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Cursor_sharing(转)

 
阅读更多

一、Cursor_sharing简介：

    这个参数是用来告诉Oracle在什么情况下可以共享游标，即SQL重用。

    Cursor_sharing参数有3个值可以设置：

     1)、EXACT：通常来说，exact值是Oracle推荐的，也是默认的，它要求SQL语句在完全相同时才会重用，否则会被重新执行硬解析操作。

     2)、SIMILAR：similar是在Oracle认为某条SQL语句的谓词条件可能会影响到它的执行计划时，才会被重新分析，否则将重用SQL。

     3)、FORCE：force是在任何情况下，无条件重用SQL。

   备注：上面所说的SQL重用，仅仅是指谓词条件不同的SQL语句，实际上这样的SQL基本上都在执行同样的业务操作。

二、在Cursor_sharing参数值不同的时对SQL的影响：

   2.1 创建实验环境：

复制代码
 ----首先创建一张jack表----
 1 SQL> create table jack (id int,name varchar2(10));
 2 
 3 Table created.
 4 
 ----产生一些数据----
 5 SQL> insert into jack values(1,'aa');
 6 
 7 1 row created.
 8 
 9 SQL> insert into jack values(2,'bb');
10 
11 1 row created.
12 
13 SQL> insert into jack values(3,'cc');
14 
15 1 row created.
16 
17 SQL> insert into jack values(4,'dd');
18 
19 1 row created.
20 
21 SQL> commit;
22 
23 Commit complete.
24 
25 SQL> select * from jack;
26 
27     ID NAME
28 ---------- ----------
29      1 aa
30      2 bb
31      3 cc
32      4 dd
33 
 ----创建下面实验将要用到的三张表----
34 SQL> create table jack_exact as select * from jack;
35 
36 Table created.
37 
38 SQL> create table jack_similar as select * from jack;
39 
40 Table created.
41 
42 SQL> create table jack_force as select * from jack;
43 
44 Table created.
45 
 ----查看该session的trace文件的路径----
46 SQL> @/u01/scripts/showtrace
47 
48 trace_file_name
49 --------------------------------------------------------------------------------
50 /u01/app/oracle/diag/rdbms/yft/yft/trace/yft_ora_5329.trc
复制代码

