Oracle优化器之基数反馈（CardinalityFeedback）功能

概述

在Oracle 11gR2的版本上推出了基数反馈（Cardinality Feedback 以后简称CFB）功能，通过这个特性，对于某些查询在第一次执行时，如果CBO发现根据统计信息估算出的基数(Computed cardinality)和SQL执行时的实际值差距很大的情况发生时，在SQL下次执行时，会根据实际值调整基数，重新生成执行计划。

另外，基数反馈（CFB）在12c版本上得到更进一步的扩展改称为统计反馈（Statistics Feedback），成为12c自动重新优化（Automatic Reoptimization）的一部分。
关于这统计反馈（Statistics Feedback）中扩展的内容和12c自动重新优化（Automatic Reoptimization）的内容，将在以后的章节中进行介绍。

下面我们将通过几个例子来了解一下CFB功能。

例子1（CFB无效）

首先我们在10.2.0.5的环境中也就是CFB无效的情况下，看看执行的情况：
（我们使用了Oracle数据库提供的样例Schema OE 及其表PRODUCT_INFORMATION和ORDER_ITEMS进行测试。）

1.首先确认相关表的统计信息和表的数据量。（基于10.2.0.5版本测试）

--统计信息能够反映出表中的数据量。
SQL> select TABLE_NAME,NUM_ROWS,BLOCKS from user_tables where TABLE_NAME in ('PRODUCT_INFORMATION','ORDER_ITEMS');

TABLE_NAME             NUM_ROWS     BLOCKS
-------------------- ---------- ----------
ORDER_ITEMS                 665          5
PRODUCT_INFORMATION         288         13

SQL> select count(*) from ORDER_ITEMS;

  COUNT(*)
----------
       665

SQL> select count(*) from PRODUCT_INFORMATION;

  COUNT(*)
----------
       288

2.设定环境参数statistics_level为ALL，以便能够通过dbms_xplan.display_cursor函数查看SQL文根据统计信息估算出的访问数据行数和SQL执行时的实际值。

SQL> alter session set statistics_level=all;

Session altered.

3.第一次执行SQL

SQL> SELECT  o.order_id, v.product_name
  2  FROM   orders o,
  3         ( SELECT order_id, product_name
  4           FROM   order_items o, product_information p
  5           WHERE  p.product_id = o.product_id
  6           AND    list_price < 50
  7           AND    min_price < 40 ) v
  8  WHERE  o.order_id = v.order_id
  9  ;

  ORDER_ID PRODUCT_NAME
---------- --------------------
      2354 Sound Card STD
...
      2457 Graphics - DIK+

269 rows selected.

4.查看第一次执行后的执行计划

SQL> select * from table(dbms_xplan.display_cursor(NULL, NULL,'typical iostats last -cost -bytes'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------
SQL_ID  bmh5hb8331u33, child number 0
-------------------------------------
SELECT  o.order_id, v.product_name FROM   orders o,        ( SELECT order_id, product_name          FROM
order_items o, product_information p          WHERE  p.product_id = o.product_id          AND    list_price < 50
       AND    min_price < 40 ) v WHERE  o.order_id = v.order_id

Plan hash value: 1906736282

---------------------------------------------------------------------------------------------------------------------------
| Id  | Operation             | Name                | Starts | E-Rows | E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
---------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                     |      1 |        |          |    269 |00:00:00.44 |    9189 |     20 |
|   1 |  NESTED LOOPS         |                     |      1 |      1 | 00:00:01 |    269 |00:00:00.44 |    9189 |     20 |
|   2 |   MERGE JOIN CARTESIAN|                     |      1 |      4 | 00:00:01 |   9135 |00:00:00.17 |      35 |     15 |
|*  3 |    TABLE ACCESS FULL  | PRODUCT_INFORMATION |      1 |      1 ★| 00:00:01 |     87 ★|00:00:00.01 |      34 |     14 |
|   4 |    BUFFER SORT        |                     |     87 |    105 | 00:00:01 |   9135 |00:00:00.07 |       1 |      1 |
|   5 |     INDEX FULL SCAN   | ORDER_PK            |      1 |    105 | 00:00:01 |    105 |00:00:00.01 |       1 |      1 |
|*  6 |   INDEX UNIQUE SCAN   | ORDER_ITEMS_UK      |   9135 |      1 |          |    269 |00:00:00.18 |    9154 |      5 |
---------------------------------------------------------------------------------------------------------------------------

