什么决定了SKIP索引扫描的出现

建表：

SQL> create table test as select 1 v_num,emp.* from emp;

Table created.

SQL> update test set v_num = 2 where empno = 7369;

1 row updated.

建索引：
SQL> create index index_n_e on test(v_num,empno);

Index created.
索引所涉及到列的基数情况：

SQL> select count(distinct v_num),count(distinct empno) from test;

COUNT(DISTINCTV_NUM) COUNT(DISTINCTEMPNO)
-------------------- --------------------
2 14
查询语句和执行计划：
1.

SQL> select ename from test where empno = 7369;

Execution Plan
----------------------------------------------------------
Plan hash value: 1357081020

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 20 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| TEST | 1 | 20 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------2.
SQL> analyze table test compute statistics; Table analyzed.

SQL> select ename from test where empno = 7369;

Execution Plan
----------------------------------------------------------
Plan hash value: 3656150759

-----------------------------------------------------------------------------------------
| Id  | Operation		    | Name	| Rows	| Bytes | Cost (%CPU)| Time	|
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	    |		|     1 |     8 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST	|     1 |     8 |     3   (0)| 00:00:01 |
|*  2 |   INDEX SKIP SCAN	    | INDEX_N_E |     1 |	|     2   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

3.
SQL> drop index index_n_e;

Index dropped.

SQL> create index index_e_n on test(empno,v_num);

Index created.

SQL> select ename from test where empno = 7369;

Execution Plan
----------------------------------------------------------
Plan hash value: 2333580441

-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 8 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 8 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | INDEX_E_N | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------4.

SQL> select ename from test where v_num = 2;

Execution Plan
----------------------------------------------------------
Plan hash value: 1357081020

--------------------------------------------------------------------------
| Id  | Operation	  | Name | Rows  | Bytes | Cost (%CPU)| Time	 |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |	 |     7 |    49 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| TEST |     7 |    49 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------

经过上面的两组对照（1，2；3，4）可以得出以下结论：
如果想要组合索引走跳跃索引扫描（INDEX SKIP SCAN）需要满足的条件有：

1.复合索引的引导列基数较低，而其他列基数高

2.对表进行分析

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

同时在做试验的时候也遇到了这样的情况，当表上有索引处于unusable的时候，是不能对这个表进行分析的。

SQL> alter index index_e_n unusable;

Index altered.

SQL> analyze table test compute statistics;
analyze table test compute statistics
*
ERROR at line 1:
ORA-01502: index 'SCOTT.INDEX_E_N' or partition of such index is in unusable state

SQL> alter index index_e_n rebuild;

Index altered.

SQL> analyze table test compute statistics;

Table analyzed.