  2.2 cursor_sharing=exact的情况：

复制代码
  ----将cursor_sharing设置为exact----
  1 SQL> alter session set cursor_sharing=exact;
  2 
  3 Session altered.
  4 
  5 SQL> alter session set sql_trace=true;
  6 
  7 Session altered.
  8 
  9 SQL> select * from jack_exact where id=1;
 10 
 11     ID NAME
 12 ---------- ----------
 13      1 aa
 14 
 15 SQL> select * from jack_exact where id=3;
 16 
 17     ID NAME
 18 ---------- ----------
 19      3 cc
 20 
 21 SQL> select * from jack_exact where id=1;
 22 
 23     ID NAME
 24 ---------- ----------
 25      1 aa
 26 
 27 SQL> alter session set sql_trace=false;
 28 
 29 Session altered.
 30 
  ----从下面的查询可以看出执行了两次硬解析----
 31 SQL> select sql_text from v$sql where sql_text like 'select * from jack_exact where%';
 32 
 33 SQL_TEXT
 34 --------------------------------------------------------------------------------
 35 select * from jack_exact where id=1
 36 select * from jack_exact where id=3
 37 
  ----查看trace文件，通过tkprof工具
[oracle@yft ~]$ tkprof /u01/app/oracle/diag/rdbms/yft/yft/trace/yft_ora_5329.trc out.txt aggregate=no sys=no----
 38 SQL ID: fnggytkynxz04
 39 Plan Hash: 4127630146
 40 select * 
 41 from
 42  jack_exact where id=1
 43 
 44 
 45 call     count       cpu    elapsed       disk      query    current        rows
 46 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 47 Parse        1      0.00       0.00          0          1          0           0
 48 Execute      1      0.00       0.00          0          0          0           0
 49 Fetch        2      0.00       0.00          0          4          0           1
 50 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 51 total        4      0.00       0.00          0          5          0           1
 52 
 53 Misses in library cache during parse: 1    ---id=1，执行一次硬解析
 54 Optimizer mode: ALL_ROWS
 55 Parsing user id: 105  
 56 
 57 Rows     Row Source Operation
 58 -------  ---------------------------------------------------
 59       1  TABLE ACCESS FULL JACK_EXACT (cr=4 pr=0 pw=0 time=0 us cost=3 size=20 card=1)
 60 
 61 ********************************************************************************
 62 
 63 SQL ID: 1n0paamkf7sup
 64 Plan Hash: 4127630146
 65 select * 
 66 from
 67  jack_exact where id=3
 68 
 69 
 70 call     count       cpu    elapsed       disk      query    current        rows
 71 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 72 Parse        1      0.00       0.00          0          1          0           0
 73 Execute      1      0.00       0.00          0          0          0           0
 74 Fetch        2      0.00       0.00          0          4          0           1
 75 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 76 total        4      0.00       0.00          0          5          0           1
 77 
 78 Misses in library cache during parse: 1     ----id=3，执行一次硬解析
 79 Optimizer mode: ALL_ROWS
 80 Parsing user id: 105  
 81 
 82 Rows     Row Source Operation
 83 -------  ---------------------------------------------------
 84       1  TABLE ACCESS FULL JACK_EXACT (cr=4 pr=0 pw=0 time=0 us cost=3 size=20 card=1)
 85 
 86 ********************************************************************************
 87 
 88 SQL ID: fnggytkynxz04
 89 Plan Hash: 4127630146
 90 select * 
 91 from
 92  jack_exact where id=1
 93 
 94 
 95 call     count       cpu    elapsed       disk      query    current        rows
 96 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 97 Parse        1      0.00       0.00          0          0          0           0
 98 Execute      1      0.00       0.00          0          0          0           0
 99 Fetch        2      0.00       0.00          0          4          0           1
100 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
101 total        4      0.00       0.00          0          4          0           1
102 
103 Misses in library cache during parse: 0   ----执行一次软解析
104 Optimizer mode: ALL_ROWS
105 Parsing user id: 105  
106 
107 Rows     Row Source Operation
108 -------  ---------------------------------------------------
109       1  TABLE ACCESS FULL JACK_EXACT (cr=4 pr=0 pw=0 time=0 us cost=3 size=20 card=1)
110 
111 ********************************************************************************
  总结：当cursor_sharing=exact时，只有当SQL语句是完全一样的情况下才能被重用。
复制代码

