有一些时候发现某些表的索引大小会超出表的大小，而且索引字段比表字段少很多,测试如下。http://www.cndba.cn/hbhe0316/article/4854

SQL> create table t1(id ,name,owner,type) as select object_id , object_name,owner,object_type from dba_objects;

Table created.

SQL> create index idx_t1 on t1(id , name);

Index created.

SQL> begin 
  2   for i in 1..1000000
  3     loop update t1 set id=id+0.1 where id=i; 
  4   end loop; 
  5   end; 
  6  /

PL/SQL procedure successfully completed.

SQL> commit;

Commit complete.

SQL> analyze index idx_t1 VALIDATE STRUCTURE;

Index analyzed.

SQL> analyze table t1 compute statistics; 

Table analyzed.

SQL> analyze index idx_t1 compute statistics;

Index analyzed.

SQL> SELECT SEGMENT_NAME,BYTES,BLOCKS FROM USER_SEGMENTS WHERE SEGMENT_NAME IN ('T1','IDX_T1') ;

SEGMENT_NAME
--------------------------------------------------------------------------------
     BYTES     BLOCKS
---------- ----------
IDX_T1
   9437184       1152

T1
   5242880        640


SQL> alter index idx_t1 rebuild;

Index altered.

SQL> SELECT SEGMENT_NAME,BYTES,BLOCKS FROM USER_SEGMENTS WHERE SEGMENT_NAME IN ('T1','IDX_T1') ;

SEGMENT_NAME
--------------------------------------------------------------------------------
     BYTES     BLOCKS
---------- ----------
IDX_T1
   5242880        640

T1
   5242880        640

当数据update操作频繁，对index的维护会导致其占用空间过大，通过rebuild可以恢复到原始状态。http://www.cndba.cn/hbhe0316/article/4854

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