Oracle 中我们知道能够使用跳跃式索引扫描(Index Skip Scan).然而,能利用跳跃式索引扫描的情况其实是有些限制的
CREATE TABLE test AS SELECT ROWNUM a,ROWNUM-1 b ,ROWNUM-2 c,ROWNUM-3 d,ROWNUM-4 e FROM all_objects;

SQL> CREATE TABLE test AS SELECT ROWNUM a,ROWNUM-1 b ,ROWNUM-2 c,ROWNUM-3 d,ROWNUM-4 e FROM all_objects;

Table created.

Elapsed: 00:00:02.78
SQL> desc test;
 Name                              Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 A                                NUMBER
 B                                NUMBER
 C                                NUMBER
 D                                NUMBER
 E                                NUMBER

SELECT DISTINCT COUNT (a) FROM test;
SQL> SELECT DISTINCT COUNT (a) FROM test;

  COUNT(A)
----------
    84394

CREATE INDEX test_idx ON test(a,b,c);

SQL> create index test_idx on test(a,b,c);

Index created.

ANALYZE TABLE test COMPUTE STATISTICS;
SET autotrace traceonly explain;
SELECT *  FROM test WHERE b = 99;

SQL> analyze table test compute statistics;

Table analyzed.

Elapsed: 00:00:02.46
SQL> set autotrace traceonly explain;
SQL> select * from test where b=99;
Elapsed: 00:00:00.00

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

--------------------------------------------------------------------------
| Id  | Operation      | Name | Rows  | Bytes | Cost (%CPU)| Time    |
--------------------------------------------------------------------------
|  0 | SELECT STATEMENT  |    |    1 |    20 |    96  (2)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| TEST |    1 |    20 |    96  (2)| 00:00:02 |
--------------------------------------------------------------------------

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

  1 - filter("B"=99)
 
 -可见这里CBO选择了全表扫描
我们接着做另一个测试
 drop table test;
 CREATE TABLE test  AS SELECT DECODE(MOD(ROWNUM,2), 0, '1', '2' ) a,ROWNUM-1 b,ROWNUM-2 c,ROWNUM-3 d,ROWNUM-4 e FROM all_objects
 set autotrace off
 select distinct a from test;
 SQL> select distinct a from test;

A
-
1
2

Elapsed: 00:00:00.08

 CREATE INDEX test_idx ON test(a,b,c)
 
 SQL> SELECT *  FROM test WHERE b = 99;
Elapsed: 00:00:00.01

Execution Plan
----------------------------------------------------------
Plan hash value: 2705879578

----------------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes | Cost (%CPU)| Time    |
----------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT        |          |    1 |    17 |    4    (0)| 00:00:01 |
|  1 |  TABLE ACCESS BY INDEX ROWID| TEST    |    1 |    17 |    4    (0)| 00:00:01 |
|*  2 |  INDEX SKIP SCAN        | TEST_IDX |    1 |      |    3    (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

  2 - access("B"=99)
      filter("B"=99)

结论：
Oracle的优化器(这里指的是CBO)能对查询应用Index Skip Scans至少要有几个条件:

1 优化器认为是合适的.
2 索引中的前导列的唯一值的数量能满足一定的条件.
3 优化器要知道前导列的值分布(通过分析/统计表得到)
4 合适的SQL语句

：

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