   2.3 cursor_sharing=similar的情况：

复制代码
 ----将curor_sharing设置为similar----
 1 SQL> alter session set cursor_sharing=similar;
 2 
 3 Session altered.
 4 
 5 SQL> alter session set sql_trace=true;
 6 
 7 Session altered.
 8 
 9 SQL> select * from jack_similar where id=1;
10 
11     ID NAME
12 ---------- ----------
13      1 aa
14 
15 SQL> select * from jack_similar where id=4;
16 
17     ID NAME
18 ---------- ----------
19      4 dd
20 
21 SQL> select * from jack_similar where id=8;
22 
23 no rows selected
24 
 ----下面查询中可以看到Oracle将SQL中的谓词条件用同一个名词的一个变量替代，尽管看起来是一样的，但是Oracle依然把它们作为两条SQL来处理----
25 SQL> select sql_text from v$sql where sql_text like 'select * from jack_similar where%';
26 
27 SQL_TEXT
28 --------------------------------------------------------------------------------
29 select * from jack_similar where id=:"SYS_B_0"
30 select * from jack_similar where id=:"SYS_B_0"
31 select * from jack_similar where id=:"SYS_B_0"
32 
33 SQL> alter session set sql_trace=false;
34 
35 Session altered.
36 
37 [oracle@yft ~]$ tkprof /u01/app/oracle/diag/rdbms/yft/yft/trace/yft_ora_5329.trc out.txt  sys=no
38 
39 TKPROF: Release 11.2.0.1.0 - Development on Tue Jan 22 10:18:16 2013
40 
41 Copyright (c) 1982, 2009, Oracle and/or its affiliates.  All rights reserved.
42 
 ----从trace文件中可以清楚看到上面的结论----
43 SQL ID: 80chtmbbwpx49
44 Plan Hash: 1559066762
45 select * 
46 from
47  jack_similar where id=:"SYS_B_0"
48 
49 
50 call     count       cpu    elapsed       disk      query    current        rows
51 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
52 Parse        3      0.00       0.00          0          0          0           0
53 Execute      3      0.00       0.00          0          3          0           0
54 Fetch        5      0.00       0.00          0         10          0           2
55 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
56 total       11      0.01       0.01          0         13          0           2
57 
58 Misses in library cache during parse: 3    ----进行三次查询，都各执行了一次硬解析。
59 Optimizer mode: ALL_ROWS
60 Parsing user id: 105  
61 
62 Rows     Row Source Operation
63 -------  ---------------------------------------------------
64       1  TABLE ACCESS FULL JACK_SIMILAR (cr=4 pr=0 pw=0 time=0 us cost=3 size=20 card=1)
65 
66 ********************************************************************************
复制代码

     对于SIMILAR的情况，如果CBO发现被绑定变量的谓词还有其他的执行计划可以选择时，如果谓词条件的值有变化，就将会产生一个新的子游标，而不是重用之前的SQL；如果谓词没有其他的执行计划可选择，则忽略谓词的值，重用之前的SQL。
     上面的例子还不能足以说明该情况，接着下面的模拟：

