Postgres 15

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

CREATE TABLE "order" (id serial primary key, customer_id int null);

CREATE TABLE

INSERT INTO "order" (customer_id) VALUES (NULL);

INSERT 0 1

SELECT id, COUNT(DISTINCT customer_id) FROM "order" GROUP BY "order"."id"

id
count

1
0

SELECT 1