Postgres 16

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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?.

select version();

version

PostgreSQL 16.0 (Debian 16.0-1.pgdg120+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit

SELECT 1

CREATE TABLE demo(
  "order" varchar(100),
  "step" int,
  "status" varchar(100)
)
;

CREATE TABLE

insert into demo values('order_1', 1 , 'done');
insert into demo values('order_1', 2 , 'done');
insert into demo values('order_1', 3 , 'done');
insert into demo values('order_1', 4 , null);

insert into demo values('order_2', 1 , 'done');
insert into demo values('order_2', 2 , null);
insert into demo values('order_2', 3 , null);

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

INSERT 0 1

select * from demo;

order
step
status

order_1
1
done

order_1
2
done

order_1
3
done

order_1
4
null

order_2
1
done

order_2
2
null

order_2
3
null

SELECT 7

order	step	status
order_1	1	done
order_1	2	done
order_1	3	done
order_1	4	null
order_2	1	done
order_2	2	null
order_2	3	null