SQL语句的缓存结果集功能是Oracle11g中最吸引我的一个新功能。
DB_BUFFER只能缓存访问过的BLOCK，部分解决了物理读的问题，查询仍然需要大量的逻辑读。
物化视图提供的是查询重写的功能，对于大部分的情况，只是简化了SQL的复杂度，即使是完全满足查询的条件，物化视图的扫描也是不可避免的。
而缓存结果集——RESULT CACHE则截然不同，它缓存的是查询的结果。不在需要大量的逻辑读，不在需要任何的复杂计算，而是直接将已经缓存的结果返回。
Oracle新增了两个HINT，RESULT_CACHE和NO_RESULT_CACHE。通过这两个提示，可以明确的指出下面的语句是否进行RESULT CACHE。
Oracle还增加了几个初始化参数来管理RESULT CACHE功能，如：RESULT_CACHE_MODE、RESULT_CACHE_MAX_SIZE等。RESULT_CACHE_MAX_SIZE指明SGA中RESULT CACHE功能可以使用的最大的内存容量。如果这个参数设置为0，则关闭RESULT CACHE功能。RESULT_CACHE_MODE参数设置Oracle如何使用RESULT CACHE，该参数有三个值：MANUAL、AUTO、FORCE。后面会通过几个例子来说明三种情况的区别。
Oracle提供了DBMS_RESULT_CACHE包来管理和维护RESULT CACHE。
Oracle还新增了几个关于RESULT CACHE的系统视图，用户可以看到和RESULT CACHE相关的各种信息，视图包括：V$RESULT_CACHE_DEPENDENCY、V$RESULT_CACHE_MEMORY、V$RESULT_CACHE_OBJECTS和V$RESULT_CACHE_STATISTICS等。
首先先看一下RESULT_CACHE所带来的性能提升：
[PHP]
SQL> create table t as select * from dba_objects;
表已创建。
SQL> set autot on
SQL> set timing on
SQL> select count(*) from t;
COUNT(*)
----------
68324
已用时间: 00: 00: 00.12
执行计划
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T | 52976 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
28 recursive calls
0 db block gets
1066 consistent gets
1006 physical reads
0 redo size
338 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select count(*) from t;
COUNT(*)
----------
68324
已用时间: 00: 00: 00.21
执行计划
----------------------------------------------------------
Plan hash value: 2966233522
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | |
| 2 | SORT AGGREGATE | | 1 | | |
| 3 | TABLE ACCESS FULL| T | 52976 | 282 (1)| 00:00:04 |
------------------------------------------------------------------------------------------
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; dependencies=(YANGTK.T); attributes=(single-row); name="select count(*) from t;
COUNT(*)
----------
68324
已用时间: 00: 00: 00.00
执行计划
----------------------------------------------------------
Plan hash value: 2966233522
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | |
| 2 | SORT AGGREGATE | | 1 | | |
| 3 | TABLE ACCESS FULL| T | 52976 | 282 (1)| 00:00:04 |
------------------------------------------------------------------------------------------
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; dependencies=(YANGTK.T); attributes=(single-row); name="select object_name from t where object_name = 'DUAL';
OBJECT_NAME
-----------------------------------------------------------
DUAL
DUAL
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE |fhp10hrt50095fufmw3s75txcx| | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
---------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DUAL')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select o
ere object_name = 'DUAL'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
28 recursive calls
0 db block gets
1067 consistent gets
0 physical reads
0 redo size
366 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> select object_name from t where object_name = 'DUAL';
OBJECT_NAME
------------------------------------------------------------
DUAL
DUAL
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | fhp10hrt50095fufmw3s75txcx| | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DUAL')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select o
ere object_name = 'DUAL'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
366 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> select object_name from t where object_name = 'DUAL';
OBJECT_NAME
-----------------------------------------------------------
DUAL
DUAL
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
|* 1 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_NAME"='DUAL')
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
4 recursive calls
0 db block gets
1065 consistent gets
0 physical reads
0 redo size
366 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
.
