数据仓库中的分区修剪
Partition Pruning
在数据仓库中分区修剪是一种非常有效的性能特性。分析修剪分析SQL中的WHERE 和FROM字句，从而在查询中消除不不必要分区。分区修剪技术能大大的减少从磁盘上读取的数据量，从而缩短运行时间，改善查询性能，减少资源浪费。即使你的索引分区和表分区不同，分区修剪也可以在索引上生效（global partition index），从而消除不必要的索引分区。
分区修剪的特性依赖SQL语句，Oracle 有两种分区修剪：动态修剪和静态修剪。静态修剪发生在编译时期，在执行计划指定的时候，已经知道那些分区会被使用。而动态修剪发生在运行时，也就是说在运行的时候，才会知道那些分区会被用到。例如，WHERE字句里面包含一个函数或者子查询用于返回分区键的值。
Information That Can Be Used for Partition Pruning
Oracle分区修剪在你使用range，like，=，inlist等谓词在range或者list分区的时候生效，以及使用=和inlist谓词在hash 分区时。
对于复合分区对象，Oracle能在每个level都实现分区修剪。例如下面的SQL， 表sales_range_hash按字段s_saledate做范围分区，按s_productid字段做hash子分区：
CREATE TABLE sales_range_hash(
s_productid NUMBER,
s_saledate DATE,
s_custid NUMBER,
s_totalprice NUMBER)
PARTITION BY RANGE (s_saledate)
SUBPARTITION BY HASH (s_productid) SUBPARTITIONS 8
(PARTITION sal99q1 VALUES LESS THAN
(TO_DATE('01-APR-1999', 'DD-MON-YYYY')),
PARTITION sal99q2 VALUES LESS THAN
(TO_DATE('01-JUL-1999', 'DD-MON-YYYY')),
PARTITION sal99q3 VALUES LESS THAN
(TO_DATE('01-OCT-1999', 'DD-MON-YYYY')),
PARTITION sal99q4 VALUES LESS THAN
(TO_DATE('01-JAN-2000', 'DD-MON-YYYY')));
SELECT * FROM sales_range_hash
WHERE s_saledate BETWEEN (TO_DATE('01-JUL-1999', 'DD-MON-YYYY'))
AND (TO_DATE('01-OCT-1999', 'DD-MON-YYYY')) AND s_productid = 1200;
Oracle的分区修剪过程如下：
Oracle访问partitions sal99q2 和 sal99q3
Oracle访问子partition 通过s_productid=1200
How to Identify Whether Partition Pruning has been Used
在EXPAIN PLAN中可以看出分区修剪是否生效。查看PLAN TABLE的字段PSTART (PARTITION_START) and PSTOP (PARTITION_STOP)。
Static Partition Pruning
大多情况下，Oracle在编译的时候判断分区的访问方式。当你使用静态的谓词的时候即发生静态分区，除了下面这些情况：
分区修剪的条件来至一个子查询的结果
优化器利用星型转换重写了查询，而分区修剪发生在转换以后
最有效的执行计划是一个NESTED LOOP
这三种情况其实就是动态修剪。
请看下面的例子:
SQL> explain plan for select * from sales where time_id = to_date('01-jan-2001', 'dd-mon-yyyy');
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------
Plan hash value: 3971874201
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 673 | 19517 | 27 (8)| 00:00:01 | | |
| 1 | PARTITION RANGE SINGLE| | 673 | 19517 | 27 (8)| 00:00:01 | 17 | 17 |
|* 2 | TABLE ACCESS FULL | SALES | 673 | 19517 | 27 (8)| 00:00:01 | 17 | 17 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("TIME_ID"=TO_DATE('2001-01-01 00:00:00', 'yyyy-mm-dd hh24:mi:ss'))
执行计划显示Oracle访问的分区号为17（PSTART 和 PSTOP）。有一点例外的是，执行计划在显示对一个间隔分区的全表扫描时候，PSTART为1，PSTOP为1048575，而不是实际的分区数量。
Dynamic Partition Pruning
动态分区发生在如果静态分区修剪无法生效的时：
Dynamic Pruning with Bind Variables
使用绑定变量会发生分区修剪. 例如:
SQL> explain plan for select * from sales s where time_id in ( :a, :b, :c, :d);
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------
Plan hash value: 513834092
---------------------------------------------------------------------------------------------------
