Postgres 15

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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?.

DROP TABLE IF EXISTS data;
CREATE TEMP TABLE data (
	id INT PRIMARY KEY,
	number INT,
	string TEXT
);
INSERT INTO data(id,number,string)
VALUES (1,1,'Apple'),(2,1,'Accordion'),(3,2,'Banana'),(4,2,'Banjo'),(5,NULL,'Whatever');

DROP TABLE

CREATE TABLE

INSERT 0 5

SELECT * FROM data WHERE number IN(1,2,NULL);

id
number
string

1
1
Apple

2
1
Accordion

3
2
Banana

4
2
Banjo

SELECT 4

SELECT * FROM data WHERE number NOT IN(1,2,NULL);

id
number
string

SELECT 0