CTAS VS create table and then insert

很多情况下我们都会需要复制源表数据以达到冗余数据的目的，那么到底是使用CREATE TABLE AS SELECT的CTAS方式，还是先建好表的结构然后再插入数据好呢？我们来看看这2种方式的不同表现：

SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE    10.2.0.4.0      Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - ProductionSQL> archive log list;
Database log mode              Archive Mode
Automatic archival             Enabled
Archive destination            USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence     1
Next log sequence to archive   2
Current log sequence           2/* 数据库处于归档模式下 */SQL> select ss.name,ms.value  from v$mystat ms,v$sysstat ss2  where3  ms.statistic#=ss.statistic#4  and ss.name in ('undo change vector size','redo size');
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                                 0
undo change vector size                                                   0SQL> create table YOUYUS as select * from dba_objects;
Table created.SQL> select ss.name,ms.value  from v$mystat ms,v$sysstat ss2  where3  ms.statistic#=ss.statistic#4  and ss.name in ('undo change vector size','redo size');
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                           5783384
undo change vector size                                               15408

/* CTAS方式产生了少量的undo，可以猜想其使用直接路径方式插入，Oracle仅产生维护数据字典的undo */SQL> drop table YOUYUS;
Table dropped.SQL> conn / as sysdba
Connected./* 清理现场 */SQL> create table YOUYUS as select * from dba_objects where 0=1;
Table created.SQL> select ss.name,ms.value  from v$mystat ms,v$sysstat ss2  where3  ms.statistic#=ss.statistic#4  and ss.name in ('undo change vector size','redo size');NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                             19492
undo change vector size                                                5680/* 建立空表YOUYUS，同样需要维护数据字典  */SQL> insert into YOUYUS select * from dba_objects;50729 rows created.SQL> commit;Commit complete.SQL> select ss.name,ms.value  from v$mystat ms,v$sysstat ss2  where3  ms.statistic#=ss.statistic#4  and ss.name in ('undo change vector size','redo size');NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                           5743540
undo change vector size                                              203904/* 普通insert操作产生了远大于CTAS的undo */SQL> drop table YOUYUS;
Table dropped.SQL> conn / as sysdba
Connected.SQL> select ss.name,ms.value  from v$mystat ms,v$sysstat ss2  where3  ms.statistic#=ss.statistic#4  and ss.name in ('undo change vector size','redo size');NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                                 0
undo change vector size                                                   0SQL> create table YOUYUS as select * from dba_objects where 0=1;
Table created.SQL> insert /*+ append */ into YOUYUS select * from dba_objects;
50729 rows created.SQL> commit;Commit complete.SQL> select ss.name,ms.value  from v$mystat ms,v$sysstat ss2  where3  ms.statistic#=ss.statistic#4  and ss.name in ('undo change vector size','redo size');
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                           5781712
undo change vector size                                               14808/* 建表后直接路径插入方式与CTAS产生的redo和undo数量大致相仿 */

从资源消耗的角度讲CTAS或直接路径插入方式有一定优势，如果启用nologging选项的话这种优势会更加显著。

转载于:https://www.cnblogs.com/macleanoracle/archive/2010/09/07/2967529.html

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