| Id | Operation | Name |Rows|Bytes|Cost (%CPU)| Time | Pstart| Pstop|
---------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | |2517|72993| 292 (0)|00:00:04| | |
| 1 | INLIST ITERATOR | | | | | | | |
| 2 | PARTITION RANGE ITERATOR | |2517|72993| 292 (0)|00:00:04|KEY(I) |KEY(I)|
| 3 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES |2517|72993| 292 (0)|00:00:04|KEY(I) |KEY(I)|
| 4 | BITMAP CONVERSION TO ROWIDS | | | | | | | |
|* 5 | BITMAP INDEX SINGLE VALUE |SALES_TIME_BIX| | | | |KEY(I) |KEY(I)|
---------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("TIME_ID"=:A OR "TIME_ID"=:B OR "TIME_ID"=:C OR "TIME_ID"=:D)
对于并行执行计划来说，只有分区START和STOP字段包含分区修剪信息。Operation字段包含的是并行操作的信息，如下例子：
SQL> explain plan for select * from sales where time_id in (:a, :b, :c, :d);
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------
Plan hash value: 4058105390
-------------------------------------------------------------------------------------------------
| Id| Operation | Name |Rows|Bytes|Cost(%CP| Time |Pstart| Pstop| TQ |INOUT| PQ Dis|
-------------------------------------------------------------------------------------------------
| 0| SELECT STATEMENT | |2517|72993| 75(36)|00:00:01| | | | | |
| 1| PX COORDINATOR | | | | | | | | | | |
| 2| PX SEND QC(RANDOM)|:TQ10000|2517|72993| 75(36)|00:00:01| | |Q1,00| P->S|QC(RAND|
| 3| PX BLOCK ITERATOR| |2517|72993| 75(36)|00:00:01|KEY(I)|KEY(I)|Q1,00| PCWC| |
|* 4| TABLE ACCESS FULL| SALES |2517|72993| 75(36)|00:00:01|KEY(I)|KEY(I)|Q1,00| PCWP| |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - filter("TIME_ID"=:A OR "TIME_ID"=:B OR "TIME_ID"=:C OR "TIME_ID"=:D)
Dynamic Pruning with Subqueries
子查询使用动态修剪的例子:
SQL> explain plan for select sum(amount_sold) from sales where time_id in
(select time_id from times where fiscal_year = 2000);
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Plan hash value: 3827742054
----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 25 | 523 (5)| 00:00:07 | | |
| 1 | SORT AGGREGATE | | 1 | 25 | | | | |
|* 2 | HASH JOIN | | 191K| 4676K| 523 (5)| 00:00:07 | | |
|* 3 | TABLE ACCESS FULL | TIMES | 304 | 3648 | 18 (0)| 00:00:01 | | |
| 4 | PARTITION RANGE SUBQUERY| | 918K| 11M| 498 (4)| 00:00:06 |KEY(SQ)|KEY(SQ)|
| 5 | TABLE ACCESS FULL | SALES | 918K| 11M| 498 (4)| 00:00:06 |KEY(SQ)|KEY(SQ)|
----------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("TIME_ID"="TIME_ID")
3 - filter("FISCAL_YEAR"=2000)
Dynamic Pruning with Star Transformation
星型转换和分区修剪的例子:
SQL> explain plan for select p.prod_name, t.time_id, sum(s.amount_sold)
from sales s, times t, products p
where s.time_id = t.time_id and s.prod_id = p.prod_id and t.fiscal_year = 2000
and t.fiscal_week_number = 3 and p.prod_category = 'Hardware'
