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 users as
    select '2019-10' as start_month, '2019-10' as end_month, 1 as user_id union all
    select '2019-10' as start_month, '2019-11' as end_month, 2 as user_id union all
    select '2019-10' as start_month, '2019-12' as end_month, 3 as user_id union all
    select '2019-10' as start_month, '2019-12' as end_month, 4 as user_id union all
    select '2019-11' as start_month, '2019-11' as end_month, 5 as user_id union all
    select '2019-11' as start_month, '2019-12' as end_month, 6 as user_id union all
    select '2019-11' as start_month, '2019-12' as end_month, 7 as user_id union all
    select '2019-11' as start_month, '2020-01' as end_month, 8 as user_id union all
    select '2019-12' as start_month, '2019-12' as end_month, 9 as user_id union all
    select '2019-12' as start_month, '2020-01' as end_month, 10 as user_id union all
    select '2019-12' as start_month, NULL as end_month, 11 as user_id union all
    select '2019-12' as start_month, '2020-02' as end_month, 12 as user_id

SELECT 12

start_month	count	month_1	month_2
2019-10	4	3	2
2019-11	4	3	1
2019-12	4	3	2

SELECT 3