Server Administration :: Exchange Partition With Update Indexes

I exchange a partition with into a normal table with UPDATE INDEXES,but i found the index of both table are marked UNUSABLE?

SQL> Create Table tb_hxl_list_part
2 (
3 statedate Number,
4 provcode Number
5 )
6 Partition By List(provcode)
7 (
8 Partition p_1 Values(1)
9 );

Table created.

SQL> Create Unique Index idx_tb_hxl_list_part On tb_hxl_list_part(provcode) Local;

Index created.

SQL> Insert Into tb_hxl_list_part Values(20111101,1);

1 row created.

SQL> commit;

Commit complete.

SQL> Select status From dba_ind_partitions aa
2 Where aa.index_name = 'IDX_TB_HXL_LIST_PART';

STATUS
--------
USABLE

SQL> Create Table tb_hxl_list_part_bak
2 (
3 statedate Number,
4 provcode Number
5 );

Table created.

SQL> Create Unique Index idx_hxl_list_part_bak On tb_hxl_list_part_bak(provcode);

Index created.

SQL> Select status From dba_indexes bb
2 Where bb.index_name = 'IDX_HXL_LIST_PART_BAK';

STATUS
--------
VALID

SQL> Alter Table tb_hxl_list_part
2 Exchange Partition p_1
3 With Table TB_HXL_LIST_PART_bak UPDATE Indexes;

Table altered.

SQL> Select status From dba_ind_partitions aa
2 Where aa.index_name = 'IDX_TB_HXL_LIST_PART';

STATUS
--------
UNUSABLE

SQL> Select status From dba_indexes bb
2 Where bb.index_name = 'IDX_HXL_LIST_PART_BAK';

STATUS
--------
UNUSABLE