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

   3 - filter(("MIN_PRICE"<40 AND "LIST_PRICE"<50))
   6 - access("O"."ORDER_ID"="ORDER_ID" AND "P"."PRODUCT_ID"="O"."PRODUCT_ID")


26 rows selected.

SQL> ---查看V$SQL的统计信息。
SQL> select sql_id,child_number, executions, buffer_gets,plan_hash_value
  2  from v$sql
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE
------------- ------------ ---------- ----------- ---------------
bmh5hb8331u33            0          1        9701      1906736282

我们发现由于访问条件(“MIN_PRICE”<40 AND “LIST_PRICE”<50)的影响，优化器认为PRODUCT_INFORMATION表的预估行数（E-Rows）为1，优化器基于预估基数在选择表PRODUCT_INFORMATION和ORDER_ITEMS结合的最优执行计划时，选择了MERGE JOIN CARTESIAN的结合方式。
但实际实际访问行数（A-Time：87），因此由于预估基数不准，很有可能导致选择的执行计划不是最优的。

5.我们再多次执行相同的SQL文

---第二次执行
SQL> SELECT  o.order_id, v.product_name
  2  FROM   orders o,
  3         ( SELECT order_id, product_name
  4           FROM   order_items o, product_information p
  5           WHERE  p.product_id = o.product_id
  6           AND    list_price < 50
  7           AND    min_price < 40 ) v
  8  WHERE  o.order_id = v.order_id
  9  ;

  ORDER_ID PRODUCT_NAME
---------- --------------------
      2354 Sound Card STD
...
      2457 Graphics - DIK+

269 rows selected.

SQL>
---第二次执行的执行计划
SQL> select * from table(dbms_xplan.display_cursor(NULL, NULL,'typical iostats last -cost -bytes'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------
SQL_ID  bmh5hb8331u33, child number 0
-------------------------------------
SELECT  o.order_id, v.product_name FROM   orders o,        ( SELECT order_id, product_name          FROM
 order_items o, product_information p          WHERE  p.product_id = o.product_id          AND
list_price < 50          AND    min_price < 40 ) v WHERE  o.order_id = v.order_id

Plan hash value: 1906736282

------------------------------------------------------------------------------------------------------------------
| Id  | Operation             | Name                | Starts | E-Rows | E-Time   | A-Rows |   A-Time   | Buffers |
------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                     |      1 |        |          |    269 |00:00:00.45 |    9189 |
|   1 |  NESTED LOOPS         |                     |      1 |      1 | 00:00:01 |    269 |00:00:00.45 |    9189 |
|   2 |   MERGE JOIN CARTESIAN|                     |      1 |      4 | 00:00:01 |   9135 |00:00:00.17 |      35 |
|*  3 |    TABLE ACCESS FULL  | PRODUCT_INFORMATION |      1 |      1 | 00:00:01 |     87 |00:00:00.01 |      34 |
|   4 |    BUFFER SORT        |                     |     87 |    105 | 00:00:01 |   9135 |00:00:00.06 |       1 |
|   5 |     INDEX FULL SCAN   | ORDER_PK            |      1 |    105 | 00:00:01 |    105 |00:00:00.01 |       1 |
|*  6 |   INDEX UNIQUE SCAN   | ORDER_ITEMS_UK      |   9135 |      1 |          |    269 |00:00:00.18 |    9154 |
------------------------------------------------------------------------------------------------------------------

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

   3 - filter(("MIN_PRICE"<40 AND "LIST_PRICE"<50))
   6 - access("O"."ORDER_ID"="ORDER_ID" AND "P"."PRODUCT_ID"="O"."PRODUCT_ID")


26 rows selected.

