SEGMENT_NAME    BLOCKS    EXTENTS  size
------------- ---------- ---------- --------
JACK            11264      82      88M

  ----在object_id上创建索引----
SQL> create index jack_ind on jack(object_id);

Index created.

  ----查看一下索引的大小----
SQL> select segment_name,segment_type,blocks,extents,bytes/1024/1024||'M' "size" from user_segments where segment_name='JACK_IND';

SEGMENT_NAME    SEGMENT_TYPE      BLOCKS    EXTENTS    size
------------ ------------------ ---------- ---------- ---------
JACK_IND          INDEX          1664        28        13M
  ----在没有收集相关的统计信息之前，查看一下index clustering factor----
SQL> select index_name,clustering_factor,num_rows from user_indexes where index_name='JACK_IND';

INDEX_NAME      CLUSTERING_FACTOR  NUM_ROWS
--------------- ----------------- ----------
JACK_IND              725460        725460

  ----简单的收集一下统计信息----
SQL> exec dbms_stats.gather_table_stats(user,'jack',cascade=>true);

PL/SQL procedure successfully completed.

  ----再次查看index clustering factor----
SQL> select index_name,clustering_factor,num_rows from user_indexes where index_name='JACK_IND';

INDEX_NAME    CLUSTERING_FACTOR  NUM_ROWS
-------------- ----------------- ----------
JACK_IND              725460      725460      ----显然统计信息收集前和后，clustering factor值不变，说在创建索引的时候，会收集表中的数据真正的行数。并且这里的clustering factor等num_rows，也说明表的clustering factor是无序的。

  ----查看一个确定值，然后查看执行计划----
SQL> explain plan for select * from jack where object_id=1501;

Explained.

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

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 2860868395

--------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Ti
--------------------------------------------------------------------------------
|  0 | SELECT STATEMENT        |          |    10 |  970 |    13    (0)| 00
|  1 |  TABLE ACCESS BY INDEX ROWID| JACK    |    10 |  970 |    13    (0)| 00
|*  2 |  INDEX RANGE SCAN        | JACK_IND |    10 |      |    3    (0)| 00
--------------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

  2 - access("OBJECT_ID"=1501)

14 rows selected.                ----在这里走了索引，cost为13.