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2011年(148)

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分类: Oracle

2011-08-17 15:28:35

cursor_sharing = similar


我们先看看在表没有分析无统计数据情况下的表现 

SQL> alter session set cursor_sharing = similar;
 
Session altered.

SQL> select name,value from v$sysstat where name like '%parse%';
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170148
parse count (hard)                                                     1619  (硬分析次数)

parse count (failures)                                                   80
 
SQL> select count(*) from t where object_id = 1000;
 
   COUNT(*)
----------
          0
 
SQL> select name,value from v$sysstat where name like '%parse%';
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170172
parse count (hard)                                                     1620
parse count (failures)                                                   80
 
SQL> /
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170176
parse count (hard)                                                     1620
parse count (failures)                                                   80
 
SQL> select count(*) from t where object_id = 1000;

 
   COUNT(*)
----------
          0
 
SQL> select name,value from v$sysstat where name like '%parse%';
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170178
parse count (hard)                                                     1620
parse count (failures)                                                   80
 
SQL> select count(*) from t where object_id = 1001;
 
   COUNT(*)
----------
          0
 
SQL> select name,value from v$sysstat where name like '%parse%';
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170180
parse count (hard)                                                     1620(即使object_id发生变化依然没有硬解析
)
parse count (failures)                                                   80

 

 


 

我们再来看分析表和字段信息后的表现

SQL> analyze table t1 compute statistics for table for columns object_id;
 
Table analyzed.
 
SQL> select name,value from v$sysstat where name like '%parse%';
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4973
parse time elapsed                                                     4495
parse count (total)                                                  170982
parse count (hard)                                                     1640

parse count (failures)                                                   80
 

SQL> select count(*) from t1 where object_id = 5000;

 
   COUNT(*)
----------
          0
 
SQL> select name,value from v$sysstat where name like '%parse%';
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4973
parse time elapsed                                                     4495
parse count (total)                                                  170984
parse count (hard)                                                     1641

parse count (failures)                                                   80
 
SQL> select count(*) from t1 where object_id = 5000;
 
   COUNT(*)
----------
          0
 
SQL> select name,value from v$sysstat where name like '%parse%';
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4973
parse time elapsed                                                     4495
parse count (total)                                                  171008
parse count (hard)                                                     1641 (重复执行没发生变化)

parse count (failures)                                                   80
 
SQL> select count(*) from t1 where object_id = 5001;
 
   COUNT(*)
----------
          0
 
SQL> select name,value from v$sysstat where name like '%parse%';
 
NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4973
parse time elapsed                                                     4495
parse count (total)                                                  171010
parse count (hard)                                                     1642 (object_id变化的时候产生硬分析)

parse count (failures)                                                   80
 
SQL>

SQL> select sql_text,child_number from v$sql where sql_text like 'select count(*) from t1 where%';
 
SQL_TEXT
--------------------------------------------------------------------------------
CHILD_NUMBER
------------
select count(*) from t1 where object_id = :"SYS_B_0"
            0
 
select count(*) from t1 where object_id = :"SYS_B_0"
            1

        可以看出若存在object_id histograms ,则每次是不同的  的时候都产生硬解析 ,若不存在 histograms ,则不产生硬解析 。换句话说,当表的字段被分析过存在histograms的时候,similar 的表现和exact一样,当表的字段没被分析不存在histograms的时候,similar的表现和force一样。这样避免了一味地如force一样转换成变量形式,因为有hostograms的情况下转换成变量之后就容易产生错误的执行计划,没有利用上统计信息。而exact呢,在没有hostograms的情况下也要分别产生硬解析,这样的话,由于执行计划不会受到数据分布的影响(因为没有统计信息)重新解析是没有实质意义的。而similar则综合了两者的优点。

 

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