From:http://blog.chinaunix.net/u/10516/showart_1009036.html

alter index rebuild与alter index rebuild online的扫描方式是不一样的。可以举个简单的例子来说明： SQL>create table sjh.test as select * from dba_users; --建一个测试表 SQL> explain plan for
2 alter index test_idx rebuild; Explained SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 2554748275
--------------------------------------------------------------------------------
| Id | Operation              | Name     | Rows | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------
|   0 | ALTER INDEX STATEMENT |          |   327 | 4251 |     1   (0)| 00:00:0
|   1 | INDEX BUILD NON UNIQUE| TEST_IDX |       |       |            |
|   2 |   SORT CREATE INDEX    |          |   327 | 4251 |            |
|   3 |    INDEX FAST FULL SCAN| TEST_IDX |   327 | 4251 |     1   (0)| 00:00:0
-------------------------------------------------------------------------------- 10 rows selected SQL> explain plan for
2 alter index test_idx rebuild online; Explained SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1149508270
--------------------------------------------------------------------------------
| Id | Operation              | Name     | Rows | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------
|   0 | ALTER INDEX STATEMENT |          |   327 | 4251 |     1   (0)| 00:00:0
|   1 | INDEX BUILD NON UNIQUE| TEST_IDX |       |       |            |
|   2 |   SORT CREATE INDEX    |          |   327 | 4251 |            |
|   3 |    TABLE ACCESS FULL   | TEST     |   327 | 4251 |     1   (0)| 00:00:0
-------------------------------------------------------------------------------- 10 rows selected SQL> --从上面例子红色标签位置可以很清楚的看到 index rebuild online实质上是扫描表而不是扫描现有的索引块来实现索引的重建，index rebuild 只扫描现有的索引块来实现索引的重建。除了扫描方式不同外，rebuild 会阻塞dml语句而rebuild online则不会(ddl语句除外)。
rebuild online时系统会产生一个SYS_JOURNAL_xxx的IOT类型的系统临时日志表，所有rebuild online时索引的变化都记录在这个表中(但要使用两倍于传统方法的空间)，当新的索引创建完成后，把这个表的记录维护到新的索引中去，然后drop掉旧的索引，rebuild online就完成了。注意并行处理，DDL，位图索引不能使用ONLINE。在rebuild online 时会报错:
ORA-08120: Need to create SYS.IND_ONLINE$ table in order to (re)build index
Cause: Alter index Build/Rebuild online require existing of SYS.IND_ONLINE$ table.
Action: User/DBA needs to create sys.ind_online$ before alter the index /rdbms/admin/catcio.sql contains script. to create ind_online$.
解决方法就是运行catcio.sql 创建SYS.IND_ONLINE$表

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alter index rebuild & alter index rebuild online

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