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db<>fiddle
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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?.
version
PostgreSQL 14.1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-4), 64-bit 
2 rows affected
user_id
user1
user2 
3 rows affected
course_id
course1
course2
course3 
2 rows affected
enrollment_id course_id user_id
enrollment1 course1 user2
enrollment2 course2 user2
enrollment_id course_id user_id course_id user_id
enrollment1 course1 user2 course1 user2
enrollment2 course2 user2 course2 user2
enrollment_id course_id user_id course_id user_id
null null null null user1
enrollment1 course1 user2 course1 user2
enrollment2 course2 user2 course2 user2
null null null course3 null
enrollment_id course_id user_id course_id user_id
enrollment1 course1 user2 course1 user1
enrollment1 course1 user2 course1 user2
enrollment2 course2 user2 course1 user1
enrollment2 course2 user2 course1 user2
enrollment1 course1 user2 course2 user1
enrollment1 course1 user2 course2 user2
enrollment2 course2 user2 course2 user1
enrollment2 course2 user2 course2 user2
enrollment1 course1 user2 course3 user1
enrollment1 course1 user2 course3 user2
enrollment2 course2 user2 course3 user1
enrollment2 course2 user2 course3 user2
user_id course_id
user1 course1
user1 course2
user1 course3
user2 course1
user2 course2
user2 course3
user_id course_id
user1 course1
user1 course2
user1 course3
user2 course1
user2 course2
user2 course3
user_id course_id enrollment_id
user1 course1 null
user1 course2 null
user1 course3 null
user2 course1 enrollment1
user2 course2 enrollment2
user2 course3 null