停止空想

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 (当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的 histograms ,则每次是不同的  值 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。

Popularity: 22% [?]