關於 cursor_sharing = similar

ZT

http://hi.baidu.com/slash_gao/blog/item/c96db24332e558179313c611.html

我們先看看在表沒有分析無統計資料情況下的表現

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 (硬分析次數)
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發生變化依然沒有硬解析)
parse count (failures)                                                   80

我們再來看分析表和欄位資訊後的表現

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 (重複執行沒發生變化)
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 (當object_id變化的時候產生硬分析)
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 ,則每次是不同的 值 的時候都產生硬解析 ,若不存在 histograms ，則不產生硬解析 。換句話說，當表的欄位被分析過存在histograms的時候，similar 的表現和exact一樣，當表的欄位沒被分析不存在histograms的時候,similar的表現和force一樣。這樣避免了一味地如force一樣轉換成變數形式，因為有hostograms的情況下轉換成變數之後就容易產生錯誤的執行計劃，沒有利用上統計資訊。而exact呢，在沒有hostograms的情況下也要分別產生硬解析，這樣的話，由於執行計劃不會受到資料分佈的影響（因為沒有統計資訊）重新解析是沒有實質意義的。而similar則綜合了兩者的優點。