Oracle 学习之性能优化（三）绑定变量

根据Oracle 学习之性能优化（二）游标中的描述，我们知道如下两条语句是不共享的。

select * from emp where empno=7698;
select * from emp where empno=7566;

这样就造成每次执行用户的查询都要进行硬解析，但是我们知道，其他这两个语句的执行计划应该是相同。那么有什么方法能避免不必要的硬解析吗？这里我们提供2种方法。
一、绑定变量

SQL> variable empno number;
SQL> exec :empno := 7839;

PL/SQL procedure successfully completed.

SQL> select ename from emp where empno = :empno;

ENAME
------------------------------
KING

SQL>  exec :empno := 7782;

PL/SQL procedure successfully completed.

SQL> select ename from emp where empno = :empno;

ENAME
------------------------------
CLARK

SQL>

我们查看一下游标

SQL> COL SQL_TEXT FOR A30
SQL> COL SQL_ID FOR A20
SQL> SET LINESIZE 200
SQL> SELECT sql_id,sql_text,executions,loads,version_count
FROM v$sqlarea
WHERE sql_text LIKE '%:empno';

SQL_ID		     SQL_TEXT			    EXECUTIONS	    LOADS VERSION_COUNT
-------------------- ------------------------------ ---------- ---------- -------------
f6r0kqk0hsa7s	     select ename from emp where em	     2		1	      1
pno = :empno

SQL> SELECT sql_id,sql_text,loads,child_number,parse_calls
FROM v$sql
WHERE sql_text LIKE '%:empno';

SQL_ID		     SQL_TEXT				 LOADS CHILD_NUMBER PARSE_CALLS
-------------------- ------------------------------ ---------- ------------ -----------
f6r0kqk0hsa7s	     select ename from emp where em	     1		  0	      2
pno = :empno
SQL>

可见，父子游标都被共享啦。
在OLTP环境中，一定要使用绑定变量以避免系统有太多的硬解析。

我们验证一下，不使用绑定变量和使用了绑定变量后，性能到底有没有提升。

1. 建立一张表

SQL> create table t(id int,text varchar2(100));

Table created.

2. 不使用绑定变量向表中插入10000行记录

SQL> set timing on
SQL> declare
begin
for i in 1 .. 10000 loop
execute immediate 'insert into t values('||i||',''test bind variable'')';
end loop;
commit;
end;
/

PL/SQL procedure successfully completed.

Elapsed: 00:00:06.43

系统产生了非常多的游标

SQL> set pause on
SQL> SELECT sql_text,
sql_id,
EXECUTIONS,
LOADS,
VERSION_COUNT
FROM v$sqlarea
WHERE sql_text LIKE 'insert into t%';

SQL_TEXT		       SQL_ID		    EXECUTIONS	    LOADS VERSION_COUNT
------------------------------ -------------------- ---------- ---------- -------------
insert into t values(9156,'tes ah7vdgtnj80b1		     1		1	      1
t bind variable')

insert into t values(8826,'tes 7yuz09vq9h0c4		     1		1	      1
t bind variable')

insert into t values(9905,'tes 97c7m0gxj80cv		     1		1	      1
t bind variable')

insert into t values(9396,'tes 9bvtw8y7080g5		     1		1	      1
t bind variable')

SQL_TEXT		       SQL_ID		    EXECUTIONS	    LOADS VERSION_COUNT
------------------------------ -------------------- ---------- ---------- -------------

insert into t values(9034,'tes ck51y8bu1c0jr		     1		1	      1
t bind variable')

insert into t values(9153,'tes 7cxb26zpcn0q9		     1		1	      1
t bind variable')

insert into t values(9783,'tes 7236x7yva40sq		     1		1	      1
t bind variable')

insert into t values(9491,'tes cn2n05f70810f		     1		1	      1

3. 使用绑定变量

SQL> set timing on
SQL> declare
begin
for i in 1 .. 10000 loop
execute immediate 'insert into t values(:x,''test bind variable'')' using i;
end loop;
commit;
end;
/

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.43

使用绑定变量，执行速度快了很多倍。

SQL> SELECT sql_text,
sql_id,
EXECUTIONS,
LOADS,
VERSION_COUNT
FROM v$sqlarea
WHERE sql_text LIKE 'insert into t%:x%';

SQL_TEXT		       SQL_ID		    EXECUTIONS	    LOADS VERSION_COUNT
------------------------------ -------------------- ---------- ---------- -------------
insert into t values(:x,'test  0nhbks92x50kk		 10000		1	      1
bind variable')

执行计划只有一个，被执行了10000次。

二、修改初始化参数
系统提供了一个初始化参数

SQL> show parameter cursor_sharing
NAME				     TYPE			       VALUE
------------------------            -------------------               ------------------
cursor_sharing			     string			       EXACT

CURSOR_SHARING determines what kind of SQL statements can share the same
cursors.
Values:
FORCE
Allows the creation of a new cursor if sharing an existing cursor, or if the cursor
plan is not optimal.