---第三次执行的执行计划

    SQL> SELECT  o.order_id, v.product_name
  2  FROM   orders o,
  3         ( SELECT order_id, product_name
  4           FROM   order_items o, product_information p
  5           WHERE  p.product_id = o.product_id
  6           AND    list_price < 50
  7           AND    min_price < 40 ) v
  8  WHERE  o.order_id = v.order_id
  9  ;

  ORDER_ID PRODUCT_NAME
---------- --------------------
      2354 Sound Card STD
...
      2457 Graphics - DIK+

269 rows selected.

--第三次执行的执行计划
SQL> select * from table(dbms_xplan.display_cursor(NULL, NULL,'typical iostats last -cost -bytes'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------
SQL_ID  bmh5hb8331u33, child number 0
-------------------------------------
SELECT  o.order_id, v.product_name FROM   orders o,        ( SELECT order_id, product_name          FROM
 order_items o, product_information p          WHERE  p.product_id = o.product_id          AND
list_price < 50          AND    min_price < 40 ) v WHERE  o.order_id = v.order_id

Plan hash value: 1906736282

------------------------------------------------------------------------------------------------------------------
| Id  | Operation             | Name                | Starts | E-Rows | E-Time   | A-Rows |   A-Time   | Buffers |
------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                     |      1 |        |          |    269 |00:00:00.45 |    9189 |
|   1 |  NESTED LOOPS         |                     |      1 |      1 | 00:00:01 |    269 |00:00:00.45 |    9189 |
|   2 |   MERGE JOIN CARTESIAN|                     |      1 |      4 | 00:00:01 |   9135 |00:00:00.16 |      35 |
|*  3 |    TABLE ACCESS FULL  | PRODUCT_INFORMATION |      1 |      1 | 00:00:01 |     87 |00:00:00.01 |      34 |
|   4 |    BUFFER SORT        |                     |     87 |    105 | 00:00:01 |   9135 |00:00:00.07 |       1 |
|   5 |     INDEX FULL SCAN   | ORDER_PK            |      1 |    105 | 00:00:01 |    105 |00:00:00.01 |       1 |
|*  6 |   INDEX UNIQUE SCAN   | ORDER_ITEMS_UK      |   9135 |      1 |          |    269 |00:00:00.19 |    9154 |
------------------------------------------------------------------------------------------------------------------

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

   3 - filter(("MIN_PRICE"<40 AND "LIST_PRICE"<50))
   6 - access("O"."ORDER_ID"="ORDER_ID" AND "P"."PRODUCT_ID"="O"."PRODUCT_ID")


26 rows selected.

我们发现，虽然根据统计信息估算出的基数(Computed cardinality)和SQL执行时的实际值不同，但是以后的执行过程中，SQL文依然会利用以前的执行计划（软解析）。
在这个情况下，很有可能由于最初优化器没有选择最优的执行计划，在以后的重复执行中也得不到改进而导致效率问题。

例子2（CFB有效）

下面我们在11.2.0.4的环境中也就是CFB有效的情况下，看看执行的情况：
（我们依然使用Oracle数据库提供的样例Schema OE 及其表PRODUCT_INFORMATION和ORDER_ITEMS进行测试。）

1.首先确认相关表的统计信息和表的数据量。（基于11.2.0.4版本测试）

--统计信息能够反映出表中的数据量。
    SQL> select TABLE_NAME,NUM_ROWS,BLOCKS from user_tables where TABLE_NAME in ('PRODUCT_INFORMATION','ORDER_ITEMS');

TABLE_NAME             NUM_ROWS     BLOCKS
-------------------- ---------- ----------
ORDER_ITEMS                 665          5
PRODUCT_INFORMATION         288         13

Elapsed: 00:00:00.04
SQL> select count(*) from ORDER_ITEMS;

  COUNT(*)
----------
       665

Elapsed: 00:00:00.02
SQL> select count(*) from PRODUCT_INFORMATION;

  COUNT(*)
----------
       288

Elapsed: 00:00:00.01
SQL>

2.设定环境参数statistics_level为ALL，以便能够通过dbms_xplan.display_cursor函数查看SQL文根据统计信息估算出的访问数据行数和SQL执行时的实际值。

SQL> alter session set statistics_level=all;

Session altered.

