计算一个语句产生的undo

归档模式下表为nologging状态，且使用/*+ append*/  insert时会提高效率。
为什么呢，因为会减少redo和undo的产生。

 

如何计算一个insert产生了多少undo呢，通过对比当前session执行insert前后

undo的数量可以知道。

 

Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.4.0

SQL> select  sid  from v$mystat  where rownum=1;
       SID
----------
       312

 

SQL>  select value  "undo change vector size"
  2   from v$statname a, v$sesstat b
  3   where a.statistic# = b.statistic#
  4   and a.name = 'undo change vector size'
  5   and b.sid = '312' ;

undo change vector size
-----------------------
                      0

SQL>  create table t  as select  rownum from  user_objects where rownum<100;

create table t  as select  rownum from  user_objects where rownum<100

ORA-00998: must name this expression with a column alias

-- 显示给rownum 指定个别名就好了

 

SQL> create table t   as select  rownum  a from  user_objects where rownum<100;
Table created

 

SQL> select value  "undo change vector size"
  2   from v$statname a, v$sesstat b
  3   where a.statistic# = b.statistic#
  4   and a.name = 'undo change vector size'
  5   and b.sid = '312' ;

undo change vector size
-----------------------
                   3128

 

SQL> insert   /*+ append */ into t   select  rownum a from  user_objects where rownum<100;

99 rows inserted

SQL>
SQL>  select value  "undo change vector size"
  2   from v$statname a, v$sesstat b
  3   where a.statistic# = b.statistic#
  4   and a.name = 'undo change vector size'
  5   and b.sid = '312' ;

undo change vector size
-----------------------
                   3204

 

--计算采用/*+ append */ 模式插入100条记录产生undo的数量

SQL> select 3204 - 3128 d from dual;

         D
----------
        76

 

SQL> insert   into t   select  rownum a from  user_objects where rownum<100;

99 rows inserted

SQL> commit;

Commit complete

SQL>  select value  "undo change vector size"
  2   from v$statname a, v$sesstat b
  3   where a.statistic# = b.statistic#
  4   and a.name = 'undo change vector size'
  5   and b.sid = '312' ;

undo change vector size
-----------------------
                   3920

 

--计算采用正常模式插入100条记录产生undo的数量

SQL> select 3920 -  3204 d from dual;

         D
----------
       716

结果表明插入同样的记录，采用/*+ append*/ 方式产生的undo会更少一些。