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;
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SQL> select count(*) from t;
COUNT(*)
----------
13456
SQL> set autot on;
SQL> select count(*) from t;
COUNT(*)
----------
13456
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id | Operation | Name |Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 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
------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |
| 1| RESULT CACHE | c8ukugu9jf5474pjzn83wj3zph | | | |
| 2| SORT AGGREGATE | | 1 | | |
| 3| TABLE ACCESS FULL| T | 12278 | 50 (2)| 00:00:01 |
---修改为force,则强制使用resultcachealter system set result_cache_mode=force;
SQL> create table t1 as select * from t;
SQL> select count(*) from t1;COUNT(*)
----------
13456
------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |
| 1| RESULT CACHE | 8q8qwtn4uk3vv7sm1ynpzz6t4k | | | |
| 2| SORT AGGREGATE | | 1 | | |
| 3| TABLE ACCESS FULL| T1 | 14242 | 50 (2)| 00:00:01 |
------------------------------------------------------------------------------------------
SQL> select count(*) from t1;
COUNT(*)
----------
13456
------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |
| 1| RESULT CACHE | 8q8qwtn4uk3vv7sm1ynpzz6t4k | | | |
| 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
------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 1 | 50 (2)| 00:00:01 |
| 1| RESULT CACHE | 8q8qwtn4uk3vv7sm1ynpzz6t4k | | | |
| 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
-------------------------------------------------------------------| Id | Operation | Name |Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 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