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?.

CREATE TABLE tablename (
  id INT GENERATED ALWAYS AS IDENTITY,
  Data VARCHAR(5),
  Date DATE,
  Time TIME
);

INSERT INTO tablename
  (Data, Date, Time)
VALUES
  ('Data1', '2020-08-19', '13:00:00'),
  ('Data1', '2020-08-19', '13:30:00'),
  ('Data1', '2020-08-19', '14:00:00'),
  ('Data1', '2020-08-21', '07:00:00'),
  ('Data1', '2020-08-21', '07:30:00'),
  ('Data2', '2020-08-20', '08:00:00'),
  ('Data2', '2020-08-20', '08:30:00'),
  ('Data1', '2020-08-19', '13:00:00'),
  ('Data1', '2020-08-19', '13:30:00'),
  ('Data1', '2020-08-19', '14:00:00'),
  ('Data1', '2020-08-21', '07:00:00'),
  ('Data1', '2020-08-21', '07:30:00'),
  ('Data2', '2020-08-20', '08:00:00'),
  ('Data2', '2020-08-20', '08:30:00'),
  ('Data1', '2020-08-19', '13:00:00'),
  ('Data1', '2020-08-19', '13:30:00'),
  ('Data1', '2020-08-19', '14:00:00'),
  ('Data1', '2020-08-21', '07:00:00'),
  ('Data1', '2020-08-21', '07:30:00'),
  ('Data2', '2020-08-20', '08:00:00'),
  ('Data2', '2020-08-20', '08:30:00'),
  ('Data1', '2020-08-19', '13:00:00'),
  ('Data1', '2020-08-19', '13:30:00'),
  ('Data1', '2020-08-19', '14:00:00'),
  ('Data1', '2020-08-21', '07:00:00'),
  ('Data1', '2020-08-21', '07:30:00'),
  ('Data2', '2020-08-20', '08:00:00'),
  ('Data2', '2020-08-20', '08:30:00');

28 rows affected

id	data	date	time	percentile
18	Data1	2020-08-21	07:00:00	1
10	Data1	2020-08-19	14:00:00	1
3	Data1	2020-08-19	14:00:00	1