oracle 提供了2种可以预估将要创建的索引大小的办法：

 ①  利用包 Dbms_space.create_index_cost 直接得到

 ②  利用11g新特性 Note raised when explain plan for create index

1、环境说明

[oracle@rhel6_lhr ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.3.0 Production on Fri Dec 26 15:58:06 2014

Copyright (c) 1982, 2011, Oracle.  All rights reserved.

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

With the Partitioning, Automatic Storage Management, OLAP, Data Mining

and Real Application Testing options

SQL> create table test_index_size as select * from dba_objects;

Table created.

SQL>  EXEC DBMS_STATS.gather_table_stats(ownname => 'SYS',tabname => 'TEST_INDEX_SIZE');

PL/SQL procedure successfully completed.

第一种办法： Dbms_space.create_index_cost

脚本：

declare

  l_index_ddl       varchar2(1000);

  l_used_bytes      number;

  l_allocated_bytes number;

begin

  dbms_space.create_index_cost(ddl         => 'create index idx_t on sys.test_index_size(object_id) ',

                               used_bytes  => l_used_bytes,

                               alloc_bytes => l_allocated_bytes);

  dbms_output.put_line('used= ' || l_used_bytes || 'bytes' ||

                       '     allocated= ' || l_allocated_bytes || 'bytes');

end;

/

实验：

SQL> set serveroutput on
SQL> declare

  2    l_index_ddl varchar2(1000);

  3    l_used_bytes number;

  4    l_allocated_bytes number;

  5  begin

  6    dbms_space.create_index_cost(ddl => 'create index idx_t on sys.test_index_size(object_id) ',

  7      used_bytes => l_used_bytes,

  8      alloc_bytes => l_allocated_bytes);

  9    dbms_output.put_line('used= ' || l_used_bytes || 'bytes' ||

10      '   allocated= ' || l_allocated_bytes || 'bytes');

11  end;

12  /

used= 383105bytes     allocated= 2097152bytes

PL/SQL procedure successfully completed.

SQL>

PL/SQL 过程已成功完成。
说明<font lucida"="" console?="" style="word-wrap: break-word;">:  used_bytes  给出索引数据实际表现的字节数。
      allocated 是当实际创建索引时在表空间中实际占用的字节数。 

第二种办法：11g新特性:Note raised when explain plan for create index

这是一个挺实用的小特性，在11g r2中使用Oracle会提示评估的索引大小<font times"="" new="" roman?="" style="word-wrap: break-word;">(estimated index size)了：

SQL> set linesize 200 pagesize 1400;

SQL>  explain plan for create index idx_t on sys.test_index_size(object_id) ;

Explained.

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT

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

Plan hash value: 32582980

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

| Id  | Operation           | Name  | Rows  | Bytes | Cost (%CPU)| Time   |

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

|   0 | CREATE INDEX STATEMENT |       | 76621 |   374K| 350(1)| 00:00:05 |

|   1 |  INDEX BUILD NON UNIQUE| IDX_T |       |       |       |          |

|   2 |   SORT CREATE INDEX    |       | 76621 |   374K|       |          |

|   3 |    INDEX FAST FULL SCAN| IDX_T |       |       |       |          |

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

Note

-----

   - estimated index size: 2097K bytes

14 rows selected.

创建真实索引

SQL> create index idx_t on sys.test_index_size(object_id) ;

Index created.

SQL>  analyze index IDX_T validate structure;

Index analyzed.

SQL> select bytes from dba_segments where segment_name='IDX_T';

     BYTES

----------

   2097152

可以看到2种办法给出的索引评估大小与实际索引占用空间差别不大，但这里有个前提条件就是预估索引大小之前必须对表进行分析过。