1.提高命中率是为了减少硬编译（hard parse），增加软编译（soft parse），从而提高sql语句的解析效率
2.sql语句运行过程
1).使用hash算法得到sql语句的hash_value值
2).如果hash_value值在内存中，叫做命中执行软解析
3).如果hash_value值不存在，执行硬解析
4).语法解析，查看是否有错误
5).语意解析，查看权限是否符合
6).若有视图，取出视图的定义
7).进行sql语句的自动改写，如将子查询改写为连接
8).选择优的执行计划
9).变量绑定
10).运行执行计划
11).返回结果给用户
因为软解析是从此11步骤中第9步开始的，因此软解析比硬解析节约大量的系统开销，应该尽量降低硬解析的次数。
3.共享池的命中率
sec@ora10g> select namespace, pins, pinhits, reloads, invalidations from v$librarycache order by namespace;
NAMESPACE PINS PINHITS RELOADS INVALIDATIONS
--------------- ---------- ---------- ---------- -------------
BODY 4380 4307 14 0
CLUSTER 942 928 3 0
INDEX 2378 1665 25 0
JAVA DATA 0 0 0 0
JAVA RESOURCE 0 0 0 0
JAVA SOURCE 0 0 0 0
OBJECT 0 0 0 0
PIPE 0 0 0 0
SQL AREA 132073 121593 370 74
TABLE/PROCEDURE 29526 24022 971 0
TRIGGER 935 926 2 0
11 rows selected.
4.实例启动以来的命中率
sec@ora10g> select sum(pinhits)/sum(pins) from v$librarycache;
SUM(PINHITS)/SUM(PINS)
----------------------
.901459105
5.如果sql的命中率小于90%就需要对其进行优化，优化方法
1).加大shared_pool_size的大小，过犹不及，太大会增加数据的额外管理负担
2).书写程序是尽量使用变量不要过多的使用常量
3).将大的包pin在内存中
4).修改cursor_sharing初始化参数
6.实验，验证cursor_sharing参数三个不同选项（exact, similar, force）的差别
1).构造一个列值分布不均匀的大表，empno列只有一行等于2000其他都为1000
sys@ora10g> conn sec/sec
Connected.
sec@ora10g>
sec@ora10g> create table t1 as select * from emp;
Table created.
sec@ora10g> insert into t1 select * from t1;
14 rows created.
sec@ora10g> /
sec@ora10g> /
57344 rows created.
sec@ora10g> commit;
Commit complete.
sec@ora10g> select count(*) from t1;
COUNT(*)
----------
114688
sec@ora10g> update t1 set empno=1000;
114688 rows updated.
sec@ora10g> commit;
Commit complete.
sec@ora10g> update t1 set empno=2000 where rownum=1;
1 row updated.
2).建立索引
sec@ora10g> create index i_t1 on t1(empno);
Index created.
3).对表进行分析，告知数据库表的大小
sec@ora10g> analyze table t1 compute statistics;
Table analyzed.
4).对列进行分析，数据库可以识别出来表中数据是分布不均匀的
sec@ora10g> analyze table t1 compute statistics for columns empno;
Table analyzed.
5).exact匹配（系统默认的模式）
sec@ora10g> show parameter cursor_sharing
NAME TYPE VALUE
------------------------------ -------------------- --------
cursor_sharing string EXACT
sec@ora10g> set autotrace traceonly explain;
sec@ora10g> select * from t1 where empno=1000;
Execution Plan
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 114K| 3471K| 168 (2)| 00:00:03 |
|* 1 | TABLE ACCESS FULL| T1 | 114K| 3471K| 168 (2)| 00:00:03 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("EMPNO"=1000)
sec@ora10g> select * from t1 where empno=2000;
Execution Plan
----------------------------------------------------------
Plan hash value: 4068921349
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 31 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 31 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | I_T1 | 1 | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMPNO"=2000)
6).similar近似匹配
sec@ora10g> conn / as sysdba
Connected.
sys@ora10g> alter system set cursor_sharing=similar scope=spfile;
System altered.
sys@ora10g> startup force;
ORACLE instance started.
Total System Global Area 167772160 bytes
Fixed Size 1266392 bytes
Variable Size 113249576 bytes
Database Buffers 50331648 bytes
Redo Buffers 2924544 bytes
Database mounted.
Database opened.
sys@ora10g> show parameter cursor_sharing
NAME TYPE VALUE
-------------------------------- -------------------- ---------
cursor_sharing string SIMILAR
sec@ora10g> set autot traceonly explain
sec@ora10g> select * from t1 where empno=1000;
Execution Plan
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 114K| 3471K| 168 (2)| 00:00:03 |
|* 1 | TABLE ACCESS FULL| T1 | 114K| 3471K| 168 (2)| 00:00:03 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("EMPNO"=1000)
sec@ora10g> select * from t1 where empno=2000;
Execution Plan
----------------------------------------------------------
Plan hash value: 4068921349
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 31 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 31 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | I_T1 | 1 | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMPNO"=2000)
7).force，强制匹配
sec@ora10g> conn / as sysdba
Connected.
sys@ora10g> alter system set cursor_sharing=force scope=spfile;
System altered.
sys@ora10g> startup force;
ORACLE instance started.
Total System Global Area 167772160 bytes
Fixed Size 1266392 bytes
Variable Size 113249576 bytes
Database Buffers 50331648 bytes
Redo Buffers 2924544 bytes
Database mounted.
Database opened.
sys@ora10g> show parameter cursor_sharing
NAME TYPE VALUE
-------------------------------- -------------------- --------
cursor_sharing string FORCE
sys@ora10g> conn sec/sec
Connected.
sec@ora10g> set autotrace traceonly explain;
sec@ora10g> select * from t1 where empno=1000;
Execution Plan
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 114K| 3471K| 168 (2)| 00:00:03 |
|* 1 | TABLE ACCESS FULL| T1 | 114K| 3471K| 168 (2)| 00:00:03 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("EMPNO"=1000)
sec@ora10g> select * from t1 where empno=2000;
Execution Plan
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 8 | 696 | 167 (1)| 00:00:03 |
|* 1 | TABLE ACCESS FULL| T1 | 8 | 696 | 167 (1)| 00:00:03 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("EMPNO"=2000)
Note
-----
- dynamic sampling used for this statement
sec@ora10g> select * from t1 where empno=2000;
Execution Plan
----------------------------------------------------------
Plan hash value: 4068921349
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 31 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 31 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | I_T1 | 1 | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMPNO"=2000)
7.小结：
1).EXACT匹配，原语句不做处理，降低了sql的命中率，但可以保证执行计划是准确的，此种模式为系统默认的模式；
2).SIMILAR近似匹配，将where条件都用变量来处理，单可以区分列值的数据敏感性，一种折中的方案，但是oracle在处理该类参数的sql语句时会有一定的问题，慎用；
3).FORCE强制匹配，将where条件都用变量来处理，提高了SQL的命中率，但不能区分列值的数据敏感性，执行计划有时是正确的，但是有时会出现错误；
4).建议尽可能的保持系统默认的EXACT匹配模式，如需调整，建议在测试环境做好充足的验证。
secooler
09.03.06
-- The End --
【实验】shared_pool的sql命中率--cursor_sharing参数研究
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