【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。

 

Cardinality Feedback2

Cardinality Feedback1

 

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;

 

 

 

  1. Cardinality feedback was introduced in Oracle Database 11g, Release 2.
    Cardinality feedback is useful for queries where the data volume being processed is stable over time.
    During query execution optimizer estimates are compared to execution statistic: if they vary significantly then a new plan will be chosen for subsequent executions
    Note: CURSOR_SHARING parameter is either EXACT or FORCE.