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扫描成本和OPTIMIZER_INDEX_COST_ADJ
我们知道，在CBO模式下，Oracle会计算各个访问路径的代价，采用最小代价的访问路径作为语句的执行计划。而对于索引的访问代价的计算，需要根据一个系统参数OPTIMIZER_INDEX_COST_ADJ来转换为与全表扫描代价等价的一个值。这是什么意思呢？我们先稍微解释一下这个参数：OPTIMIZER_INDEX_COST_ADJ。它的值是一个百分比，默认是100，取值范围是1～10000。当估算索引扫描代价时，会将索引的原始代价值乘以这个百分比，将换算后的值作为与全表扫描代价比较的值。也就是说，当这个值为100时，计算出的索引扫描代价就是它的原始代价：
COST_COM = COST_ORG * OPTIMIZER_INDEX_COST_ADJ/100 |
看以下例子：
SQL> create table T_PEEKING (a NUMBER, b char(1), c char(2000)); Table created. SQL> SQL> create index T_PEEKING_IDX1 on T_PEEKING(b); Index created. SQL> begin 2 for i in 1..1000 loop 3 insert into T_PEEKING values (i, 'A', i); 4 end loop; 5 6 insert into T_PEEKING values (1001, 'B', 1001); 7 insert into T_PEEKING values (1002, 'B', 1002); 8 insert into T_PEEKING values (1003, 'C', 1003); 9 10 commit; 11 end; 12 / PL/SQL procedure successfully completed. |
注意，我们给索引字段B插入的值中只有3个distinct值，记录数是1003，它的集的势很高（1003/3）=334。
SQL> SQL> analyze table T_PEEKING compute statistics for table for all indexes for all indexed columns; Table analyzed. SQL> |
我们看下索引扫描的代价是多少：
SQL> show parameter OPTIMIZER_INDEX_COST_ADJ
NAME TYPE VALUE
------------------------------------ ----------- ------
optimizer_index_cost_adj integer 100
SQL> delete from plan_table;
0 rows deleted.
SQL>
SQL> explain plan for select
/*+index(a T_PEEKING_IDX1)*/ * from T_PEEKING a where b = :V;
Explained.
SQL> select lpad(' ', 2*(level-1))||operation||' '||options||' '||
2 object_name||' '||decode(id, 0, 'Cost='||position) "Query
3 Plan_Table"
4 from plan_table
5 start with id = 0
6 connect by prior id = parent_id
7 ;
Query
Plan_Table
-----------------------------------------------------
SELECT STATEMENT Cost=113
TABLE ACCESS BY INDEX ROWID T_PEEKING
INDEX RANGE SCAN T_PEEKING_IDX1
SQL> |
再看全表扫描的代价是多少：
SQL> delete from plan_table;
3 rows deleted.
SQL>
SQL> explain plan for select
/*+full(a)*/ * from T_PEEKING a where b = :V;
Explained.
SQL>
SQL> select lpad(' ', 2*(level-1))||operation||' '||options||' '||
2 object_name||' '||decode(id, 0, 'Cost='||position) "Query
3 Plan_Table"
4 from plan_table
5 start with id = 0
6 connect by prior id = parent_id
7 ;
Query
Plan_Table
----------------------------------------------------
SELECT STATEMENT Cost=75
TABLE ACCESS FULL T_PEEKING
SQL> |
这时，我们可以计算得出让优化器使用索引（无提示强制）的OPTIMIZER_INDEX_COST_ADJ值应该< ROUND(COST_FTS/COST_IDX*100) = ROUND(75/113*100) = 66，而大于66则会使用全表扫描：
SQL> alter system set OPTIMIZER_INDEX_COST_ADJ=67;
System altered.
SQL>
SQL> delete from plan_table;
2 rows deleted.
SQL>
SQL> explain plan for select * from T_PEEKING a where b = :V;
Explained.
SQL>
SQL> select lpad(' ', 2*(level-1))||operation||' '||options||' '||
2 object_name||' '||decode(id, 0, 'Cost='||position) "Query
3 Plan_Table"
4 from plan_table
5 start with id = 0
6 connect by prior id = parent_id;
Query
Plan_Table
-----------------------------------------------------------------
SELECT STATEMENT Cost=75
TABLE ACCESS FULL T_PEEKING
SQL>
SQL>
SQL> alter system set OPTIMIZER_INDEX_COST_ADJ=66;
System altered.
SQL>
SQL> delete from plan_table;
2 rows deleted.
SQL>
SQL> explain plan for select * from T_PEEKING a where b = :V;
Explained.
SQL>
SQL> select lpad(' ', 2*(level-1))||operation||' '||options||' '||
2 object_name||' '||decode(id, 0, 'Cost='||position) "Query
3 Plan_Table"
4 from plan_table
5 start with id = 0
6 connect by prior id = parent_id;
Query
Plan_Table
---------------------------------------------------------
SELECT STATEMENT Cost=75
TABLE ACCESS BY INDEX ROWID T_PEEKING
INDEX RANGE SCAN T_PEEKING_IDX1 |
可以看出，在使用绑定变量时，参数OPTIMIZER_INDEX_COST_ADJ对于是否选择索引会有重要的影响。
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