Postgres 9.6

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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 talent_speciality (
    userid int,
    speciality int
);

INSERT INTO talent_speciality VALUES
(840, 1), (840, 2), (840, 3), (42, 42);

SELECT * FROM talent_speciality;

userid
speciality

840
1

840
2

840
3

42
42

SELECT 4

-- json array
SELECT json_agg(speciality) 
FROM talent_speciality
WHERE userid = 840

json_agg

[1, 2, 3]

SELECT 1

-- simple array
SELECT array_agg(speciality) 
FROM talent_speciality
WHERE userid = 840

array_agg

{1,2,3}

SELECT 1