oracle分区表运行计划

分区表有非常多优点，以大化小，一小化了，加上并行的使用，在loap中能往往能提高几十倍甚至几百倍的效果。

当然表设计得不好也会适得其反。效果比普通表跟糟糕。
为了更好的使用分区表，这里看一下分区表的运行计划。
PARTITION RANGE ALL：扫描全部分区
PARTITION RANGE ITERATOR：扫描多个分区，小于全部个分区数量
PARTITION RANGE SINGLE：扫描单一的分区
KEY，表示运行时才知道哪个分区

看到keywordALL的时候就要注意了，扫描的是全部分区。

写sql的时候在where条件中能充分利用分区字段来限制的话最好，这样能起到分区裁剪的作用，不是必需的分区就不用扫描了。

SQL> create table t1
  2  partition by range(created)(
  3  partition p1 values less than (to_date('20140101','yyyymmdd')),
  4  partition p2 values less than (to_date('20140201','yyyymmdd')),
  5  partition p3 values less than (to_date('20140301','yyyymmdd')),
  6  partition p4 values less than (to_date('20140401','yyyymmdd')),
  7  partition p5 values less than (to_date('20140501','yyyymmdd')),
  8  partition p6 values less than (to_date('20140601','yyyymmdd')),
  9  partition p7 values less than (to_date('20140701','yyyymmdd')),
 10  partition p8 values less than (to_date('20140801','yyyymmdd')),
 11  partition p9 values less than (to_date('20140901','yyyymmdd')),
 12  partition p10 values less than (to_date('20141001','yyyymmdd')),
 13  partition p11 values less than (to_date('20141101','yyyymmdd')),
 14  partition p12 values less than (to_date('20141201','yyyymmdd')),
 15  partition p13 values less than (maxvalue)
 16  )
 17  as select * from dba_objects where created>=to_date('20131001','yyyymmdd');

--PARTITION RANGE ALL：扫描全部分区
SQL> explain plan for select count(*) from t1;
-------------------------------------------------------------------------------------
| Id  | Operation            | Name | Rows  | Cost (%CPU)| Time     | Pstart| Pstop |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      |     1 |   106   (1)| 00:00:02 |       |       |
|   1 |  SORT AGGREGATE      |      |     1 |            |          |       |       |
|   2 |   PARTITION RANGE ALL|      | 41973 |   106   (1)| 00:00:02 |     1 |    13 |
|   3 |    TABLE ACCESS FULL | T1   | 41973 |   106   (1)| 00:00:02 |     1 |    13 |
-------------------------------------------------------------------------------------

--PARTITION RANGE ITERATOR：扫描多个分区。小于全部个分区数量
SQL> explain plan for select * from t1 where created>=to_date('20141101','yyyymmdd');
-------------------------------------------------------------------------------------------------
| Id  | Operation                | Name | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |      | 13121 |  2267K|    39   (6)| 00:00:01 |       |       |
|   1 |  PARTITION RANGE ITERATOR|      | 13121 |  2267K|    39   (6)| 00:00:01 |    12 |    13 |
|*  2 |   TABLE ACCESS FULL      | T1   | 13121 |  2267K|    39   (6)| 00:00:01 |    12 |    13 |
-------------------------------------------------------------------------------------------------

--PARTITION RANGE SINGLE：扫描单一的分区
SQL> explain plan for select * from t1 where created>=to_date('20141217','yyyymmdd');
-----------------------------------------------------------------------------------------------
| Id  | Operation              | Name | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |      |   947 |   163K|    28   (0)| 00:00:01 |       |       |
|   1 |  PARTITION RANGE SINGLE|      |   947 |   163K|    28   (0)| 00:00:01 |    13 |    13 |
|*  2 |   TABLE ACCESS FULL    | T1   |   947 |   163K|    28   (0)| 00:00:01 |    13 |    13 |
-----------------------------------------------------------------------------------------------

--KEY，表示运行时才知道哪个分区
SQL> explain plan for select * from t1 where created>=sysdate-1;
-------------------------------------------------------------------------------------------------
| Id  | Operation                | Name | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |      |   947 |   163K|    33  (16)| 00:00:01 |       |       |
|   1 |  PARTITION RANGE ITERATOR|      |   947 |   163K|    33  (16)| 00:00:01 |   KEY |    13 |
|*  2 |   TABLE ACCESS FULL      | T1   |   947 |   163K|    33  (16)| 00:00:01 |   KEY |    13 |
-------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("CREATED">=SYSDATE@!-1)

SQL> variable x varchar2;
SQL> explain plan for select * from t1 where created>=to_date(:x,'yyyymmdd');
-------------------------------------------------------------------------------------------------
| Id  | Operation                | Name | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |      |  2099 |   362K|   107   (2)| 00:00:02 |       |       |
|   1 |  PARTITION RANGE ITERATOR|      |  2099 |   362K|   107   (2)| 00:00:02 |   KEY |    13 |
|*  2 |   TABLE ACCESS FULL      | T1   |  2099 |   362K|   107   (2)| 00:00:02 |   KEY |    13 |
-------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("CREATED">=TO_DATE(:X,'yyyymmdd'))