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 A (
    x INT,
    y INT,
    UNIQUE(x, y)
);
INSERT INTO A (x, y) VALUES (1, 1), (1, null), (1, null);
3 rows affected

SELECT * FROM A;


x
y


1
1

1
null

1
null

x	y
1	1
1	null
1	null