Postgres 9.6

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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 links(id1, id2, id3, id4, content) as
values (1,2,3,4,'Text')
     , (1,2,5,4,'Other thing')
     , (8,4,3,2,'More stuff');
3 rows affected

explain
select *
from links
where (id1,id2,id3,id4) in ((1,2,3,4),(1,2,5,4),(8,4,3,2));


QUERY PLAN


Seq Scan on links  (cost=0.00..52.80 rows=1 width=48)

  Filter: (((id1 = 1) AND (id2 = 2) AND (id3 = 3) AND (id4 = 4)) OR ((id1 = 1) AND (id2 = 2) AND (id3 = 5) AND (id4 = 4)) OR ((id1 = 8) AND (id2 = 4) AND (id3 = 3) AND (id4 = 2)))


explain
select *
from links
where (id1,id2,id3,id4) in ((1,2,3,4),(1,2,5,4));


QUERY PLAN


Seq Scan on links  (cost=0.00..34.08 rows=1 width=48)

  Filter: ((id1 = 1) AND (id2 = 2) AND (id4 = 4) AND ((id3 = 3) OR (id3 = 5)))