OLTP系统最主要的核心还是数据的录入操作，而这些应用的场景并不适合于并行计算的方式。对于OLAP业务场景更适合并行的操作，并行也成为数据库仓库调优利器；
【使用类型和场景】
Oracle并行处理（Parallel Processing）特性主要是针对SQL语句处理的并行。目前Oracle提供支持并行的操作包括如下类型：
- 并行查询操作；
- 并行DDL，对数据对象的DDL操作；
- 并行DML，进行并行的数据更新修改；
在具体的应用场景上，有如下场景：
- 数据表、索引范围扫描的查询；
- 批量增加、修改和删除操作；
- 数据表、索引对象的创建；
- "DBMS_STATS"收集统计量；
-
备份恢复管理器Recovery Manager的使用；
【设置并行和并行度】
- Alter session force parallel query parallel n;（ALTER SESSION enable parallel query;） 单个SESSION里面
- Alter table tab1 parallel n; 单个TABLE
- Select /*+parallel(tab n)*/ from tab; Hint设置单个SQL
优先级：Hint > session > object
【并行度设置】并行度的设置是以多核CPU为核心的，所以并行度不能超过CPU的逻辑数量，Oracle官方文档介绍如下
If the PARALLEL clause is specified but no degree of parallelism is listed, the object gets the default DOP. Default parallelism uses a formula to determine the DOP based on the system configuration, as in the following:
- For a single instance, DOP = PARALLEL_THREADS_PER_CPU x CPU_COUNT
- For an Oracle RAC configuration, DOP = PARALLEL_THREADS_PER_CPU x CPU_COUNT x INSTANCE_COUNT
【实验测试】
环境说明：
数据库信息：
SQL> select * from v$version where rownum<2;
BANNER
-----------------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
CPU信息
SQL> show parameter cpu
NAME TYPE VALUE
------------------------------------ ----------- -------------
cpu_count integer 8
parallel_threads_per_cpu integer 1
resource_manager_cpu_allocation integer 8
【实验步骤】
正常情况下的执行计划
SQL> alter session set STATISTICS_LEVEL=ALL;
Session altered.
SQL> select count(1) from edidc;
COUNT(1)
----------
47262191
SQL> select * from table(dbms_xplan.DISPLAY_CURSOR(null, null, 'ALLSTATS'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID 8h0snkxwm7x0w, child number 0
-------------------------------------
select count(1) from sapsr3.edidc
Plan hash value: 2138151876
----------------------------------------------------------------------------------------------------------------------
Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:15.52| 351K|
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:15.52| 351K|
| 2 | INDEX FAST FULL SCAN | EDIDC~1 | 1 | 46M | 47M |00:00:09.95| 351K|
--------------------------------------------------------------------------------------------------------------------
修改session的方式
SQL> alter session set STATISTICS_LEVEL=ALL;
SQL> Alter session force parallel query;
SQL> select count(1) from edidc;
COUNT(1)
----------
47262191
SQL> select * from table(dbms_xplan.DISPLAY_CURSOR(null, null, 'ALLSTATS'));
-------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |
-------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | |
| 1 | SORT AGGREGATE | | 1 |
| 2 | PX COORDINATOR | | |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 |
| 4 | SORT AGGREGATE | | 1 |
| 5 | PX BLOCK ITERATOR | | 46M |
|* 6 | INDEX FAST FULL SCAN| EDIDC~1 | 46M |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
----------------------------------------------------------------------------
6 - access(:Z>=:Z AND :Z<=:Z)
------------------------------------------------------
用hint的方式进行
SQL> select /*+ PARALLEL(4) */ count(1) from edidc;
COUNT(1)
----------
47262191
SQL> select * from table(dbms_xplan.DISPLAY_CURSOR(null, null, 'ALLSTATS'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID gr4rp3q9c4qu3, child number 0
-------------------------------------
select /*+ PARALLEL(4) */ count(1) from sapsr3.edidc
Plan hash value: 152749150
-------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads |
-------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:13.86 | 25 | 0 |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:13.86 | 25 | 0 |
| 2 | PX COORDINATOR | | 1 | | 4 |00:00:13.86 | 25 | 0 |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 0 | 1 | 0 |00:00:00.01 | 0 | 0 |
| 4 | SORT AGGREGATE | | 4 | 1 | 4 |00:00:52.76 | 357K | 351K |
| 5 | PX BLOCK ITERATOR | | 4 | 46M | 47M |00:00:41.69 | 357K | 351K |
|* 6 | INDEX FAST FULL SCAN | EDIDC~1 | 136 | 46M | 47M |00:00:20.42 | 357K | 351K |
--------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
6 - access(:Z>=:Z AND :Z<=:Z)
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
修改table的方式
SQL> ALTER TABLE edidc PARALLEL 2; #设置完成后需要更新统计信息
SQL> SELECT TABLE_NAME, degree FROM dba_tables WHERE TABLE_NAME='EDIDC';
TABLE_NAME DEGREE
------------------------------ ------------------------------
EDIDC 2
SQL> alter session set STATISTICS_LEVEL=ALL;
Session altered.
SQL> select count(1) from edidc;
COUNT(1)
----------
47262191
SQL> select * from table(dbms_xplan.DISPLAY_CURSOR(null, null, 'ALLSTATS'));
--------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads |
-------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:12.93 | 25 | 0 |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:12.93 | 25 | 0 |
| 2 | PX COORDINATOR | | 1 | | 8 |00:00:12.93 | 25 | 0 |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 0 | 1 | 0 |00:00:00.01 | 0 | 0 |
| 4 | SORT AGGREGATE | | 8 | 1 | 8 |00:01:39.17 | 359K| 351K|
| 5 | PX BLOCK ITERATOR | | 8 | 46M | 47M|00:01:23.50 | 359K| 351K|
|* 6 | INDEX FAST FULL SCAN | EDIDC~1 | 164 | 46M| 47M|00:00:54.01 | 359K| 351K|
------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
6 - access(:Z>=:Z AND :Z<=:Z)
PLAN_TABLE_OUTPUT
---------------------------------------------------
SQL>ALTER TABLE TABLE_NAME NOPARALLEL; 取消并行
以上是针对查询的操作，同样也是适用于DML操作的；
并行的使用并是简单的以上的几个语句的套用，在OLTP系统中使用需谨慎，用好了是一把调优的利器，用不好可能反而会被利器所伤。本文档也是是针对并行的一篇基础文章，后续会针对更深入的应用继续说明。