SIMILAR
Causes statements that may differ in some literals, but are otherwise identical, to
share a cursor, unless the literals affect either the meaning of the statement or the
degree to which the plan is optimized.

EXACT
Only allows statements with identical text to share the same cursor

1. 清空shared_pool

SQL> set pagesize 10000
SQL> set linesize 200
SQL> col SQL_TEXT for a50
SQL> col SQL_ID for 520
SQL> col SQL_ID for a20
SQL> alter system flush shared_pool;

System altered.

SQL> SELECT sql_text,
sql_id,
EXECUTIONS,
LOADS,
VERSION_COUNT
FROM v$sqlarea
WHERE sql_text LIKE 'insert into t%';   2    3    4    5    6    7

no rows selected

SQL>

2. 将cursor_sharing改为FORCE，执行查询

SQL>  alter session set cursor_sharing=force;

Session altered.

Elapsed: 00:00:00.02
SQL> declare
begin
for i in 1 .. 10000 loop
execute immediate 'insert into t values('||i||',''test bind variable'')';
end loop;
commit;
end;
/  2    3    4    5    6    7    8

PL/SQL procedure successfully completed.

Elapsed: 00:00:01.15

3. 查看游标情况

SQL> SELECT sql_text,
sql_id,
EXECUTIONS,
LOADS,
VERSION_COUNT
FROM v$sqlarea
WHERE sql_text LIKE 'insert into t%';

no rows selected

SQL> /

SQL_TEXT					   SQL_ID		EXECUTIONS	LOADS VERSION_COUNT
-------------------------------------------------- -------------------- ---------- ---------- -------------
insert into t values(9966,'test bind variable')    07xgdm0uwn5gb		 1	    3		  1
insert into t values(10000,'test bind variable')   76rf2hx2w45m1		 1	    3		  1
insert into t values(9969,'test bind variable')    bfayz4q1j4b37		 1	    3		  1
insert into t values(9984,'test bind variable')    48t1dy0tahgxh		 1	    3		  1
insert into t values(9998,'test bind variable')    04bhmfjq8hhtu		 1	    3		  1
insert into t values(9967,'test bind variable')    0njsupf834kn0		 1	    3		  1
insert into t values(9999,'test bind variable')    6uhdudx8k4rv3		 1	    3		  1
insert into t values(9975,'test bind variable')    0tqj0jcamsspu		 1	    3		  1
insert into t values(9957,'test bind variable')    a6as35h2wwu00		 1	    3		  1
insert into t values(9970,'test bind variable')    58m581pqq8v4j		 1	    3		  1
insert into t values(9982,'test bind variable')    3gh1q9f2wnxr8		 1	    3		  1
insert into t values(9977,'test bind variable')    1t0t0jz9y0zg9		 1	    3		  1
insert into t values(9988,'test bind variable')    111vappsrszy7		 1	    3		  1
insert into t values(9994,'test bind variable')    8kvjy7tns10vq		 1	    3		  1
insert into t values(9963,'test bind variable')    c1w951tadx4tb		 1	    3		  1
insert into t values(9993,'test bind variable')    10h1mbxvtt9tm		 1	    3		  1
insert into t values(9981,'test bind variable')    gv77ng7kndhty		 1	    3		  1
insert into t values(9978,'test bind variable')    0v7773365tj70		 1	    3		  1
insert into t values(9974,'test bind variable')    astu71gzn1uw5		 1	    3		  1
insert into t values(9960,'test bind variable')    09d0bxcsndxzm		 1	    3		  1
insert into t values(9959,'test bind variable')    7ht6qzyy0jz5w		 1	    3		  1
insert into t values(9965,'test bind variable')    dgz4fbhzgpzh4		 1	    3		  1
insert into t values(9989,'test bind variable')    3srf852y19zx6		 1	    3		  1
insert into t values(9995,'test bind variable')    1r6tp423v613x		 1	    3		  1
insert into t values(9976,'test bind variable')    9vxdayk3yq1nn		 1	    3		  1
insert into t values(9958,'test bind variable')    9ptg2jd30k6d8		 1	    3		  1
insert into t values(9968,'test bind variable')    akt2u5gn1y9kp		 1	    3		  1
insert into t values(9992,'test bind variable')    ch4rx2b3ja9x8		 1	    3		  1
insert into t values(9962,'test bind variable')    a2p68fsk6abwz		 1	    3		  1
insert into t values(9997,'test bind variable')    f0474tah8ubzq		 1	    3		  1
insert into t values(9972,'test bind variable')    gzqpvbrsn6ggk		 1	    3		  1
insert into t values(9983,'test bind variable')    ah9r6ghzsugmp		 1	    3		  1
insert into t values(9979,'test bind variable')    2cvqu9h4wagva		 1	    3		  1
insert into t values(9996,'test bind variable')    3h90mc46sqmzr		 1	    3		  1
insert into t values(9961,'test bind variable')    7t8njvfx8fn4y		 1	    3		  1
insert into t values(9987,'test bind variable')    1qxhj0g7cuw8u		 1	    3		  1
insert into t values(9991,'test bind variable')    5n2jahrk5z258		 1	    3		  1
insert into t values(:"SYS_B_0",:"SYS_B_1")	   950r47takm3c4	      9953	    1		  1
insert into t values(9971,'test bind variable')    fyb5pvjuqz4d0		 1	    3		  1
insert into t values(9955,'test bind variable')    1adu3pctt76bp		 1	    3		  1
insert into t values(9990,'test bind variable')    62pp4zqc9r767		 1	    3		  1
insert into t values(9973,'test bind variable')    adb60k3nxr9mk		 1	    3		  1
insert into t values(9985,'test bind variable')    gz4hry47rzhvt		 1	    3		  1
insert into t values(9986,'test bind variable')    b54fdtcu47v0d		 1	    3		  1
insert into t values(9980,'test bind variable')    fvwh43nh6zvhk		 1	    3		  1
insert into t values(9956,'test bind variable')    1vcjq6rm9gx72		 1	    3		  1
insert into t values(9964,'test bind variable')    a06un7tf1rxgu		 1	    3		  1
insert into t values(9954,'test bind variable')    0nb4synx6bxqv		 1	    3		  1

