Plan hash value: 2966233522
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 105 | 7 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 105 | | |
|* 2 | TABLE ACCESS FULL| T | 991 | 101K| 7 (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("N"<=990)
--返回小部分数据时，使用的是索引扫描
scott@ORA11G> select count(pad) from t where n<=10;
Execution Plan
----------------------------------------------------------
Plan hash value: 4270555908
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| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 105 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 105 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| T | 10 | 1050 | 3 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | T_PK | 10 | | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("N"<=10)
d、使用并行方式访问表时使用了全表扫描
scott@ORA11G> select /*+ parallel(3) */ count(pad) from t where n<=10;
Execution Plan
----------------------------------------------------------
Plan hash value: 3126468333
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| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 105 | 3 (0)| 00:00:01 | | | |
| 1 | SORT AGGREGATE | | 1 | 105 | | | | | |
| 2 | PX COORDINATOR | | | | | | | | |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 | 105 | | | Q1,00 | P->S | QC (RAND) |
| 4 | SORT AGGREGATE | | 1 | 105 | | | Q1,00 | PCWP | |
| 5 | PX BLOCK ITERATOR | | 10 | 1050 | 3 (0)| 00:00:01 | Q1,00 | PCWC | |
|* 6 | TABLE ACCESS FULL| T | 10 | 1050 | 3 (0)| 00:00:01 | Q1,00 | PCWP | |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
6 - filter("N"<=10)
Note
-----
- Degree of Parallelism is 3 because of hint
--Author : Robinson
--Blog : http://blog.csdn.net/robinson_0612
e、使用full提示时使用了全表扫描
scott@ORA11G> select /*+ full(t) */ count(pad) from t where n<=10;
Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (