Oracle 自适应游标共享--adaptive cursor sharing - 数据库编程

自适应游标共享功能的引入，可以有效的解决这个问题。

首先看一下我们的测试环境：

SQL> desc acs_test_tab
名称是否为空类型
----------------------------------------------------- -------- ------------------------------------
ID NOT NULL NUMBER
RECORD_TYPE NUMBER
DESCRIPTION VARCHAR2(50)

SQL> select count(*) from acs_test_tab;

COUNT(*)
----------
100000

SQL> select count(*) from acs_test_tab where record_type=2;

COUNT(*)
----------
50000

SQL> select count(distinct record_type) from acs_test_tab;

COUNT(DISTINCTRECORD_TYPE)
--------------------------
50001

表acs_test_Tab在列record_type上分布式是倾斜的。收集统计信息：

SQL> exec dbms_stats.gather_Table_Stats(user,'acs_test_Tab',cascade=>true,method_opt=>'for all columns size auto');

PL/SQL 过程已成功完成。

SQL> select column_name,histogram from user_tab_cols where table_name='ACS_TEST_TAB';

COLUMN_NAME HISTOGRAM
------------------------------ ---------------
ID NONE
RECORD_TYPE HEIGHT BALANCED
DESCRIPTION NONE

首先我们对record_type 为1 的列进行查询

SQL> select count(*) from acs_test_tab where record_type = 1;

COUNT(*)
----------
1

SQL> alter system flush shared_pool;

系统已更改。

SQL> var v number;
SQL> exec :v := 1

PL/SQL 过程已成功完成。

SQL> select sum(id) from acs_test_tab where record_type = :v;

SUM(ID)
----------
1

SQL> select * from table(dbms_xplan.display_cursor);

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 3p66zbwtm19bs, child number 0
-------------------------------------
select sum(id) from acs_test_tab where record_type = :v

Plan hash value: 3987223107

-----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 4 (100)| |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| ACS_TEST_TAB | 1 | 9 | 4 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | ACS_TEST_TAB_RECORD_TYPE_I | 1 | | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

3 - access("RECORD_TYPE"=:V)

已选择20行。

SQL> select child_number,executions,buffer_gets,is_bind_sensitive,is_bind_aware
2 from v$sql
3 where sql_text like 'select sum(id)%';

CHILD_NUMBER EXECUTIONS BUFFER_GETS I I
------------ ---------- ----------- - -
0 1 218 Y N

下面我们在查询一下record_type为2的记录，

SQL> exec :v := 2

PL/SQL 过程已成功完成。

SQL> select sum(id) from acs_test_tab where record_type = :v;

SUM(ID)
----------
2500050000

SQL> select * from table(dbms_xplan.display_cursor);

Plan hash value: 3987223107

-----------------------------------------------------------------------------

Oracle 自适应游标共享--adaptive cursor sharing(一)