全部博文(389)
分类: Oracle
2014-01-18 11:58:32
ORACLE分区访问常用方式
对于常规的表(HEAP)访问方法主要是TABLE ACCESS FULL(全表扫描)和ROWID的方式(不考虑Exadata).第一个方法扫描
整个表的所有数据块,然后再过滤或是下一步处理;第二种方法主要是通过索引访问得到rowid,再通过rowid的方式得到相应
的行.对于分区表由于数据被分隔在各个分区里面,带来的分区访问方式有稍有不同。
假设有表
dongdongtang>create table t1 (a integer,b varchar2(100))
2 partition by range(a)
3 (partition t1_p1 values less than (5),
4 partition t1_p2 values less than (10),
5 partition t1_p3 values less than (15),
6 partition t1_p4 values less than (20));
Table created.
dongdongtang>select * from t1;
A B
---------- ----------
2 a
7 b
12 c
17 c
insert 4个值,保证每个分区都不为空
1,先看全表扫描的方式
dongdongtang>set autotrace on;
dongdongtang>select count(a) from t1;
COUNT(A)
----------
4
Execution Plan
----------------------------------------------------------
Plan hash value: 2705263620
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 3 (0)| 00:00:01 | | |
| 1 | SORT AGGREGATE | | 1 | 13 | | | | |
| 2 | PARTITION RANGE ALL| | 4 | 52 | 3 (0)| 00:00:01 | 1 | 4 |
| 3 | TABLE ACCESS FULL | T1 | 4 | 52 | 3 (0)| 00:00:01 | 1 | 4 |
---------------------------------------------------------------------------------------------
从id 2中可以看到,分区访问的方式是PARTITION RANGE ALL,pstart到pstop是1-4,表示是访问该表的所有的
分区.类似于非分区表的全表扫描.
2,分区最重要特性之一就是CBO可以根据where条件中的partition key来访问相应的分区,对于
不需要的分区不会被访问,这种特性称为partition purging,
dongdongtang>select * from t1 where a=2;
A B
---------- ------------------------------------------------
2 a
Execution Plan
----------------------------------------------------------
Plan hash value: 1737577267
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 65 | 2 (0)| 00:00:01 | | |
| 1 | PARTITION RANGE SINGLE| | 1 | 65 | 2 (0)| 00:00:01 | 1 | 1 |
|* 2 | TABLE ACCESS FULL | T1 | 1 | 65 | 2 (0)| 00:00:01 | 1 | 1 |
-----------------------------------------------------------------------------------------------
id 1中表示oracle通过PARTITION RANGE SINGLE,只访了一个分区,这种情况下pstart和pstop也是相等的.
pstart和pstop分别对应的起始分区和结束分区.
3,多个连续分区的访问,有时候查询可能需要从两个或两个以上的连续分区中来读取数据,比如:
dongdongtang>select * from t1 where a<13;
A B
---------- ---------------------------------------------
2 a
7 b
12 c
Execution Plan
----------------------------------------------------------
Plan hash value: 277861402
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 195 | 3 (0)| 00:00:01 | | |
| 1 | PARTITION RANGE ITERATOR| | 3 | 195 | 3 (0)| 00:00:01 | 1 | 3 |
|* 2 | TABLE ACCESS FULL | T1 | 3 | 195 | 3 (0)| 00:00:01 | 1 | 3 |
-------------------------------------------------------------------------------------------------
该查询使用的访问方法为PARTITION RANGE ITERATOR,oracle通过在连续的分区中进行迭代来获取数据。pstart
和pstop分别对应了1-3个分区.
4,满足where条件的partition key的数据分布在不连续中的分区中.比如,通过第1,2,4个分区来读数据
dongdongtang>select * from t1 where a<8 or a>13;
A B
---------- -----------------------------------------------------
2 a
7 b
17 c
Execution Plan
----------------------------------------------------------
Plan hash value: 2818491992
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 195 | 3 (0)| 00:00:01 | | |
| 1 | PARTITION RANGE OR| | 3 | 195 | 3 (0)| 00:00:01 |KEY(OR)|KEY(OR)|
|* 2 | TABLE ACCESS FULL| T1 | 3 | 195 | 3 (0)| 00:00:01 |KEY(OR)|KEY(OR)|
-------------------------------------------------------------------------------------------
PARTITION RANGE OR表明where条件是or的,该查询使用了不连续的分区访问法,pstart和pstop没有对应的
分区位置信息.
5,in操作的访问方法
dongdongtang>select * from t1 where a in (2,7,12);
A B
---------- --------------------------------------------
2 a
7 b
12 c
Execution Plan
----------------------------------------------------------
Plan hash value: 594219520
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 195 | 3 (0)| 00:00:01 | | |
| 1 | PARTITION RANGE INLIST| | 3 | 195 | 3 (0)| 00:00:01 |KEY(I) |KEY(I) |
|* 2 | TABLE ACCESS FULL | T1 | 3 | 195 | 3 (0)| 00:00:01 |KEY(I) |KEY(I) |
-----------------------------------------------------------------------------------------------
该执行计划也是需要访问1,2,4个分区,但是执行计划中分区的访问不一样,PARTITION RANGE INLIST也无法通
过pstart和pstop一眼看出分区的起始和结束位置。但是低层也是通过or来转换的.
6,对于where条件中的partition key不存在的情况,这时候CBO直接判定条件无效或是最后一个分区的连界值
来决定结果.
dongdongtang>select * from t1 where a<6 and a>16;
no rows selected
Execution Plan
----------------------------------------------------------
Plan hash value: 1006818538
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 65 | 0 (0)| | | |
|* 1 | FILTER | | | | | | | |
| 2 | PARTITION RANGE EMPTY| | 1 | 65 | 2 (0)| 00:00:01 |INVALID|INVALID|
|* 3 | TABLE ACCESS FULL | T1 | 1 | 65 | 2 (0)| 00:00:01 |INVALID|INVALID|
-----------------------------------------------------------------------------------------------
pstart和pstop都是invalid表示分区无效,PARTITION RANGE EMPTY不需要访问分区.对于非分区表也采用类似的
方法的话需要走索或是全表扫描,而在分区表中,oracle可以直接通过partition的特性可以判断出不存在该条件
,可以大大增加类似查询的速度.
7,对于not in和!=(<>)这种运算很小心,这种运算操作无法使用partition purging
select * from t1 where a not in (2,7)
select * from t1 where a!=7
dongdongtang>select * from t1 where a not in (2,7);
A B
---------- ----------------------------------------------
12 c
17 c
Execution Plan
----------------------------------------------------------
Plan hash value: 589593414
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 130 | 3 (0)| 00:00:01 | | |
| 1 | PARTITION RANGE ALL| | 2 | 130 | 3 (0)| 00:00:01 | 1 | 4 |
|* 2 | TABLE ACCESS FULL | T1 | 2 | 130 | 3 (0)| 00:00:01 | 1 | 4 |
人工可能很容易看出该语句只要访问3,4个分区就可以了,但是从执行中我们还是很容易看出访问了所有的
分区.