11g新特性：result cache 结果缓存

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result cache是oracle 11g新推出的特性，通过把查询结果还存在内存中来提高查询性能。缓存分为clinet和server端缓存。本文主要讨论server端缓存。serverresult cache是shared pool的内存的一部分。结果缓存更适合于数据变化不频繁的系统，例如OLAP。简单原理如下：当sql执行时，数据库先去result cache去查找相关结果；当结果相关的对象发生变化，例如相关表的数据被删除一条，则结果缓存失效。

相关参数主要如下

SQL> show parameter result_cache

NAME TYPE VALUE

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

client_result_cache_lag big integer 3000

client_result_cache_size big integer 0

result_cache_max_result integer 5 ---一个result占总比例最大大小

result_cache_max_size big integer 21568K --缓存的最大大小

result_cache_mode string MANUAL ---模式

result_cache_remote_expiration integer 0

result_cache_mode：

manual：查询时候要手动加hintresult_cache 通知优化器才可以

auto:优化器会判断是否将使用已经存在的result cache（用hint产生的result cache）（依据查询执行的频率、生成结果的成本以及针对底层数据库对象更改的频率。）,另外只有使用hint的时候产生result cache，oracle才会使用

force:尽可能的cache 查询结果，将result_cache加到select语句中（no_result_cache可拒绝cache）

备注：比较奇怪的是，官方文档没有auto的介绍，只有manual和auto

在以下两种情况下会使用resultcache，分别是系统参数和table进行设置

1，result_cache_mode参数；设置为manual，则需加hint才能使用resultcache；force直接使用；使用auto则由数据库决定，测试了一次，结果为不适用；

参数为result_cache_mode为manual时,只有使用才使用hint result_cahce

SQL> connect wj/oracle;

ò?á??ó?￡

SQL> select count(*) from t;

COUNT(*)

----------

13456

SQL> set autot on;

SQL> select count(*) from t;

COUNT(*)

----------

13456

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

Plan hash value: 2966233522

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

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

| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |

| 1| SORT AGGREGATE | | 1 | | |

| 2| TABLE ACCESS FULL| T | 12278 | 50 (2)| 00:00:01 |

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

SQL> select /*+ result_cache */ count(*)from t;

COUNT(*)

----------

13456

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

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

| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |

| 2| SORT AGGREGATE | | 1 | | |

| 3| TABLE ACCESS FULL| T | 12278 | 50 (2)| 00:00:01 |

---修改为force，则强制使用resultcache

alter system set result_cache_mode=force;

SQL> create table t1 as select * from t;

SQL> select count(*) from t1;

COUNT(*)

----------

13456

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

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

| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |

| 2| SORT AGGREGATE | | 1 | | |

| 3| TABLE ACCESS FULL| T1 | 14242 | 50 (2)| 00:00:01 |

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

SQL> select count(*) from t1;

COUNT(*)

----------

13456

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

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

| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |

| 2| SORT AGGREGATE | | 1 | | |

| 3| TABLE ACCESS FULL| T1 | 14242 | 50 (2)| 00:00:01 |

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

查询V$RESULT_CACHE_OBJECTS可以得到目前缓存的情况

1*select name,cache_id from V$RESULT_CACHE_OBJECTS

SQL> select name,cache_id fromV$RESULT_CACHE_OBJECTS;

NAME CACHE_ID

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

WJ.T1 WJ.T1

WJ.T WJ.T

select count(*) from t1 8q8qwtn4uk3vv7sm1ynpzz6t4k

select /*+ result_cache */ count(*) fromt c8ukugu9jf5474pjzn83wj3zph

2，使用Table Annotations。表注释的优先级低于sql语句。通过关键字RESULT_CACHE来实现。有DEFAULT和FORCE两个数值

DEFAULT

If at least one table in a query is set to DEFAULT, then result cachingis not enabled at the table level for this query, unless the RESULT_CACHE_MODEinitialization parameter is set to FORCE or the RESULT_CACHE hint is specified.This is the default value.

FORCE

If all the tables of a query are marked as FORCE, then the query resultis considered for caching. The table annotation FORCE takes precedence over theRESULT_CACHE_MODE parameter value of MANUAL set at the session level.

测试结果如下

SQL> alter table t1 result_cache (MODEforce);

SQL> alter session set result_cache_mode= MANUAL;

--查询T1使用resultcache

SQL> select count(*) from t1;

COUNT(*)

----------

13456

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

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

| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |

| 2| SORT AGGREGATE | | 1 | | |

| 3| TABLE ACCESS FULL| T1 | 14242 | 50 (2)| 00:00:01 |

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

--查询表T没有使用resultcache

SQL> select count(*) from t;

COUNT(*)

----------

13456

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

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

| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |

| 1| SORT AGGREGATE | | 1 | | |

| 2| TABLE ACCESS FULL| T | 12278 | 50 (2)| 00:00:01 |

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

参考文献

Oracle? Database Performance Tuning Guide11g Release 2 (11.2)

http://space.itpub.net/12020513/viewspace-622483

11g New Feature : SQL Query Result Cache[ID 453567.1]

http://yangtingkun.itpub.net/post/468/391015