (random() * (hival - loval) + loval)::int 
             from generate_series(1, valcount));
3 rows affected

select *
from my_table;


loval
hival
valcount
vals


1
10
9
{9,4,3,9,5,6,4,3,2}

3
22
6
{21,6,19,20,9,9}

2
42
8
{7,15,27,5,11,22,2,10}

loval	hival	valcount	vals
1	10	9	{9,4,3,9,5,6,4,3,2}
3	22	6	{21,6,19,20,9,9}
2	42	8	{7,15,27,5,11,22,2,10}