Elapsed: 00:00:00.01

3.第一次执行SQL

SQL>
SQL> SELECT  o.order_id, v.product_name
  2  FROM   orders o,
  3         ( SELECT order_id, product_name
  4           FROM   order_items o, product_information p
  5           WHERE  p.product_id = o.product_id
  6           AND    list_price < 50
  7           AND    min_price < 40 ) v
  8  WHERE  o.order_id = v.order_id
  9  ;

  ORDER_ID PRODUCT_NAME
---------- --------------------
      2403 Battery - EL
...
      2450 Plastic Stock - W/HD

269 rows selected.

Elapsed: 00:00:00.22
SQL>

4.查看第一次执行后的执行计划

SQL> select * from table(dbms_xplan.display_cursor(format=>'typical iostats last -cost -bytes'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
SQL_ID  bmh5hb8331u33, child number 0
-------------------------------------
SELECT  o.order_id, v.product_name FROM   orders o,        ( SELECT
order_id, product_name          FROM   order_items o,
product_information p          WHERE  p.product_id = o.product_id
   AND    list_price < 50          AND    min_price < 40 ) v WHERE
o.order_id = v.order_id

Plan hash value: 1906736282

---------------------------------------------------------------------------------------------------------------------------
| Id  | Operation             | Name                | Starts | E-Rows | E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
---------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                     |      1 |        |          |    269 |00:00:00.17 |    1337 |     20 |
|   1 |  NESTED LOOPS         |                     |      1 |      1 | 00:00:01 |    269 |00:00:00.17 |    1337 |     20 |
|   2 |   MERGE JOIN CARTESIAN|                     |      1 |      4 | 00:00:01 |   9135 |00:00:00.06 |      33 |     15 |
|*  3 |    TABLE ACCESS FULL  | PRODUCT_INFORMATION |      1 |   ★ 1 | 00:00:01 |  ★ 87 |00:00:00.01 |      32 |     14 |
|   4 |    BUFFER SORT        |                     |     87 |    105 | 00:00:01 |   9135 |00:00:00.02 |       1 |      1 |
|   5 |     INDEX FULL SCAN   | ORDER_PK            |      1 |    105 | 00:00:01 |    105 |00:00:00.01 |       1 |      1 |
|*  6 |   INDEX UNIQUE SCAN   | ORDER_ITEMS_UK      |   9135 |      1 |          |    269 |00:00:00.05 |    1304 |      5 |
---------------------------------------------------------------------------------------------------------------------------

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

   3 - filter(("MIN_PRICE"<40 AND "LIST_PRICE"<50))
   6 - access("O"."ORDER_ID"="ORDER_ID" AND "P"."PRODUCT_ID"="O"."PRODUCT_ID")


28 rows selected.

Elapsed: 00:00:00.19

我们发现和10.2.0.5环境一样，由于访问条件(“MIN_PRICE”<40 AND “LIST_PRICE”<50)的影响，优化器认为PRODUCT_INFORMATION表的预估行数（E-Rows）为1，优化器基于预估基数在选择表PRODUCT_INFORMATION和ORDER_ITEMS结合的最优执行计划时，选择了MERGE JOIN CARTESIAN的结合方式。

5.查看动态视图VSQL和VSQL_SHARED_CURSOR

SQL> ---sql_id:bmh5hb8331u33
SQL> select sql_id,child_number, executions, buffer_gets,plan_hash_value
  2  from v$sql
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE
------------- ------------ ---------- ----------- ---------------
bmh5hb8331u33            0          1        1604      1906736282

Elapsed: 00:00:00.01
SQL>
SQL> select sql_id, child_number, USE_FEEDBACK_STATS
  2  from V$SQL_SHARED_CURSOR
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER U
------------- ------------ -
bmh5hb8331u33            0 Y

Elapsed: 00:00:00.04
SQL>

我们发现V$SQL_SHARED_CURSOR的USE_FEEDBACK_STATS列标记为Y。
（USE_FEEDBACK_STATS列是在11.2.0.4 的版本上新追加的列，用于标示当根据统计信息估算出的基数(Computed cardinality)和SQL执行时的实际值差距很大时，下次执行时重新生成执行计划）

6.我们再次次执行相同的SQL文

---第二次执行
SQL> SELECT  o.order_id, v.product_name
  2  FROM   orders o,
  3         ( SELECT order_id, product_name
  4           FROM   order_items o, product_information p
  5           WHERE  p.product_id = o.product_id
  6           AND    list_price < 50
  7           AND    min_price < 40 ) v
  8  WHERE  o.order_id = v.order_id
  9  ;

  ORDER_ID PRODUCT_NAME
---------- --------------------
      2403 Battery - EL
...
      2401 SPNIX3.3 AU

269 rows selected.

Elapsed: 00:00:00.03★
SQL>

我们发现执行时间变短了。

7.再次查看执行计划

SQL> select * from table(dbms_xplan.display_cursor(format=>'typical iostats last -cost -bytes'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
SQL_ID  bmh5hb8331u33, child number 1
-------------------------------------
SELECT  o.order_id, v.product_name FROM   orders o,        ( SELECT
order_id, product_name          FROM   order_items o,
product_information p          WHERE  p.product_id = o.product_id
   AND    list_price < 50          AND    min_price < 40 ) v WHERE
o.order_id = v.order_id

Plan hash value: 35479787

----------------------------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name                | Starts | E-Rows | E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
----------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |                     |      1 |        |          |    269 |00:00:00.01 |      61 |      1 |
|   1 |  NESTED LOOPS          |                     |      1 |    313 | 00:00:01 |    269 |00:00:00.01 |      61 |      1 |
|*  2 |   HASH JOIN            |                     |      1 |    313 | 00:00:01 |    269 |00:00:00.01 |      40 |      1 |
|*  3 |    TABLE ACCESS FULL   | PRODUCT_INFORMATION |      1 |     ★87 | 00:00:01 |     ★87 |00:00:00.01 |      15 |      0 |
|   4 |    INDEX FAST FULL SCAN| ORDER_ITEMS_UK      |      1 |    665 | 00:00:01 |    665 |00:00:00.01 |      25 |      1 |
|*  5 |   INDEX UNIQUE SCAN    | ORDER_PK            |    269 |      1 |          |    269 |00:00:00.01 |      21 |      0 |
----------------------------------------------------------------------------------------------------------------------------

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

   2 - access("P"."PRODUCT_ID"="O"."PRODUCT_ID")
   3 - filter(("MIN_PRICE"<40 AND "LIST_PRICE"<50))
   5 - access("O"."ORDER_ID"="ORDER_ID")

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


32 rows selected.

Elapsed: 00:00:00.03

我们发现SQL文进行了硬解析，并且表PRODUCT_INFORMATION的预估信息（E-Rows）调整为第一次执行时收集的实际值（87），用于优化器选择执行计划。因此，优化器基于调整后预估基数在选择表PRODUCT_INFORMATION和ORDER_ITEMS结合的最优执行计划时，选择了HASH JOIN的结合方式，从而更有效的执行了SQL文。

8.再次查看动态视图VSQL和VSQL_SHARED_CURSOR

SQL> ---sql_id:bmh5hb8331u33
SQL> select sql_id,child_number, executions, buffer_gets,plan_hash_value,is_shareable
  2  from v$sql
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE I
------------- ------------ ---------- ----------- --------------- -
bmh5hb8331u33            0          1        1604      1906736282 N★
bmh5hb8331u33            1          1          61        35479787 Y★

