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?.

drop table if exists meltuniverse;
create table meltuniverse(customer_id text);
insert into meltuniverse values
('N12300'),
('245007.00'); 

DROP TABLE

CREATE TABLE

INSERT 0 2

UPDATE meltuniverse 
SET customer_id = regexp_replace(customer_id, '\.00$', '')

UPDATE 2

select *
from meltuniverse 

customer_id

N12300

245007

SELECT 2