Plan hash value: 2508449852

    ------------------------------------------------------------------------------------

    | Id  | Operation               | Name     | Rows  | Bytes | Cost  | Pstart| Pstop |

    ------------------------------------------------------------------------------------

    |   0 | ALTER INDEX STATEMENT   |          | 50617 |   247K|    56 |       |       |

    |   1 |  INDEX BUILD NON UNIQUE | T_ID_IDX |       |       |       |       |       |

    |   2 |   SORT CREATE INDEX     |          | 50617 |   247K|       |       |       |

    |   3 |    PARTITION HASH SINGLE|          |       |       |       |     1 |     1 |

    |   4 |     INDEX FAST FULL SCAN| T_ID_IDX |       |       |       |     1 |     1 |

    ------------------------------------------------------------------------------------

    Note

    -----

    - cpu costing is off (consider enabling it)

    15 rows selected.

    SQL> EXPLAIN PLAN FOR ALTER INDEX T_ID_IDX REBUILD PARTITION SYS_P225 ONLINE;

    Explained.

    SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

    PLAN_TABLE_OUTPUT

    ----------------------------------------------------------------------------------------------------

    Plan hash value: 78911014

    -----------------------------------------------------------------------------------

    | Id  | Operation              | Name     | Rows  | Bytes | Cost  | Pstart| Pstop |

    -----------------------------------------------------------------------------------

    |   0 | ALTER INDEX STATEMENT  |          | 50617 |   247K|    56 |       |       |

    |   1 |  INDEX BUILD NON UNIQUE| T_ID_IDX |       |       |       |       |       |

    |   2 |   SORT CREATE INDEX    |          | 50617 |   247K|       |       |       |

    |   3 |    PARTITION RANGE ALL |          | 50617 |   247K|    56 |     1 |     6 |

    |*  4 |     TABLE ACCESS FULL  | T        | 50617 |   247K|    56 |     1 |     6 |

    -----------------------------------------------------------------------------------

    Predicate Information (identified by operation id):

    ---------------------------------------------------

    4 - filter(TBL$OR$IDX$PART$NUM("TEST"."T",58596,0,1048576,"ID")=1)

    Note

    -----

    - cpu costing is off (consider enabling it)

    20 rows selected.

    SQL> EXPLAIN PLAN FOR ALTER INDEX T_ID_IDX REBUILD PARTITION SYS_P226;

    Explained.

    SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

    PLAN_TABLE_OUTPUT

    ----------------------------------------------------------------------------------------------------

    Plan hash value: 2508449852

    ------------------------------------------------------------------------------------

    | Id  | Operation               | Name     | Rows  | Bytes | Cost  | Pstart| Pstop |

    ------------------------------------------------------------------------------------

    |   0 | ALTER INDEX STATEMENT   |          | 50617 |   247K|    56 |       |       |

 [NextPage]

    |   1 |  INDEX BUILD NON UNIQUE | T_ID_IDX |       |       |       |       |       |

    |   2 |   SORT CREATE INDEX     |          | 50617 |   247K|       |       |       |

    |   3 |    PARTITION HASH SINGLE|          |       |       |       |     2 |     2 |

    |   4 |     INDEX FAST FULL SCAN| T_ID_IDX |       |       |       |     2 |     2 |

    ------------------------------------------------------------------------------------

    Note

    -----

    - cpu costing is off (consider enabling it)

    15 rows selected.

    SQL> EXPLAIN PLAN FOR ALTER INDEX T_ID_IDX REBUILD PARTITION SYS_P226 ONLINE;

    Explained.

    SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

    PLAN_TABLE_OUTPUT

    ----------------------------------------------------------------------------------------------------

    Plan hash value: 78911014

    -----------------------------------------------------------------------------------

    | Id  | Operation              | Name     | Rows  | Bytes | Cost  | Pstart| Pstop |

    -----------------------------------------------------------------------------------

    |   0 | ALTER INDEX STATEMENT  |          | 50617 |   247K|    56 |       |       |

    |   1 |  INDEX BUILD NON UNIQUE| T_ID_IDX |       |       |       |       |       |

    |   2 |   SORT CREATE INDEX    |          | 50617 |   247K|       |       |       |

    |   3 |    PARTITION RANGE ALL |          | 50617 |   247K|    56 |     1 |     6 |

    |*  4 |     TABLE ACCESS FULL  | T        | 50617 |   247K|    56 |     1 |     6 |

    -----------------------------------------------------------------------------------

    Predicate Information (identified by operation id):

    ---------------------------------------------------

    4 - filter(TBL$OR$IDX$PART$NUM("TEST"."T",58596,0,1048576,"ID")=2)

    Note

    -----

    - cpu costing is off (consider enabling it)

    20 rows selected.

    可以看到，如果要ONLINE重建这个索引，将会对表T执行32全表扫描。如果要对比较大的表进行在线重建索引，全局索引的重建代价是比较高的。