group by t.time_id, p.prod_name;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------
Plan hash value: 4020965003
------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Pstart| Pstop |
------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 79 | | |
| 1 | HASH GROUP BY | | 1 | 79 | | |
|* 2 | HASH JOIN | | 1 | 79 | | |
|* 3 | HASH JOIN | | 2 | 64 | | |
|* 4 | TABLE ACCESS FULL | TIMES | 6 | 90 | | |
| 5 | PARTITION RANGE SUBQUERY | | 587 | 9979 |KEY(SQ)|KEY(SQ)|
| 6 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES | 587 | 9979 |KEY(SQ)|KEY(SQ)|
| 7 | BITMAP CONVERSION TO ROWIDS | | | | | |
| 8 | BITMAP AND | | | | | |
| 9 | BITMAP MERGE | | | | | |
| 10 | BITMAP KEY ITERATION | | | | | |
| 11 | BUFFER SORT | | | | | |
|* 12 | TABLE ACCESS FULL | TIMES | 6 | 90 | | |
|* 13 | BITMAP INDEX RANGE SCAN | SALES_TIME_BIX | | |KEY(SQ)|KEY(SQ)|
| 14 | BITMAP MERGE | | | | | |
| 15 | BITMAP KEY ITERATION | | | | | |
| 16 | BUFFER SORT | | | | | |
| 17 | TABLE ACCESS BY INDEX ROWID| PRODUCTS | 14 | 658 | | |
|* 18 | INDEX RANGE SCAN | PRODUCTS_PROD_CAT_IX | 14 | | | |
|* 19 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX | | |KEY(SQ)|KEY(SQ)|
| 20 | TABLE ACCESS BY INDEX ROWID | PRODUCTS | 14 | 658 | | |
|* 21 | INDEX RANGE SCAN | PRODUCTS_PROD_CAT_IX | 14 | | | |
------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("S"."PROD_ID"="P"."PROD_ID")
3 - access("S"."TIME_ID"="T"."TIME_ID")
4 - filter("T"."FISCAL_WEEK_NUMBER"=3 AND "T"."FISCAL_YEAR"=2000)
12 - filter("T"."FISCAL_WEEK_NUMBER"=3 AND "T"."FISCAL_YEAR"=2000)
13 - access("S"."TIME_ID"="T"."TIME_ID")
18 - access("P"."PROD_CATEGORY"='Hardware')
19 - access("S"."PROD_ID"="P"."PROD_ID")
21 - access("P"."PROD_CATEGORY"='Hardware')
Note
-----
- star transformation used for this statement
Dynamic Pruning with Nested Loop Joins
NESTED LOOP JOIN和分区修剪的例子:
SQL> explain plan for select t.time_id, sum(s.amount_sold)
from sales s, times t
where s.time_id = t.time_id and t.fiscal_year = 2000 and t.fiscal_week_number = 3
group by t.time_id;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Plan hash value: 50737729
----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 6 | 168 | 126 (4)| 00:00:02 | | |
| 1 | HASH GROUP BY | | 6 | 168 | 126 (4)| 00:00:02 | | |
| 2 | NESTED LOOPS | | 3683 | 100K| 125 (4)| 00:00:02 | | |
|* 3 | TABLE ACCESS FULL | TIMES | 6 | 90 | 18 (0)| 00:00:01 | | |
| 4 | PARTITION RANGE ITERATOR| | 629 | 8177 | 18 (6)| 00:00:01 | KEY | KEY |
|* 5 | TABLE ACCESS FULL | SALES | 629 | 8177 | 18 (6)| 00:00:01 | KEY | KEY |
----------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("T"."FISCAL_WEEK_NUMBER"=3 AND "T"."FISCAL_YEAR"=2000)
5 - filter("S"."TIME_ID"="T"."TIME_ID")
