Oracle - 创建表没有并行选项

Question

出于某种原因,当我在一个大表(4000万〜记录)上尝试CTAS(创建表为select)时,我在v $ session 18中看到了我的sql语句的活动会话.

当我试图暗示优化器使用更少的CPU时

create table table_name parallel (degree 2) as 
       select * from large_table;

我看到6个活跃的会话.

学位3我看到8个活跃的课程.我尝试了学位默认,但它也创造了18个课程.

以前在代码中,在执行CREATE TABLE语句之前,我在会话中更改了一些属性:

alter session set workarea_size_policy = manual;
alter session set hash_area_size = 1048576000;

我想在1个会话中创建表,我该怎么做？

谢谢 !

Answer 1

在CTAS语句的select子句中使用 NO_PARALLEL提示.另请参见并行提示说明

create table t as
select /*+ NO_PARALLEL */ * 
from large_table;

调查和测试:

SQL*Plus版本和Oracle数据库版本:

SQL*Plus: Release 11.1.0.6.0 - Production on Fri Sep 21 11:56:58 2012

Copyright (c) 1982, 2007, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Data Mining and Real Application Testing options

我使用两个不同的sqlplus进程通过不同的服务名称连接到同一个数据库.一个正在运行的sqlplus副本将用于执行CTAS.sqlplus的另一个副本将用于查询会话数.

在其他任何事情之前获取会话计数和结果的查询:

SQL> select service_name, count(*)
  2  from v$session
  3  where username = 'ME'
  4  group by service_name
  5  order by service_name;

SERVICE_NAME                                                       COUNT(*)
---------------------------------------------------------------- ----------
aaaaaa2                                                                   1
aaaaaa3                                                                   1

创建large_table:

SQL> create table LAO as select * from all_objects;

Table created.

SQL> exec dbms_stats.gather_table_stats(user, 'LAO');

PL/SQL procedure successfully completed.

SQL> create table large_table parallel 4 as
  2  select N.N, LAO.*
  3  from LAO
  4  cross join (select level as N from dual connect by level <= 400) N
  5  /

Table created.

SQL> select to_char(count(*), 'fm999,999,999')
  2  from large_table;

TO_CHAR(COUN
------------
42,695,200

自large_table创建以来parallel 4,一个简单的CTAS默认为4个工作进程,CTAS:

create table t as
select * from large_table;

会议:

SERVICE_NAME                                                       COUNT(*)
---------------------------------------------------------------- ----------
aaaaaa2                                                                   5
aaaaaa3                                                                   1

现在,noparallel表选项.(创建一个默认情况下不会有DML并行计划的表,但并行性large_table导致CTAS并行运行.)

drop table t;
create table t noparallel as
select * from large_table;

会话(SYS $ BACKGROUND显示在最后一个查询的末尾,然后只是停留.我相信如果在一定时间内不需要它会死掉.):

SERVICE_NAME                                                       COUNT(*)
---------------------------------------------------------------- ----------
SYS$BACKGROUND                                                            1
aaaaaa2                                                                   5
aaaaaa3                                                                   1

select子句中的NO_PARALLEL提示确实有效:

drop table t;
create table t as
select /*+ NO_PARALLEL */ * from large_table;

会议:

SERVICE_NAME                                                       COUNT(*)
---------------------------------------------------------------- ----------
SYS$BACKGROUND                                                            1
aaaaaa2                                                                   1
aaaaaa3                                                                   1

最后一个CTAS,其中创建了一个默认使用多个进程的表,但在CTAS期间不会:

drop table t;
create table t parallel 4 as
select /*+ NO_PARALLEL */ * from large_table;

会议:

SERVICE_NAME                                                       COUNT(*)
---------------------------------------------------------------- ----------
SYS$BACKGROUND                                                            1
aaaaaa2                                                                   1
aaaaaa3                                                                   1