oracle 多库表建立dblink查询

select /*+ driving_site(a) driving_site(b) driving_site(c) */ a.object_id,b.object_type,c.object_sid
  from a1@dblink1 a,a2@dblink2 b,a3@dblink3 c,d4
 where a.object_id=b.object_id
   and b.object_ccid=c.object_ccid
   and c.object_sod=d.object_sod

　　加了     '/*+ driving_site(关联表1别名) driving_site(关联表2别名) driving_site(关联表3别名) */'  速度快甚至几十倍