Postgres 12

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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 product (
                id serial NOT NULL,
                opid int4 NULL,
                opvalue int4 NULL,
                info varchar NULL,
                CONSTRAINT product_pkey PRIMARY KEY (id)
            );
INSERT INTO product (id,opid,opvalue,info) VALUES 
            (1,1,1,'s1')
            ,(2,1,1,'s2')
            ,(3,2,1,'s2')
            ,(4,1,1,'s3')
            ,(6,3,2,'s3')
            ,(5,2,1,'s3')
            ;
6 rows affected

select *
from product p
where opid = 1 and opvalue = 1
and not exists (
  select *
  from product x
  where x.info = p.info
  and (x.opid != p.opid or x.opvalue != p.opvalue)
);


id
opid
opvalue
info


1
1
1
s1