Postgres 14

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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?.

select version();


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


with users (user_id, name) as 
     ( values (1,'user 1')
            , (2,'user 2')
            , (3,'user 3')
     ) 
     
   , followers ( user_id, other_userid) as 
     (values (1,2)
           , (3,1)
           , (1,3) 
     )
-- your query beging here 
select u.user_id
     , u.name
     , exists(select null 
                from followers f 
               where f.user_id = u.user_id
              ) is_followed 
  from users u 
 order by u.user_id; 


user_id
name
is_followed


1
user 1
t

2
user 2
f

3
user 3
t


with users (user_id, name) as 
     ( values (1,'user 1')
            , (2,'user 2')
            , (3,'user 3')
     ) 
     
   , followers ( user_id, other_userid) as 
     (values (1,2)
           , (3,1)
           , (1,3) 
     )
-- your query beging here 
select distinct on (u.user_id) u.user_id
     , u.name
     , f.user_id is not null is_followed
  from users u
  left join followers f 
    on (  f.user_id = u.user_id) 
 order by u.user_id; 


user_id
name
is_followed


1
user 1
t

2
user 2
f

3
user 3
t