Cover NEST query

I have a query like

   ARRAY {} for bItem IN b END 
FROM bucket a
NEST bucket b ON AND b.type='B'
WHERE a.type='A'

If I use JOIN instead of NEST the query used covering index , But in NEST it fetches b too
How can I change query or add index to cover NEST?

NEST query can’t be covered . if need covering use JOIN + GROUP BY+ARRAY_AGG()