复制代码
  ----清楚一下shared_pool中的内容，否则会影响后面的输出----
  1 SQL> alter system flush shared_pool;
  2 
  3 System altered.
  4 
  5 SQL> select * from jack_similar;
  6 
  7     ID NAME
  8 ---------- ----------
  9      1 aa
 10      2 bb
 11      3 cc
 12      4 dd
 13 
 14 SQL> insert into jack_similar values(1,'gg');
 15 
 16 1 row created.
 17 
 18 SQL> commit;
 19 
 20 Commit complete.
 21 
  ----创建索引，并进行分析----
 22 SQL> create index jack_similar_ind on jack_similar(id);
 23 
 24 Index created.
 25 SQL> exec dbms_stats.gather_table_stats(user,'jack_similar',cascade=>true);
 26 
 27 PL/SQL procedure successfully completed.
 28 
 29 SQL> select * from jack_similar;
 30 
 31     ID NAME
 32 ---------- ----------
 33      1 aa
 34      2 bb
 35      3 cc
 36      4 dd
 37      1 gg
 38 
 39 SQL> alter session set cursor_sharing=similar;
 40 
 41 Session altered.
 42 
 43 SQL> alter session set sql_trace=true;
 44 
 45 Session altered.
 46 
 47 SQL> select * from jack_similar where id=1 and name='aa';
 48 
 49     ID NAME
 50 ---------- ----------
 51      1 aa
 52 
 53 SQL> select * from jack_similar where id=1 and name='gg';
 54 
 55     ID NAME
 56 ---------- ----------
 57      1 gg
 58 
 59 SQL> alter session set sql_trace=false;
 60 
 61 Session altered.
 62 
  ----在这里可以看到执行两次SQL查询，只进行了一个硬解析----
 63 SQL> select sql_text from v$sql where sql_text like 'select * from jack_similar where%';
 64 
 65 SQL_TEXT
 66 --------------------------------------------------------------------------------
 67 select * from jack_similar where id=:"SYS_B_0" and name=:"SYS_B_1"
 68 
 69 
  ----在trace文件也验证了这一点。----
  ----虽然name的值发生了改变，但是id的值没有发生变，而id上有索引的，name上没有索引，CBO认为这样的情况不会改变SQL的执行计划。
 70 SQL ID: 10ku2kuy1sqaj
 71 Plan Hash: 2730352089
 72 select * 
 73 from
 74  jack_similar where id=:"SYS_B_0" and name=:"SYS_B_1"
 75 
 76 
 77 call     count       cpu    elapsed       disk      query    current        rows
 78 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 79 Parse        1      0.00       0.00          0          0          0           0
 80 Execute      1      0.00       0.00          0          0          0           0
 81 Fetch        2      0.00       0.00          0          4          0           1
 82 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 83 total        4      0.00       0.00          0          4          0           1
 84 
 85 Misses in library cache during parse: 1
 86 Optimizer mode: ALL_ROWS
 87 Parsing user id: 105  
 88 
 89 Rows     Row Source Operation
 90 -------  ---------------------------------------------------
 91       1  TABLE ACCESS BY INDEX ROWID JACK_SIMILAR (cr=4 pr=0 pw=0 time=0 us cost=3 size=6 card=1)
 92       2   INDEX RANGE SCAN JACK_SIMILAR_IND (cr=2 pr=0 pw=0 time=17 us cost=1 size=0 card=2)(object id 75044)
 93 
 94 ********************************************************************************
 95 
 96 SQL ID: 10ku2kuy1sqaj
 97 Plan Hash: 2730352089
 98 select * 
 99 from
100  jack_similar where id=:"SYS_B_0" and name=:"SYS_B_1"
101 
102 
103 call     count       cpu    elapsed       disk      query    current        rows
104 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
105 Parse        1      0.00       0.00          0          0          0           0
106 Execute      1      0.00       0.00          0          0          0           0
107 Fetch        2      0.00       0.00          0          4          0           1
108 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
109 total        4      0.00       0.00          0          4          0           1
110 
111 Misses in library cache during parse: 0
112 Optimizer mode: ALL_ROWS
113 Parsing user id: 105  
114 
115 Rows     Row Source Operation
116 -------  ---------------------------------------------------
117       1  TABLE ACCESS BY INDEX ROWID JACK_SIMILAR (cr=4 pr=0 pw=0 time=0 us cost=3 size=6 card=1)
118       2   INDEX RANGE SCAN JACK_SIMILAR_IND (cr=2 pr=0 pw=0 time=3 us cost=1 size=0 card=2)(object id 75044)
119 
120 ********************************************************************************
复制代码

