测试在cursor_sharing为force情况下执行计划的共享情况
1、首先，我们确定cursor_sharing参数
SQL> show parameter cursor_sharing
NAME TYPE
------------------------------------ --------------------------------
VALUE
------------------------------
cursor_sharing string
FORCE
SQL> select spid from v$process where addr in(select paddr from v$session where
sid in(select sid from v$mystat where rownum=1));
SPID
------------
8392
2、测试常量发生变化时，执行计划的共享情况
SQL> alter session set sql_trace=true;
Session altered.
SQL> select * from tb_test_connect_by where id=4;
no rows selected
SQL> select * from tb_test_connect_by where id=5;
ID
----------
5
SQL> select * from tb_test_connect_by where id=6;
ID
----------
6
SQL> alter session set sql_trace off
2 ;
alter session set sql_trace off
*
ERROR at line 1:
ORA-00922: missing or invalid option
SQL> alter session set sql_trace=off;
alter session set sql_trace=off
*
ERROR at line 1:
ORA-00922: missing or invalid option
SQL> alter session set sql_trace=false;
Session altered.
trace结果：
.....
select *
from
tb_test_connect_by where id=:"SYS_B_0"
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 4 0.00 0.01 0 0 0 0
Execute 4 0.00 0.00 0 0 0 0
Fetch 5 0.00 0.00 0 3 0 2
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 13 0.00 0.01 0 3 0 2
Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 61
Rows Row Source Operation
------- ---------------------------------------------------
0 INDEX UNIQUE SCAN SYS_C0025401 (cr=1 pr=0 pw=0 time=24 us)(object id 57978)
.....
从上面的分析结果来看，在cursor_sharing为force时，确实会把常量替换成一个系统自带的绑定变量，这样，就可以减少硬解析次数，只是当列的值有倾斜时，可能会导致执行计划出现问题。
3、测试在绑定变量名称不一样的情况下，执行计划的共享情况
SQL> alter session set sql_trace=true;
Session altered.
SQL> variable id number;
SQL> variable id10 number;
SQL> variable id11 number;
SQL> exec :id=1;
BEGIN :id=1; END;
*
ERROR at line 1:
ORA-06550: line 1, column 10:
PLS-00103: Encountered the symbol "=" when expecting one of the following:
:= . ( @ % ; indicator
The symbol ":= was inserted before "=" to continue.
SQL> exec :id:=1
PL/SQL procedure successfully completed.
SQL> exec :id10:=2
PL/SQL procedure successfully completed.
SQL> exec :id11:=3
PL/SQL procedure successfully completed.
SQL> select * from tb_test_connect_by where id=:id;
ID
----------
1
SQL> select * from tb_test_connect_by where id=:id10;
ID
----------
2
SQL> select * from tb_test_connect_by where id=:id11;
ID
----------
3
SQL> alter session set sql_trace=false;
Session altered.
trace文件：
......
select *
from
tb_test_connect_by where id=:id
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.04 0 0 0 0
Fetch 2 0.00 0.00 0 1 0 1
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 4 0.00 0.04 0 1 0 1
Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 61
Rows Row Source Operation
------- ---------------------------------------------------
1 INDEX UNIQUE SCAN SYS_C0025401 (cr=1 pr=0 pw=0 time=79 us)(object id 57978)
********************************************************************************
select *
from
tb_test_connect_by where id=:id10
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.01 0 0 0 0
Fetch 2 0.00 0.00 0 1 0 1
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 4 0.00 0.01 0 1 0 1
Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 61
Rows Row Source Operation
------- ---------------------------------------------------
1 INDEX UNIQUE SCAN SYS_C0025401 (cr=1 pr=0 pw=0 time=16 us)(object id 57978)
********************************************************************************
select *
from
tb_test_connect_by where id=:id11
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.01 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 0 1 0 1
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 4 0.00 0.01 0 1 0 1
Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 61
Rows Row Source Operation
------- ---------------------------------------------------
1 INDEX UNIQUE SCAN SYS_C0025401 (cr=1 pr=0 pw=0 time=28 us)(object id 57978)
.....
从测试结果分析来看，当绑定变量的名称不一样时，即使在cursor_sharing为force的情况一下，执行计划也是无法共享的。
来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/12350275/viewspace-690250/，如需转载，请注明出处，否则将追究法律责任。
转载于:http://blog.itpub.net/12350275/viewspace-690250/