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 categories (
  id int,
  name varchar(10)
);

insert into categories values
(1, 'A'),
(1, 'B'),
(1, 'C'),
(1, 'D'),
(2, 'A'),
(2, 'B'),
(2, 'E');

CREATE TABLE



INSERT 0 7


SELECT *, row_number() over (partition by id order by name) as rn
FROM categories


id
name
rn


1
A
1

1
B
2

1
C
3

1
D
4

2
A
1

2
B
2

2
E
3



SELECT 7


SELECT g.id, jsonb_agg (g.*) FILTER (WHERE g.name IS NOT NULL) AS categories
FROM (
  SELECT *, row_number() OVER (PARTITION BY id ORDER BY name) as rn
  FROM categories
) g
WHERE rn <=2
GROUP BY id


id
categories


1
[{"id": 1, "rn": 1, "name": "A"}, {"id": 1, "rn": 2, "name": "B"}]

2
[{"id": 2, "rn": 1, "name": "A"}, {"id": 2, "rn": 2, "name": "B"}]



SELECT 2

id	categories
1	[{"id": 1, "rn": 1, "name": "A"}, {"id": 1, "rn": 2, "name": "B"}]
2	[{"id": 2, "rn": 1, "name": "A"}, {"id": 2, "rn": 2, "name": "B"}]