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?.

CREATE TABLE parent (
    id          TEXT PRIMARY KEY,
    name        TEXT,
    created_at  TIMESTAMPTZ DEFAULT NOW() NOT NULL,
    updated_at  TIMESTAMPTZ DEFAULT NOW() NOT NULL
);
INSERT INTO parent (id, name) VALUES (1, 'name 1'), (2, 'name 2');

CREATE TABLE

INSERT 0 2

CREATE TABLE child_n (
    id        BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
    parent_id TEXT NOT NULL REFERENCES parent (id) ON DELETE CASCADE
);
INSERT INTO child_n (id, parent_id) VALUES (1,1), (2,2), (3,1), (4,2);

CREATE TABLE

INSERT 0 4

SELECT * FROM parent;
SELECT * FROM child_n;

id
name
created_at
updated_at

1
name 1
2023-12-18 12:43:14.303489+00
2023-12-18 12:43:14.303489+00

2
name 2
2023-12-18 12:43:14.303489+00
2023-12-18 12:43:14.303489+00

SELECT 2

id
parent_id

1
1

2
2

3
1

4
2

SELECT 4

WITH cte AS ( DELETE
              FROM parent
              WHERE id = '1'
              RETURNING * )
INSERT INTO parent
SELECT *
FROM cte;

INSERT 0 1

SELECT * FROM parent;
SELECT * FROM child_n;

id
name
created_at
updated_at

2
name 2
2023-12-18 12:43:14.303489+00
2023-12-18 12:43:14.303489+00

1
name 1
2023-12-18 12:43:14.303489+00
2023-12-18 12:43:14.303489+00

SELECT 2

id
parent_id

2
2

4
2

SELECT 2

id	name	created_at	updated_at
1	name 1	2023-12-18 12:43:14.303489+00	2023-12-18 12:43:14.303489+00
2	name 2	2023-12-18 12:43:14.303489+00	2023-12-18 12:43:14.303489+00