关于Optimizer_index_cost_adj参数的设置
Thomas建议:
对于许多系统,应到考虑设置这两个参数为非默认值,至少测试一下两种极端情形:
1. optimizer_index_caching=0 和 optimizer_index_cost_adj=100的默认值. 他们一般适用于许多数据仓库/报表系统
2. otpimizer_index_caching=90和optimizer_index_cost_adj=25的设置,他们一般适用于许多事物处理系统/oltp系统.
对于数据仓库和DSS系统要反复调整来取一个合理值。
Oracle在选择不同的访问路径时，会对全表扫描和索引扫描进行比较评估，在比较的时候，
Oracle会把索引扫描的成本转换为全表扫描的成本，和全表扫描的COST进行比较。这个转换需要一个转换因子，就是Optimizer_index_cost_adj;
Optimizer_index_cost_adj*(index scan cost)=等价的Full Scan cost
所以 optimizer_index_cost_adj = Full Scan Cost / Index Scan Cost
在缺省情况下：Optimizer_index_cost_adj=100
SQL> show parameter optimizer_index_cost_adj
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
optimizer_index_cost_adj integer 100
SQL> create table t3 as select * from dba_objects;
Table created
SQL>
SQL> create index ind_owner on t3(owner);
Index created
SQL> analyze table t3 compute statistics;
SQL> set autotrace on
SQL> set timing on
SQL> set autotrace traceonly
SQL> alter session set optimizer_index_cost_adj=43;
Session altered.
select /*+ FULL(t3)*/* from bia_stg.t3 t3 where owner='BIA_STG';
322 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2574254479
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3044 | 255K| 130 (2)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| T3 | 3044 | 255K| 130 (2)| 00:00:02 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OWNER"='BIA_STG')
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
739 consistent gets
0 physical reads
0 redo size
23185 bytes sent via SQL*Net to client
700 bytes received via SQL*Net from client
23 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
322 rows processed
SQL> select /*+ index(t3 ind_owner)*/* from bia_stg.t3 t3 where owner='BIA_STG';
322 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2790462862
--------------------------------------------------------------------------------
---------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| T
ime |
--------------------------------------------------------------------------------
---------
| 0 | SELECT STATEMENT | | 3044 | 255K| 89 (0)| 0
0:00:02 |
| 1 | TABLE ACCESS BY INDEX ROWID| T3 | 3044 | 255K| 89 (0)| 0
0:00:02 |
|* 2 | INDEX RANGE SCAN | IND_OWNER | 3044 | | 8 (0)| 0
0:00:01 |
--------------------------------------------------------------------------------
---------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OWNER"='BIA_STG')
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
65 consistent gets
0 physical reads
0 redo size
23185 bytes sent via SQL*Net to client
700 bytes received via SQL*Net from client
23 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
322 rows processed
optimizer_index_cost_adj = Full Scan Cost / Index Scan Cost* 100=130/89 * 100=1.460 * 100
=146
及optimizer_index_cost_adj调整为147