【11g新特性】Cardinality Feedback基数反馈

Cardinality Feedback基数反馈是版本11.2中引入的关于SQL 性能优化的新特性，该特性主要针对 统计信息陈旧、无直方图或虽然有直方图但仍基数计算不准确的情况， Cardinality基数的计算直接影响到后续的JOIN COST等重要的成本计算评估，造成CBO选择不当的执行计划。以上是Cardinality Feedback特性引入的初衷。

 

 

但是每一个Oracle新版本引入的新特性 都被一些老外DBA称之为buggy ，Cardinality Feedback基数反馈多少也造成了一些麻烦，典型的情况是测试语句性能时，第一次的性能最好，之后再运行其性能变差。

 

我们来看一下 Cardinality Feedback基数反馈是如何作用的：
注意使用普通用户来测试Cardinality Feedback，sys用户被默认禁用该特性

 

 

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> conn maclean/oracle
已连接。

SQL> show parameter dynamic

NAME                                 TYPE                   VALUE
------------------------------------ ---------------------- ------------------------------
optimizer_dynamic_sampling           integer                0

SQL> create table test as select * from dba_tables;

表已创建。

SQL> select /*+ gather_plan_statistics */ count(*) from test;

  COUNT(*)
----------
      2873

SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------
SQL_ID  0p4u1wqwg6t9z, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from test

Plan hash value: 1950795681

-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      1 |00:00:00.01 |     104 |
|   1 |  SORT AGGREGATE    |      |      1 |      1 |      1 |00:00:00.01 |     104 |
|   2 |   TABLE ACCESS FULL| TEST |      1 |   8904 |   2873 |00:00:00.01 |     104 |
-------------------------------------------------------------------------------------

已选择14行。

SQL> select /*+ gather_plan_statistics */ count(*) from test;

  COUNT(*)
----------
      2873

SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------
SQL_ID  0p4u1wqwg6t9z, child number 1
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from test

Plan hash value: 1950795681

-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      1 |00:00:00.01 |     104 |
|   1 |  SORT AGGREGATE    |      |      1 |      1 |      1 |00:00:00.01 |     104 |
|   2 |   TABLE ACCESS FULL| TEST |      1 |   2873 |   2873 |00:00:00.01 |     104 |
-------------------------------------------------------------------------------------

Note
-----
   - cardinality feedback used for this statement

已选择18行。

 

 

上例中第一次运行时，由于未收集表上的统计信息且optimizer_dynamic_sampling=0 关闭了动态采样所以基数评估值(1)和实际值(2873)有着较大的差距。

 

 

cardinality feedback used for this statement这个信息说明第二次执行时使用了Cardinality Feedback基数反馈，且其基数评估也十分精确了，这是因为第二次执行时考虑到第一次执行时的基数反馈，我们来看看Oracle到底是如何做到的：

 

 

 

SQL> alter system flush shared_pool;

系统已更改。

SQL>
SQL> alter session set events '10053 trace name context forever, level 1';

会话已更改。

SQL>  select /*+ gather_plan_statistics */ count(*) from test;

  COUNT(*)
----------
      2873

SQL>  select /*+ gather_plan_statistics */ count(*) from test;

  COUNT(*)
----------
      2873

10053 trace:

第一次执行：

sql= select /*+ gather_plan_statistics */ count(*) from test
----- Explain Plan Dump -----
----- Plan Table -----

============
Plan Table
============
--------------------------------------+-----------------------------------+
| Id  | Operation           | Name    | Rows  | Bytes | Cost  | Time      |
--------------------------------------+-----------------------------------+
| 0   | SELECT STATEMENT    |         |       |       |    31 |           |
| 1   |  SORT AGGREGATE     |         |     1 |       |       |           |
| 2   |   TABLE ACCESS FULL | TEST    |  8904 |       |    31 |  00:00:01 |
--------------------------------------+-----------------------------------+

SELECT /*+ OPT_ESTIMATE (TABLE "TEST" ROWS=2873.000000 ) */ COUNT(*) "COUNT(*)" FROM "MACLEAN"."TEST" "TEST"

SINGLE TABLE ACCESS PATH 
  Single Table Cardinality Estimation for TEST[TEST] 
  Table: TEST  Alias: TEST
    Card: Original: 8904.000000    >> Single Tab Card adjusted from:8904.000000 to:2873.000000
  Rounded: 2873  Computed: 2873.00  Non Adjusted: 8904.00
  Access Path: TableScan
    Cost:  31.10  Resp: 31.10  Degree: 0
      Cost_io: 31.00  Cost_cpu: 1991217
      Resp_io: 31.00  Resp_cpu: 1991217
  Best:: AccessPath: TableScan
         Cost: 31.10  Degree: 1  Resp: 31.10  Card: 2873.00  Bytes: 0

sql= select /*+ gather_plan_statistics */ count(*) from test
----- Explain Plan Dump -----
----- Plan Table -----

============
Plan Table
============
--------------------------------------+-----------------------------------+
| Id  | Operation           | Name    | Rows  | Bytes | Cost  | Time      |
--------------------------------------+-----------------------------------+
| 0   | SELECT STATEMENT    |         |       |       |    31 |           |
| 1   |  SORT AGGREGATE     |         |     1 |       |       |           |
| 2   |   TABLE ACCESS FULL | TEST    |  2873 |       |    31 |  00:00:01 |
--------------------------------------+-----------------------------------+

 

 

 

 

可以看到第二次执行时SQL最终转换加入了 OPT_ESTIMATE (TABLE “TEST” ROWS=2873.000000 )的HINT ，OPT_ESTIMATE HINT一般由 kestsaFinalRound()内核函数生成。该HINT用以纠正各种类型的优化器评估，例如某表上的基数或某个列的最大、最小值。反应出优化的不足或者BUG。

 

可以通过V$SQL_SHARED_CURSOR和来找出现有系统shared pool中仍存在的 使用了Cardinality Feedback基数反馈的子游标：

 

 

SQL> select sql_ID,USE_FEEDBACK_STATS  FROM V$SQL_SHARED_CURSOR where USE_FEEDBACK_STATS ='Y';

SQL_ID                     US
-------------------------- --
159sjt1f6khp2              Y

 

 

 

还可以使用cardinality HINT来强制使用Cardinality Feedback 。

select /*+ cardinality(test,  1) */ count(*) from test;

 

 

如何禁用Cardinality Feedback基数反馈

 

对于这些”惹火”特性，为了stable，往往考虑关闭该特性。

可以通过多种方法禁用该特性

1. 使用 _optimizer_use_feedback 隐藏参数

session 级别

SQL> alter session set “_optimizer_use_feedback”=false;

会话已更改。

system级别

SQL> alter system set “_optimizer_use_feedback”=false;

系统已更改。

 

2. 使用opt_param(‘_optimizer_use_feedback’ ‘false’) HINT

例如：

select /*+ opt_param(‘_optimizer_use_feedback’ ‘false’) cardinality(test,1) */ count(*) from test;