Slow performance to use 'UNION'

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I don't see how forcing the index with a hint would improve the performance if the CBO determined a FTS to be more efficient from the get go.

Perhaps if you explained what each query is trying to accomplish.
Is there any reason an OR statement in the where-clause could not
accomplish what you need?

Does this improve anything?

select decode(a, null, b, a) supplier_id from 
(SELECT s.supplier_id a
  FROM company_default_permissions cdp, username u, supplier s
 WHERE u.username_id = 117120
   AND u.company_id = cdp.company_id
   AND u.company_id = s.company_id
   AND s.supplier_id NOT IN (
          SELECT supplier_id
            FROM supplier_spm_acl ssa
           WHERE ssa.supplier_id = s.supplier_id
             AND (   ssa.username_id = 117120
                  OR ssa.user_group_id IN (
SELECT user_group_id FROM username_group_map 
WHERE username_id = 117120)))
   AND cdp.can_read = 1
   AND cdp.object_type_id = 9)
full outer join
(SELECT s.supplier_id b
  FROM supplier_spm_acl ssa, username u, supplier s
 WHERE u.username_id = 117120
   AND u.username_id = ssa.username_id
   AND u.company_id = s.company_id
   AND s.supplier_id = ssa.supplier_id
   AND ssa.can_read = 1)
on a = b