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?.

select version();

version

PostgreSQL 12.3 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.3.1 20191121 (Red Hat 8.3.1-5), 64-bit

CREATE TABLE people
    (
        id              bigserial,
        timestamp       timestamptz DEFAULT now() NOT NULL,
        PRIMARY KEY     (id)
    );

INSERT INTO people (timestamp) VALUES (NOW()) RETURNING id;

id

1

SELECT * FROM people;

id
timestamp

1
2020-08-03 13:45:04.844899+01

--
-- @a_horse_with_no_name's solutions are below
--
INSERT INTO people DEFAULT VALUES RETURNING id;

id

2

SELECT * FROM people;

id
timestamp

1
2020-08-03 13:45:04.844899+01

2
2020-08-03 13:45:04.849085+01

INSERT INTO people (timestamp) VALUES (DEFAULT) RETURNING id;

id

3

SELECT * FROM people;

id
timestamp

1
2020-08-03 13:45:04.844899+01

2
2020-08-03 13:45:04.849085+01

3
2020-08-03 13:45:04.850986+01

id	timestamp
1	2020-08-03 13:45:04.844899+01
2	2020-08-03 13:45:04.849085+01

id	timestamp
1	2020-08-03 13:45:04.844899+01
2	2020-08-03 13:45:04.849085+01
3	2020-08-03 13:45:04.850986+01