Elapsed: 00:00:00.02
SQL>
SQL> select sql_id, child_number, USE_FEEDBACK_STATS
  2  from V$SQL_SHARED_CURSOR
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER U
------------- ------------ -
bmh5hb8331u33            0 Y
bmh5hb8331u33            1 N

Elapsed: 00:00:00.00

通过视图V$SQL我们发现，新生成的游标CHILD#1比以前的游标CHILD#1会使用更少的BUFFER_GETS，效率更高。并且以前游标CHILD#0的is_shareable列标记为N，不在被共享。
新生成的游标CHILD#1的is_shareable列标记为Y，供以后的执行重用。

9.再多次执行SQL文

--第三次执行
SQL> SELECT  o.order_id, v.product_name
  2  FROM   orders o,
  3         ( SELECT order_id, product_name
  4           FROM   order_items o, product_information p
  5           WHERE  p.product_id = o.product_id
  6           AND    list_price < 50
  7           AND    min_price < 40 ) v
  8  WHERE  o.order_id = v.order_id
  9  ;

  ORDER_ID PRODUCT_NAME
---------- --------------------
      2403 Battery - EL
...
      2401 SPNIX3.3 AU

269 rows selected.

Elapsed: 00:00:00.06
SQL>

--查看执行计划

SQL> set line 200
SQL> set pagesize 9999
SQL>
SQL> select * from table(dbms_xplan.display_cursor(format=>'typical iostats last -cost -bytes'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
SQL_ID  bmh5hb8331u33, child number 1
-------------------------------------
SELECT  o.order_id, v.product_name FROM   orders o,        ( SELECT
order_id, product_name          FROM   order_items o,
product_information p          WHERE  p.product_id = o.product_id
   AND    list_price < 50          AND    min_price < 40 ) v WHERE
o.order_id = v.order_id

Plan hash value: 35479787

-------------------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name                | Starts | E-Rows | E-Time   | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |                     |      1 |        |          |    269 |00:00:00.03 |      61 |
|   1 |  NESTED LOOPS          |                     |      1 |    313 | 00:00:01 |    269 |00:00:00.03 |      61 |
|*  2 |   HASH JOIN            |                     |      1 |    313 | 00:00:01 |    269 |00:00:00.02 |      40 |
|*  3 |    TABLE ACCESS FULL   | PRODUCT_INFORMATION |      1 |     87 | 00:00:01 |     87 |00:00:00.01 |      15 |
|   4 |    INDEX FAST FULL SCAN| ORDER_ITEMS_UK      |      1 |    665 | 00:00:01 |    665 |00:00:00.01 |      25 |
|*  5 |   INDEX UNIQUE SCAN    | ORDER_PK            |    269 |      1 |          |    269 |00:00:00.01 |      21 |
-------------------------------------------------------------------------------------------------------------------

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

   2 - access("P"."PRODUCT_ID"="O"."PRODUCT_ID")
   3 - filter(("MIN_PRICE"<40 AND "LIST_PRICE"<50))
   5 - access("O"."ORDER_ID"="ORDER_ID")

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


32 rows selected.

Elapsed: 00:00:00.07

--查看动态视图
SQL> ---sql_id:bmh5hb8331u33
SQL> select sql_id,child_number, executions, buffer_gets,plan_hash_value,is_shareable
  2  from v$sql
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE I
------------- ------------ ---------- ----------- --------------- -
bmh5hb8331u33            0          1        1604      1906736282 N
bmh5hb8331u33            1          2         122        35479787 Y

Elapsed: 00:00:00.00
SQL>
SQL> select sql_id, child_number, USE_FEEDBACK_STATS
  2  from V$SQL_SHARED_CURSOR
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER U
------------- ------------ -
bmh5hb8331u33            0 Y
bmh5hb8331u33            1 N

Elapsed: 00:00:00.00
--第四次执行
SQL> SELECT  o.order_id, v.product_name
  2  FROM   orders o,
  3         ( SELECT order_id, product_name
  4           FROM   order_items o, product_information p
  5           WHERE  p.product_id = o.product_id
  6           AND    list_price < 50
  7           AND    min_price < 40 ) v
  8  WHERE  o.order_id = v.order_id
  9  ;

  ORDER_ID PRODUCT_NAME
---------- --------------------
      2403 Battery - EL
...
      2401 SPNIX3.3 AU

269 rows selected.