[/PHP]
当设置为MANUAL时，只有使用HINT的情况下，Oracle才会利用缓存结果集。而对于AUTO模式，Oracle如果发现缓冲结果集已经存在，那么就会使用。但是如果缓冲结果集不存在，Oracle并不会自动进行缓冲。只有使用HINT的情况下，Oracle才会将执行的结果集缓存。
[PHP]
SQL> alter session set result_cache_mode = auto;
会话已更改。
SQL> select object_name from t where object_name = 'DUAL';
OBJECT_NAME
------------------------------
DUAL
DUAL
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE |fhp10hrt50095fufmw3s75txcx| | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DUAL')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam
t_name = 'DUAL'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
5 recursive calls
0 db block gets
53 consistent gets
0 physical reads
0 redo size
366 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> select object_name from t where object_name = 'DUAL';
OBJECT_NAME
------------------------------
DUAL
DUAL
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | fhp10hrt50095fufmw3s75txcx | | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DUAL')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam
t_name = 'DUAL'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
366 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> select object_name from t where object_name = 'DBA_TABLES';
OBJECT_NAME
------------------------------
DBA_TABLES
DBA_TABLES
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | 14buvcmfum6h9622vpkxm7sj2w| | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DBA_TABLES')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam
t_name = 'DBA_TABLES'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
4 recursive calls
0 db block gets
1065 consistent gets
0 physical reads
0 redo size
372 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> select object_name from t where object_name = 'DBA_TABLES';
OBJECT_NAME
------------------------------
DBA_TABLES
DBA_TABLES
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | 14buvcmfum6h9622vpkxm7sj2w | | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DBA_TABLES')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam
t_name = 'DBA_TABLES'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1012 consistent gets
0 physical reads
0 redo size
372 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
.
[/PHP]
对比上面两种情况，由于第一个SQL的缓存结果集已经存在，Oracle采用了缓存结果集，而对于第二个SQL，Oracle是不会自动使用缓存结果集的。
[PHP]
SQL> select object_name from t where object_name = 'DBA_TABLES';
OBJECT_NAME
------------------------------
DBA_TABLES
DBA_TABLES
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | 14buvcmfum6h9622vpkxm7sj2w | | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DBA_TABLES')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select o
ere object_name = 'DBA_TABLES'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
4 recursive calls
0 db block gets
1065 consistent gets
0 physical reads
0 redo size
372 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> select object_name from t where object_name = 'DBA_TABLES';
OBJECT_NAME
------------------------------
DBA_TABLES
DBA_TABLES
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | 14buvcmfum6h9622vpkxm7sj2w | | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DBA_TABLES')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam
t_name = 'DBA_TABLES'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
372 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
.
[/PHP]
当使用HINT后，结果集被缓存，随后的相同的查询就会使用缓存的结果集。
对于FORCE参数，顾名思义，就是会对所有的SQL进行缓存，除非明确使用NO_RESULT_CACHE提示：
[PHP]
SQL> alter session set result_cache_mode = force;
会话已更改。
SQL> select object_name from t where object_name = 'DBA_VIEWS';
OBJECT_NAME
------------------------------
DBA_VIEWS
DBA_VIEWS
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | bdkgz2jxpb7tc8pkwn478qt3p0 | | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DBA_VIEWS')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select object_name from t wh
BA_VIEWS'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
5 recursive calls
0 db block gets
1065 consistent gets
0 physical reads
0 redo size
371 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> select object_name from t where object_name = 'DBA_VIEWS';
OBJECT_NAME
------------------------------
DBA_VIEWS
DBA_VIEWS
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
| 1 | RESULT CACHE | bdkgz2jxpb7tc8pkwn478qt3p0 | | | | |
|* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_NAME"='DBA_VIEWS')
Result Cache Information (identified by operation id):
------------------------------------------------------
1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select object_name from t wh
BA_VIEWS'"
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
371 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> select object_name from t where object_name = 'DBA_VIEWS';
OBJECT_NAME
------------------------------
DBA_VIEWS
DBA_VIEWS
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 |
|* 1 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_NAME"='DBA_VIEWS')
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
4 recursive calls
0 db block gets
1065 consistent gets
0 physical reads
0 redo size
371 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
.
[/PHP]
原文出自：http://yangtingkun.itpub.net/post/468/391015