Postgres 10

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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 tbl(id serial, size integer);

CREATE TABLE

-- insert 10 random values
insert into tbl (size)
select 
   (random()*200000)::int AS id 
from  
   generate_series (1,10) AS x(n);

INSERT 0 10

select * from tbl;

id
size

1
133668

2
103285

3
139021

4
81445

5
16058

6
88128

7
129225

8
108070

9
87526

10
14765

SELECT 10

-- change size data type
alter table tbl
    alter column "size" set data type varchar(20) using size::varchar(20);

ALTER TABLE

select * from tbl;

id
size

1
133668

2
103285

3
139021

4
81445

5
16058

6
88128

7
129225

8
108070

9
87526

10
14765

SELECT 10

id	size
1	133668
2	103285
3	139021
4	81445
5	16058
6	88128
7	129225
8	108070
9	87526
10	14765