  2.4 cursor_sharing=force的情况

复制代码
  ----设置cursor_sharing=force----
  1 SQL> alter session set cursor_sharing=force;
  2 
  3 Session altered.
  4 
  5 SQL> alter session set sql_trace=true;
  6 
  7 Session altered.
  8 
  9 SQL> select * from jack_force where id=1;
 10 
 11     ID NAME
 12 ---------- ----------
 13      1 aa
 14 
 15 SQL> select * from jack_force where id=4;
 16 
 17     ID NAME
 18 ---------- ----------
 19      4 dd
 20 
 21 SQL> select * from jack_force where id=1;
 22 
 23     ID NAME
 24 ---------- ----------
 25      1 aa
 26 
 27 SQL> alter session set sql_trace=false;
 28 
 29 Session altered.
 30 
  ----从下面的查询中可以看出只进行了一次硬解析，而且使用了绑定变量----
 31 SQL> select sql_text from v$sql where sql_text like 'select * from jack_force where%';
 32 
 33 SQL_TEXT
 34 --------------------------------------------------------------------------------
 35 select * from jack_force where id=:"SYS_B_0"
 36 
 37 
  ----查看trace文件内容----
 38 [oracle@yft ~]$ tkprof /u01/app/oracle/diag/rdbms/yft/yft/trace/yft_ora_5551.trc  aggregate=no sys=no
 39 
 40 SQL ID: 38vy9d4quwdwk
 41 Plan Hash: 1272021682
 42 select * 
 43 from
 44  jack_force where id=:"SYS_B_0"
 45 
 46 
 47 call     count       cpu    elapsed       disk      query    current        rows
 48 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 49 Parse        1      0.01       0.01          0          1          0           0
 50 Execute      1      0.00       0.00          0          1          0           0
 51 Fetch        2      0.00       0.00          0          4          0           1
 52 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 53 total        4      0.01       0.01          0          6          0           1
 54 
 55 Misses in library cache during parse: 1     ----id=1的时候进行一次硬解析
 56 Optimizer mode: ALL_ROWS
 57 Parsing user id: 105  
 58 
 59 Rows     Row Source Operation
 60 -------  ---------------------------------------------------
 61       1  TABLE ACCESS FULL JACK_FORCE (cr=4 pr=0 pw=0 time=0 us cost=3 size=20 card=1)
 62 
 63 ********************************************************************************
 64 
 65 SQL ID: 38vy9d4quwdwk
 66 Plan Hash: 1272021682
 67 select * 
 68 from
 69  jack_force where id=:"SYS_B_0"
 70 
 71 
 72 call     count       cpu    elapsed       disk      query    current        rows
 73 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 74 Parse        1      0.00       0.00          0          0          0           0
 75 Execute      1      0.00       0.00          0          0          0           0
 76 Fetch        2      0.00       0.00          0          3          0           1
 77 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 78 total        4      0.00       0.00          0          3          0           1
 79 
 80 Misses in library cache during parse: 0     ----id=4的时候进行0次硬解析，一次软解析
 81 Optimizer mode: ALL_ROWS
 82 Parsing user id: 105  
 83 
 84 Rows     Row Source Operation
 85 -------  ---------------------------------------------------
 86       1  TABLE ACCESS FULL JACK_FORCE (cr=3 pr=0 pw=0 time=0 us cost=3 size=20 card=1)
 87 
 88 ********************************************************************************
 89 
 90 SQL ID: 38vy9d4quwdwk
 91 Plan Hash: 1272021682
 92 select * 
 93 from
 94  jack_force where id=:"SYS_B_0"
 95 
 96 
 97 call     count       cpu    elapsed       disk      query    current        rows
 98 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
 99 Parse        1      0.00       0.00          0          0          0           0
100 Execute      1      0.00       0.00          0          0          0           0
101 Fetch        2      0.00       0.00          0          4          0           1
102 ------- ------  -------- ---------- ---------- ---------- ----------  ----------
103 total        4      0.00       0.00          0          4          0           1
104 
105 Misses in library cache during parse: 0    ----id=1时进行0次硬解析，一次软解析
106 Optimizer mode: ALL_ROWS
107 Parsing user id: 105  
108 
109 Rows     Row Source Operation
110 -------  ---------------------------------------------------
111       1  TABLE ACCESS FULL JACK_FORCE (cr=4 pr=0 pw=0 time=0 us cost=3 size=20 card=1)
112 
113 ********************************************************************************
  总结：force是在任何情况下，无条件重用SQL。
复制代码

 三、总结：

   FORCE和SIMIALR最大的区别在于，FORCE会把所有的谓词用变量代替，并且不管变量的值如何，一律重用第一条SQL语句，而SIMILAR会根据谓词的不同，来重新选择SQL的执行计划。

   如果一个系统，它存在变量绑定的问题，并且这种问题已经影响到了系统的性能，这时候可以考虑将参数cursor_sharing的值设置为SIMILAR或FORCE来改善这种局面，不过在改成SIMILAR或FORCE都可能带来一些Bug以及很多未知的东西，所以需要慎用。

   最后需要说明一点，对于OLTP系统，如果绑定变量情况不好的话，也许可以考虑通过设置这个参数来缓解一下问题；对于是在OLAP系统上，这个参数应该设置成EXACT，并且不应该使用绑定变量，因为在OLAP系统中，SQL的解析对于SQL的执行来看，话费的代价几乎可以忽略，而正确的SQL执行计划才是OLAP数据库最需要关注的。

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