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 mytable (id text primary key, name text, email text);

insert into mytable values 
	('userid_1', 'A', 'A@gmail.com'),
	('userid_2', 'B', 'B@gmail.com')
;

CREATE TABLE

INSERT 0 2

select * from mytable;

id
name
email

userid_1
A
A@gmail.com

userid_2
B
B@gmail.com

SELECT 2

select jsonb_object_agg(id, to_jsonb(t) - 'id') res
from mytable t

res

{"userid_1": {"name": "A", "email": "A@gmail.com"}, "userid_2": {"name": "B", "email": "B@gmail.com"}}

SELECT 1