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 abc (id INT, letter text, rank smallint);
INSERT INTO abc (id,letter,rank) 
VALUES 
(1,'A',1),
(2,'B',2),
(3,'C',3),
(4,'D',4),
(5,'E',5);
5 rows affected

select g.name, 
       sum(abc.rank)
from (
  values 
     ('Gruppe 1', array['A','B','C']), 
     ('Gruppe 2', array['D','E'])
) as g(name, letters)  
  left join abc on abc.letter = any(g.letters)
group by g.name
order by g.name;


name
sum


Gruppe 1
6

Gruppe 2
9


select case 
          when letter in ('A','B','C') then 'A-C'
          when letter in ('D', 'E') then 'D-E'
       end as grp ,
       sum(abc.rank)
from abc 
group by grp 
order by grp;


grp
sum


A-C
6

D-E
9