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 t ( a int, b int, c int, d int) ;
alter table t add unique(b, c);
insert into t (a, b, c, d) values (1, 1, 1, 1), (2, 2, null, 2), (3, 2, null, null);
3 rows affected

select * from t;


a
b
c
d


1
1
1
1

2
2
null
2

3
2
null
null

a	b	c	d
1	1	1	1
2	2	null	2
3	2	null	null