Postgres 11

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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 vanities as 
    select 'v2' as version union all
    select 'v3'

SELECT 2

select *
from vanities

version

v2

v3

SELECT 2

alter table vanities alter column version TYPE INTEGER USING (substr(version, 2)::integer);

ALTER TABLE

select *
from vanities

version

2

3

SELECT 2