Oracle表的并行度
查看dba_tables数据字典时，
可以发现有“DEGREE”字段，这个字段表示的就是数据表的并行度。这个参数的设置，关系着数据库的I/O，以及sql的执行效率。
并行度的优点就是能够最大限度的利用机器的多个cpu资源，是多个cpu同时工作，从而达到提高数据库工作效率的目的。在系统空闲时间，使用并行是个不错的选择，但是好东西总是相对而言，没有绝对的好坏，不当的使用，同样会引起数据库的新的问题产生。
1、 此参数的大小设置
orcl@ SCOTT> select table_name,degree from user_tables;
TABLE_NAME DEGREE
------------------------------ --------------------
T1 1
TAB_REGISTER 1
EMP 1
EMP_BAK 1
SALGRADE 1
BONUS 1
DEPT 1
LETTER_USER 1
T2 1
BASE_LOG 1
T 1
通过上例的例子大家可以观察，此参数的默认值为1，这个数值，我们认为的增加，当设置表的并行度非常高的时候，sql优化器将可能对表进行全表扫描，引起 Direct Path Read 等待 。
在使用并行查询前需要慎重考虑， 因为并行查询尽管能提高程序的响应时间， 但是会
消耗比较多的资源。 对于低配置的数据库服务器需要慎重。 此外， 需要确认并行度的设置要与 IO 系统的配置相符
（建议并行度为 2~4 * CPU 数） 。
2、 并行度的修改
alter table t parallel(degree 1);------直接指定表的并行度
alter table t parallel; ----------设置表的并行度为default
3、 如何在sql语句中使用表的并行度，并选择合适的并行等级
示例：使用并行查询的执行计划
并行度为4
orcl@ SCOTT> SELECT /*+ PARALLEL(4) */
2 MAX(sal),
3 AVG(comm)
4 FROM emp,dept
5 WHERE emp.deptno=dept.deptno
6 GROUP BY 1
7 ;
已用时间: 00: 00: 00.09
执行计划
----------------------------------------------------------
Plan hash value: 1358624651
---------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
---------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 126 | 2 (0)| 00:00:01 | | | |
| 1 | HASH GROUP BY | | 14 | 126 | 2 (0)| 00:00:01 | | | |
| 2 | PX COORDINATOR | | | | | | | | |
| 3 | PX SEND QC (RANDOM)| :TQ10000 | 14 | 126 | 2 (0)| 00:00:01 | Q1,00 | P->S | QC (RAND) |
| 4 | PX BLOCK ITERATOR | | 14 | 126 | 2 (0)| 00:00:01 | Q1,00 | PCWC | |
|* 5 | TABLE ACCESS FULL| EMP | 14 | 126 | 2 (0)| 00:00:01 | Q1,00 | PCWP | |
---------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - filter("EMP"."DEPTNO" IS NOT NULL)
Note
-----
- automatic DOP: computed degree of parallelism is 4
统计信息
----------------------------------------------------------
20 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
482 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed
非并行度
SELECT /*+ no_parallel */ ename, dname FROM emp e, dept d
WHERE e.deptno=d.deptno;
自动并行度
SELECT /*+ parallel(auto) */ ename, dname FROM emp e, dept d
WHERE e.deptno=d.deptno;
4、 并行查询的使用范围
适用于：
大表查询，join,分区索引的查询，
创建大量的index，
创建大量的表（包括固化视图），
批量的insert,update,delete；
查行执行适合场景：
对称多处理器，集群，并行系统，
cpu利用不足，
足够的内存用于其他操作，排序，hash,缓存，
查行执行适合与dss与数据仓库，也适合于批量操作的OLTP系统，不适合OLTP简介的dml或select操作；
并行执行不适合场景：
非常短的查询或事务
基本硬件要求：
并行执行设计需要多个cpu与io来实现快速的查询，每个硬件都应该维持在同一个吞吐量
哪些操作可以用并行
全表查询，分区查询，索引快速查询
join操作
nested loop, sort merge, hash, and star transformation
DDL语句
CREATE TABLE AS SELECT, CREATEINDEX, REBUILDINDEX,
REBUILD INDEX PARTITION,
And MOVE/SPLIT/COALESCEPARTITION
DML语句
INSERT AS SELECT,UPDATE,DELETE,
And MERGE operations
5、 在并行操作中默认情况并行查询和并行DDL操作可以无障碍使用并行，但是如果想使用并行DML，需要先修改dml并行配置
alter session enable parallel dml;
11g new feature:
For a statement-level PARALLEL hint:
■ PARALLEL: The statement always is run parallel, and the database computes the
degree of parallelism, which can be 2 or greater.
■ PARALLEL (DEFAULT): The same as PARALLEL. The DEFAULT keyword is
included for completeness.
■ PARALLEL (AUTO): The database computes the degree of parallelism, which can be
1 or greater. If the computed degree of parallelism is 1, then the statement runs
serially.
■ PARALLEL (MANUAL): The optimizer is forced to use the parallel settings of the
objects in the statement.
■ PARALLEL (integer): The optimizer uses the degree of parallelism specified by
integer.
In the following example, the optimizer calculates the degree of parallelism. The
statement always runs in parallel.
SELECT /*+ PARALLEL */ last_name
FROM employees;
In the following example, the optimizer calculates the degree of parallelism, but that
degree may be 1, in which case the statement will run serially.
SELECT /*+ PARALLEL (AUTO) */ last_name
FROM employees;
11g new feature:
For a statement-level PARALLEL hint:
■ PARALLEL: The statement always is run parallel, and the database computes the
degree of parallelism, which can be 2 or greater.
■ PARALLEL (DEFAULT): The same as PARALLEL. The DEFAULT keyword is
included for completeness.
■ PARALLEL (AUTO): The database computes the degree of parallelism, which can be
1 or greater. If the computed degree of parallelism is 1, then the statement runs
serially.
■ PARALLEL (MANUAL): The optimizer is forced to use the parallel settings of the
objects in the statement.
■ PARALLEL (integer): The optimizer uses the degree of parallelism specified by
integer.
In the following example, the optimizer calculates the degree of parallelism. The
statement always runs in parallel.
SELECT /*+ PARALLEL */ last_name
FROM employees;
In the following example, the optimizer calculates the degree of parallelism, but that
degree may be 1, in which case the statement will run serially.
SELECT /*+ PARALLEL (AUTO) */ last_name
FROM employees;