规则如下：
测试如下：
hr@ORCL> drop table t purge;
Table dropped.
hr@ORCL> create table t as select * from dba_objects;
Table created.
hr@ORCL> alter table t add (object_id_1 number);
Table altered.
hr@ORCL> update t set object_id_1=object_id;
50363 rows updated.
hr@ORCL> commit;
Commit complete.
hr@ORCL> create index idx_t_a on t(object_id);
Index created.
hr@ORCL> create index idx_t_b on t(object_id_1);
Index created.
/* 统计信息相同、意味着同类型执行计划的Cost值会相同*/
hr@ORCL> exec dbms_stats.gather_table_stats(ownname=>'HR',tabname=>'T',estimate_percent=>100,cascade=>TRUE,no_invalidate=>false);
PL/SQL procedure successfully completed.
/* 叶子块数量相同、CBO按字母顺序在前走索引*/
hr@ORCL> select index_name,leaf_blocks from user_indexes where table_name='T' and index_name in ('IDX_T_A','IDX_T_B');
INDEX_NAME LEAF_BLOCKS
------------------------------ -----------
IDX_T_A 111
IDX_T_B 111
hr@ORCL> set autot trace exp
hr@ORCL> select * from t where object_id=1000 and object_id_1=1000;
Execution Plan
----------------------------------------------------------
Plan hash value: 1194865126
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 98 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 98 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_T_A | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID_1"=1000)
2 - access("OBJECT_ID"=1000)
/* 把idx_t_b叶子块数量从111改为110*/
hr@ORCL> set autot off
hr@ORCL> exec dbms_stats.set_index_stats(ownname=>'HR',indname=>'IDX_T_B',numlblks=>110);
PL/SQL procedure successfully completed.
hr@ORCL> select index_name,leaf_blocks from user_indexes where table_name='T' and index_name in ('IDX_T_A','IDX_T_B');
INDEX_NAME LEAF_BLOCKS
------------------------------ -----------
IDX_T_A 111
IDX_T_B 110
/* Cost 值相同、CBO 选择叶子块数量较少的索引*/
hr@ORCL> set autot trace exp
hr@ORCL> select * from t where object_id=1000 and object_id_1=1000;
Execution Plan
----------------------------------------------------------
Plan hash value: 3073359464
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 98 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 98 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_T_B | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID"=1000)
2 - access("OBJECT_ID_1"=1000)
By David Lin
20113-06-05
Good Luck
