oracle跳跃式索引扫描测试

iyut

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