值分布不均衡的列建索引不收集直方图信息的结果

在一个值分布不均衡的列上建索引，但是收集统计信息的时候没有收集直方图信息的情况下会走索引吗？
SQL> create table t as select * from dba_objects;


Table created.


SQL> create index idx_t1 on t(status);
create index idx_t1 on t(status)
             *
ERROR at line 1:
ORA-00955: name is already used by an existing object




SQL> create index idx_t on t(status);
create index idx_t on t(status)
             *
ERROR at line 1:
ORA-00955: name is already used by an existing object




SQL> drop index idx_t;


Index dropped.


SQL>  create index idx_t on t(status);


Index created.


SQL> select status,count(1) from t group by status;


STATUS    COUNT(1)
------- ----------
VALID        71983
INVALID          2


SQL> exec dbms_stats.gather_table_stats(ownname=>'SYS',TABNAME=>'T',cascade=>true,method_opt=>'for all columns size auto for columns status size 1');


PL/SQL procedure successfully completed.


SQL> 
SQL> 
SQL> 
SQL> 
SQL> set autotrace on
SQL> select object_id ,status from t where status='INVALID';


 OBJECT_ID STATUS
---------- -------
     73871 INVALID
     73846 INVALID




Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873


--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      | 35993 |   421K|   280   (1)| 00:00:04 |
|*  1 |  TABLE ACCESS FULL| T    | 35993 |   421K|   280   (1)| 00:00:04 |
--------------------------------------------------------------------------


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


   1 - filter("STATUS"='INVALID')




Statistics
----------------------------------------------------------
        164  recursive calls
          0  db block gets
       1049  consistent gets
          0  physical reads
          0  redo size
        669  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          4  sorts (memory)
          0  sorts (disk)
          2  rows processed


SQL> 
从上面可以看出是不会的，那么为什么没有走索引那？

做一个10053的trace看一下
***************************************
BASE STATISTICAL INFORMATION
***********************
Table Stats::
  Table:  T  Alias:  T
    #Rows: 71985  #Blks:  1024  AvgRowLen:  97.00
Index Stats::
  Index: IDX_T  Col#: 10
    LVLS: 1  #LB: 171  #DK: 2  LB/K: 85.00  DB/K: 512.00  CLUF: 1025.00
Access path analysis for T
***************************************
SINGLE TABLE ACCESS PATH 
  Single Table Cardinality Estimation for T[T] 
  Table: T  Alias: T
    Card: Original: 71985.000000  Rounded: 35993  Computed: 35992.50  Non Adjusted: 35992.50
  Access Path: TableScan
    Cost:  280.09  Resp: 280.09  Degree: 0
      Cost_io: 279.00  Cost_cpu: 34646655
      Resp_io: 279.00  Resp_cpu: 34646655
  Access Path: index (AllEqRange)
    Index: IDX_T
    resc_io: 599.00  resc_cpu: 23342883
    ix_sel: 0.500000  ix_sel_with_filters: 0.500000 
    Cost: 599.73  Resp: 599.73  Degree: 1
  Best:: AccessPath: TableScan
         Cost: 280.09  Degree: 1  Resp: 280.09  Card: 35992.50  Bytes: 0
***************************************
得出的还是全表扫描比较好---，11g的10053的trace好像变了，cost的计算方法好像是变了啊，计算方法有待研究
原来的样子：

Access path analysis for MACLEAN1***************************************SINGLE TABLE ACCESS PATH  Single Table Cardinality Estimation for MACLEAN1[MACLEAN1]  Column (#10): STATUS(    AvgLen: 7 NDV: 2 Nulls: 0 Density: 0.500000  Table: MACLEAN1  Alias: MACLEAN1    Card: Original: 22639.000000  Rounded: 11320  Computed: 11319.50  Non Adjusted: 11319.50  Access Path: TableScan    Cost:  85.33  Resp: 85.33  Degree: 0      Cost_io: 85.00  Cost_cpu: 11935345      Resp_io: 85.00  Resp_cpu: 11935345  Access Path: index (AllEqRange)    Index: IND_MACLEAN1    resc_io: 185.00  resc_cpu: 8449916    ix_sel: 0.500000  ix_sel_with_filters: 0.500000    Cost: 185.24  Resp: 185.24  Degree: 1  Best:: AccessPath: TableScan         Cost: 85.33  Degree: 1  Resp: 85.33  Card: 11319.50  Bytes: 0

当然进行直方图收集之后就没有问题