如果一个表上有索引，那么直接加载又会是什么情况呢？产生的redo量跟传统加载又相差多少呢？例子：

sys@ORCL> create table t

2 as

3 select * from dba_objects where 1=0;

Table created.

sys@ORCL> create index t_ind on t(object_id);

Index created.

sys@ORCL> set autotrace trace stat;

首先看下传统加载在有索引列产生的redo量

sys@ORCL> insert into t

2 select * from dba_objects where rownum<10000;

9999 rows created.

Statistics

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

1067 recursive calls

1608 db block gets

2926 consistent gets

78 physical reads

1746900 redo size

918 bytes sent via SQL*Net to client

973 bytes received via SQL*Net from client

6 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

9999 rows processed

sys@ORCL> rollback;

Rollback complete.

再看直接加载在有索引列产生的redo

sys@ORCL> insert/*+append*/ into t

2 select * from dba_objects where rownum<10000;

9999 rows created.

Statistics

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

40 recursive calls

295 db block gets

2283 consistent gets

22 physical reads

1326520 redo size

903 bytes sent via SQL*Net to client

987 bytes received via SQL*Net from client

6 SQL*Net roundtrips to/from client

2 sorts (memory)

0 sorts (disk)

9999 rows processed

通过对比可以发现两者相差的redo量并不是很多。因此使用直接加载时，通常是针对非常大的表。如果这些表存在索引，将会带来很大的性能影响，可以考虑将索引disable或者drop掉，然后加载数据，再建立索引。