Elapsed: 00:00:00.05
SQL>
 --查看执行计划
SQL> set line 200
SQL> set pagesize 9999
SQL>
SQL> select * from table(dbms_xplan.display_cursor(format=>'typical iostats last -cost -bytes'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
SQL_ID  bmh5hb8331u33, child number 1
-------------------------------------
SELECT  o.order_id, v.product_name FROM   orders o,        ( SELECT
order_id, product_name          FROM   order_items o,
product_information p          WHERE  p.product_id = o.product_id
   AND    list_price < 50          AND    min_price < 40 ) v WHERE
o.order_id = v.order_id

Plan hash value: 35479787

-------------------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name                | Starts | E-Rows | E-Time   | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |                     |      1 |        |          |    269 |00:00:00.02 |      61 |
|   1 |  NESTED LOOPS          |                     |      1 |    313 | 00:00:01 |    269 |00:00:00.02 |      61 |
|*  2 |   HASH JOIN            |                     |      1 |    313 | 00:00:01 |    269 |00:00:00.01 |      40 |
|*  3 |    TABLE ACCESS FULL   | PRODUCT_INFORMATION |      1 |     87 | 00:00:01 |     87 |00:00:00.01 |      15 |
|   4 |    INDEX FAST FULL SCAN| ORDER_ITEMS_UK      |      1 |    665 | 00:00:01 |    665 |00:00:00.01 |      25 |
|*  5 |   INDEX UNIQUE SCAN    | ORDER_PK            |    269 |      1 |          |    269 |00:00:00.01 |      21 |
-------------------------------------------------------------------------------------------------------------------

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

   2 - access("P"."PRODUCT_ID"="O"."PRODUCT_ID")
   3 - filter(("MIN_PRICE"<40 AND "LIST_PRICE"<50))
   5 - access("O"."ORDER_ID"="ORDER_ID")

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


32 rows selected.

Elapsed: 00:00:00.02
SQL>
--查看动态视图
SQL>
SQL> ---sql_id:bmh5hb8331u33
SQL> select sql_id,child_number, executions, buffer_gets,plan_hash_value,is_shareable
  2  from v$sql
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER EXECUTIONS BUFFER_GETS PLAN_HASH_VALUE I
------------- ------------ ---------- ----------- --------------- -
bmh5hb8331u33            0          1        1604      1906736282 N
bmh5hb8331u33            1          3         183        35479787 Y

Elapsed: 00:00:00.00
SQL>
SQL> select sql_id, child_number, USE_FEEDBACK_STATS
  2  from V$SQL_SHARED_CURSOR
  3  where sql_id = 'bmh5hb8331u33';

SQL_ID        CHILD_NUMBER U
------------- ------------ -
bmh5hb8331u33            0 Y
bmh5hb8331u33            1 N

Elapsed: 00:00:00.00

我们发现以后的执行都会变成软解析，使用第二次产生的执行计划。
通过CFB功能使优化器能够在以后的执行中选择更优的执行计划，从得到更好的执行效率。

CFB的处理流程

下面通过以下流程图来总体的回顾一下CFB的处理过程。

Oracle优化器之基数反馈（CardinalityFeedback）功能-LMLPHP

在下列情况CBO可能无法估算出准确的Cardinality，Oracle会启用CFB功能：

针对上述情况，Oracle会采取如下的CFB流程处理：

hnairdb