USER            PREV_ORDER                       FINAL_ORDER
  1    [doc_user1_1, ..., doc_user1_10]  [doc_user1_3, ..., doc_user1_9]
  2    [doc_user2_1, ..., doc_user1_10]  [doc_user2_5, ..., doc_user2_3]
  3    [doc_user3_1, ..., doc_user1_10]  [doc_user3_8, ..., doc_user3_1]
 ...