Postgres 8.4

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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 Tbl1
(ID int, Person varchar(5), Date_ varchar(10), Where_ varchar(6));

INSERT INTO Tbl1
(ID, Person, Date_, Where_)
VALUES
(1, 'Laura', '17/10/2019', 'School'),
(2, 'Laura', '18/10/2019', 'School'),
(3, 'Laura', '19/10/2019', 'School'),
(4, 'Laura', '19/10/2019', 'Park'),
(5, 'Laura', '20/10/2019', 'Home'),
(6, 'Laura', '21/10/2019', 'School'),
(7, 'Laura', '21/10/2019', 'School'),
(8, 'Laura', '21/10/2019', 'Home'),
(9, 'Tom', '17/10/2019', 'Home'),
(10, 'Tom', '18/10/2019', 'Home'),
(11, 'Tom', '18/10/2019', 'Home'),
(12, 'Tom', '19/10/2019', 'Home'),
(13, 'Tom', '19/10/2019', 'School'),
(14, 'Tom', '20/10/2019', 'Home');
14 rows affected

select person, date_, max(where_) from tbl1
group by person, date_
having count(distinct where_) < 2


person
date_
max


Laura
17/10/2019
School

Laura
18/10/2019
School

Laura
20/10/2019
Home

Tom
17/10/2019
Home

Tom
18/10/2019
Home

Tom
20/10/2019
Home

person	date_	max
Laura	17/10/2019	School
Laura	18/10/2019	School
Laura	20/10/2019	Home
Tom	17/10/2019	Home
Tom	18/10/2019	Home
Tom	20/10/2019	Home