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正确答案：B

 

Oracle 11g 新特性：Result Cache ，它是存在SHARED POOL中，作用SQL提高性能,如果结果缓存起来，下次查询直接就返回数据，无须再解析、执行SQL了。大大提高了性能。但缺点是需要在共享池留一块空间出来。

结果集缓存是Oracle 11g新引入的功能，除了可以在服务器端缓存结果之外，还可以在客户端缓存结果。服务器端的Result Cache Memory由两部分组成。

■SQL Query Result Cache：存储SQL查询的结果集。

■PL/SQL Function Result Cache：用于存储过程PL/SQL函数的结果集。

Oracle通过一个新引入初始化参数来控制该Cache的大小（result_cache_max_size）

sys@OCM> show parameter result

NAME                                 TYPE        VALUE

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

client_result_cache_lag              big integer 3000

client_result_cache_size             big integer 0

result_cache_max_result              integer     5

result_cache_max_size                big integer 3360K

result_cache_mode                    string      MANUAL

result_cache_remote_expiration       integer     0

如果result_cache_max_size=0则表示禁用该特性。参数result_cache_max_result则控制单个缓存结果可以占总的Server Result Cache大小的百分比。

上面显示的参数中result_cache_mode用于控制Server Result Cache的模式，该参数有3个可选设置。

● result_cache_mode=AUTO：则优化器自动判断是否将查询结果缓存。

●result_cache_mode=MANUAL：则需要通过查询提示来告诉优化器是否缓存结果。

●result_cache_mode=FORCE：则尽可能地缓存查询结果，通过提示：no_result_cache可以拒绝缓存。

Ok，下面我们来测Result Cache的性能。

(1)当前我的result_cache_mode为manual，如上所说result_cache_mode=MANUAL：则需要通过查询提示来告诉优化器是否缓存结果。

sys@OCM> show parameter result

NAME                                 TYPE        VALUE

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

client_result_cache_lag              big integer 3000

client_result_cache_size             big integer 0

result_cache_max_result              integer     5

result_cache_max_size                big integer 3360K

result_cache_mode                    string      MANUAL

result_cache_remote_expiration       integer     0

（2）在没有缓存结果的况情下的测试

 gyj@OCM> set autot traceonly;

gyj@OCM> select  count(*) from t1;

Execution Plan

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

Plan hash value: 3724264953

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

| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT   |      |     1 |     3   (0)| 00:00:01 |

|   1 |  SORT AGGREGATE    |      |     1 |            |          |

|   2 |   TABLE ACCESS FULL| T1   |     2 |     3   (0)| 00:00:01 |

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

Statistics

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

          1  recursive calls

          0  db block gets

          6  consistent gets

          0  physical reads

          0  redo size

        526  bytes sent via SQL*Net to client

        520  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

1	rows processed

 gyj@OCM> set autot traceonly;

gyj@OCM> select  count(*) from t1;

Execution Plan

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

Plan hash value: 3724264953

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

| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT   |      |     1 |     3   (0)| 00:00:01 |

|   1 |  SORT AGGREGATE    |      |     1 |            |          |

|   2 |   TABLE ACCESS FULL| T1   |     2 |     3   (0)| 00:00:01 |

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

Statistics

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

          1  recursive calls

          0  db block gets

          6  consistent gets

          0  physical reads

          0  redo size

        526  bytes sent via SQL*Net to client

        520  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

1	rows processed

不管你执行几次select  count(*)from t1;，它的consistent gets值都为6.从下面可以看出逻辑读是6（逻辑读= 当前读db block gets+一致性读consistent gets）

（2）在有缓存结果的况情下的测试

 gyj@OCM>  select /*+ result_cache */ count(*) from t1;

Execution Plan

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

Plan hash value: 3724264953

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

| Id  | Operation           | Name                       | Rows  | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT    |                            |     1 |     3   (0)| 00:00:01 |

|   1 |  RESULT CACHE       | 83r168smxh22sgfhqrq2u524uv |       |            |          |

|   2 |   SORT AGGREGATE    |                            |     1 |            |          |

|   3 |    TABLE ACCESS FULL| T1                         |     2 |     3   (0)| 00:00:01 |

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

Result Cache Information (identified by operation id):

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

   1 - column-count=1; dependencies=(GYJ.T1); attributes=(single-row); name="select /*+ result_cache */ count(*) from t1"

Statistics

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

          1  recursive calls

          0  db block gets

          6  consistent gets

          0  physical reads

          0  redo size

        526  bytes sent via SQL*Net to client

        520  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

gyj@OCM>  select /*+ result_cache */ count(*) from t1;

Execution Plan

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

Plan hash value: 3724264953

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

| Id  | Operation           | Name                       | Rows  | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT    |                            |     1 |     3   (0)| 00:00:01 |

|   1 |  RESULT CACHE       | 83r168smxh22sgfhqrq2u524uv |       |            |          |

|   2 |   SORT AGGREGATE    |                            |     1 |            |          |

|   3 |    TABLE ACCESS FULL| T1                         |     2 |     3   (0)| 00:00:01 |

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

Result Cache Information (identified by operation id):

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

   1 - column-count=1; dependencies=(GYJ.T1); attributes=(single-row); name="select /*+ result_cache */ count(*) from t1"

Statistics

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

          0  recursive calls

          0  db block gets

          0  consistent gets

          0  physical reads

          0  redo size

        526  bytes sent via SQL*Net to client

        520  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

1	rows processed

看到了吗，第二次执行consistent gets=0，整个逻辑读为0，说明没有执行SQL。发出SQL语句直接从共享池的Result Cache（结果集缓）存中取数据返回给客户端。

一道OCP的题目就引出了好多东西，这道题原本只是考结果缓存池在哪？实际了我们不但要知道它在共享池shared pool中，更重要的，我们要知道它的作用，Result Cache（结果集缓）是提高SQL的查询性能。哈哈。。OCP题库确实是个好东西啊，读懂每道题，转化为自己的知识，1000多道这样折腾下来，你的理论知识不扎实都不可能了。

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