Postgres 10

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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?.

with recursive b as (
      select v.*, row_number() over (order by items desc, id) as seqnum
      from (values (3834, 7206), (78350, 6151), (73525, 5996), (3838, 5192), (71331, 5184), (76842, 3982), (76854, 3982)
           ) v(id, items)
     ),
     cte as (
      select (b.id::text) as ids, b.items as items, 1 as grp, b.seqnum
      from b
      where seqnum = 1
      union all
      select (case when b.items + cte.items < 15000
                   then cte.ids || ',' || b.id
                   else b.id::text
              end) as ids,
             (case when b.items + cte.items < 15000
                   then cte.items + b.items
                   else b.items
              end) as ids,
             (case when b.items + cte.items < 15000
                   then cte.grp
                   else cte.grp + 1
              end) as grp,
             b.seqnum
      from cte join
           b
           on b.seqnum = cte.seqnum + 1
     )
select distinct on (grp) cte.*
from cte
order by grp, seqnum desc;

ids	items	grp	seqnum
3834,78350	13357	1	2
73525,3838	11188	2	4
71331,76842,76854	13148	3	7