Predicate Information (identified by operation id):
---------------------------------------------------

  1 - access("OWNER"='SCOTT')

Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      4  consistent gets
      0  physical reads
      0  redo size
    610  bytes sent via SQL*Net to client
    524  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed

因为这个索引不是唯一索引，where后面的列用到了索引000，所以进行index range scan

SQL> select owner, object_name,object_type from test where object_name='EMP' ;

no rows selected

Execution Plan
----------------------------------------------------------
Plan hash value: 1799988433

-------------------------------------------------------------------------
| Id  | Operation    | Name | Rows    | Bytes | Cost (%CPU)| Time    |
-------------------------------------------------------------------------
|  0 | SELECT STATEMENT |    |    2 |    80 |    26  (0)| 00:00:01 |
|*  1 |  INDEX SKIP SCAN | OOO    |    2 |    80 |    26  (0)| 00:00:01 |
-------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

  1 - access("OBJECT_NAME"='EMP')
      filter("OBJECT_NAME"='EMP')

Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
    28  consistent gets
      0  physical reads
      0  redo size
    479  bytes sent via SQL*Net to client
    513  bytes received via SQL*Net from client
      1  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      0  rows processed

上面的查询可以通过索引000来得到，并且where后面没有用到索引列，而且返回的行数很少（。）所以cbo选择index skip scan

select owner, object_name,object_type from test where object_type='INDEX';
Execution Plan
----------------------------------------------------------
Plan hash value: 3464522019