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