游标共享之cursor_sharing=EXACT

cursor sharing用来告诉oracle什么情况下可以共享游标，即SQL重用。oracle默认cursor_sharing 是exact 指的是SQL语句必须绝对一样的情况下才能共享游标，否则作为新的SQL语句处理。

适合场景：使用exact 高效的前提是应用代码中使用了绑定变量，是最好的实践方式，这也是oracle推荐的。

参数还有两种选择：

游标共享之cursor_sharing=similar

游标共享之cursor_sharing=force

SQL> Drop table t purge;

表已删除。

SQL> show parameter cursor_sharing;
NAME                                                                  TYPE        VALUE
------------------------------------                            ----------- ------------------------------
_optimizer_extended_cursor_sharing          string      UDO
cursor_sharing                                                  string      EXACT


SQL> create table t as select * from dba_objects;
SQL> alter system flush shared_pool;

SQL> select /*+test_exact*/count(1) from t where object_id=100;
  COUNT(1)
----------
         1
SQL> select /*+test_exact*/count(1) from t where object_id=200;
  COUNT(1)
----------
         1

SQL> col sql_text format a80;

----可以看到谓词不同则产生两条解析的SQL

SQL> select sql_text from v$sql where sql_text like '%/*+test_exact*/%'   and  sql_text not like '%sql_text%';
SQL_TEXT
--------------------------------------------------------------------------------
select /*+test_exact*/count(1) from t where object_id=100
select /*+test_exact*/count(1) from t where object_id=200

SQL> alter system flush shared_pool;
SQL> var x number;
SQL> exec :x:=100;
SQL> select /*+test_exact*/count(1) from t where object_id=:x;
  COUNT(1)
----------
         1

SQL> exec :x:=200;
SQL> select /*+test_exact*/count(1) from t where object_id=:x;
  COUNT(1)
----------
         1
----在使用绑定变量的情况下只产生一次硬解析
SQL> select sql_text from v$sql where sql_text like '%/*+test_exact*/%'   and  sql_text not like '%sql_text%';
SQL_TEXT
--------------------------------------------------------------------------------
select /*+test_exact*/count(1) from t where object_id=:x