Postgres 13

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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 my_table as
       select 'Chemistry' as subject, '2021-02-04'::date as attendance_date, 'Chris' as student union all
       select 'Chemistry' as subject, '2021-02-04'::date as attendance_date, 'Joseph' as student union all
       select 'English  ' as subject, '2021-02-04'::date as attendance_date, 'Chris' as student union all
       select 'English  ' as subject, '2021-02-04'::date as attendance_date, 'Joseph' as student union all
       select 'Chemistry' as subject, '2021-02-05'::date as attendance_date, 'Chris' as student union all
       select 'Chemistry' as subject, '2021-02-05'::date as attendance_date, 'Joseph' as student union all
       select 'English  ' as subject, '2021-02-05'::date as attendance_date, 'Joseph' as student union all
       select 'Chemistry' as subject, '2021-02-07'::date as attendance_date, 'Chris' as student union all
       select 'Chemistry' as subject, '2021-02-07'::date as attendance_date, 'Joseph' as student union all
       select 'English  ' as subject, '2021-02-07'::date as attendance_date, 'Chris' as student union all
       select 'English  ' as subject, '2021-02-07'::date as attendance_date, 'Joseph' as student

11 rows affected

date	subject	student	attended_flag
2021-02-04	English	Chris	t
2021-02-07	English	Chris	t
2021-02-06	English	Chris	f
2021-02-05	English	Chris	f
2021-02-04	Chemistry	Chris	t
2021-02-05	Chemistry	Chris	t
2021-02-07	Chemistry	Chris	t
2021-02-06	Chemistry	Chris	f
2021-02-04	English	Joseph	t
2021-02-05	English	Joseph	t
2021-02-07	English	Joseph	t
2021-02-06	English	Joseph	f
2021-02-04	Chemistry	Joseph	t
2021-02-05	Chemistry	Joseph	t
2021-02-07	Chemistry	Joseph	t
2021-02-06	Chemistry	Joseph	f