48 rows selected.

SQL>

4. 再次清空shared_pool

SQL> alter system flush shared_pool;

System altered.

5. 将cursor_sharing改为SIMILAR，执行查询

SQL> alter session set cursor_sharing=similar;

Session altered.

Elapsed: 00:00:00.03
SQL> declare
begin
for i in 1 .. 10000 loop
execute immediate 'insert into t values('||i||',''test bind variable'')';
end loop;
commit;
end;
/  2    3    4    5    6    7    8

PL/SQL procedure successfully completed.

Elapsed: 00:00:01.14

5. 查看共享游标

SQL> SELECT sql_text,
sql_id,
EXECUTIONS,
LOADS,
VERSION_COUNT
FROM v$sqlarea
WHERE sql_text LIKE 'insert into t%';  2    3    4    5    6    7

SQL_TEXT					   SQL_ID		EXECUTIONS	LOADS VERSION_COUNT
-------------------------------------------------- -------------------- ---------- ---------- -------------
insert into t values(9966,'test bind variable')    07xgdm0uwn5gb		 1	    4		  1
insert into t values(10000,'test bind variable')   76rf2hx2w45m1		 1	    4		  1
insert into t values(9969,'test bind variable')    bfayz4q1j4b37		 1	    4		  1
insert into t values(9984,'test bind variable')    48t1dy0tahgxh		 1	    4		  1
insert into t values(9998,'test bind variable')    04bhmfjq8hhtu		 1	    4		  1
insert into t values(9967,'test bind variable')    0njsupf834kn0		 1	    4		  1
insert into t values(9999,'test bind variable')    6uhdudx8k4rv3		 1	    4		  1
insert into t values(9975,'test bind variable')    0tqj0jcamsspu		 1	    4		  1
insert into t values(9957,'test bind variable')    a6as35h2wwu00		 1	    4		  1
insert into t values(9970,'test bind variable')    58m581pqq8v4j		 1	    4		  1
insert into t values(9982,'test bind variable')    3gh1q9f2wnxr8		 1	    4		  1
insert into t values(9977,'test bind variable')    1t0t0jz9y0zg9		 1	    4		  1
insert into t values(9988,'test bind variable')    111vappsrszy7		 1	    4		  1
insert into t values(9994,'test bind variable')    8kvjy7tns10vq		 1	    4		  1
insert into t values(9963,'test bind variable')    c1w951tadx4tb		 1	    4		  1
insert into t values(9993,'test bind variable')    10h1mbxvtt9tm		 1	    4		  1
insert into t values(9981,'test bind variable')    gv77ng7kndhty		 1	    4		  1
insert into t values(9978,'test bind variable')    0v7773365tj70		 1	    4		  1
insert into t values(9974,'test bind variable')    astu71gzn1uw5		 1	    4		  1
insert into t values(9960,'test bind variable')    09d0bxcsndxzm		 1	    4		  1
insert into t values(9959,'test bind variable')    7ht6qzyy0jz5w		 1	    4		  1
insert into t values(9965,'test bind variable')    dgz4fbhzgpzh4		 1	    4		  1
insert into t values(9989,'test bind variable')    3srf852y19zx6		 1	    4		  1
insert into t values(9995,'test bind variable')    1r6tp423v613x		 1	    4		  1
insert into t values(9976,'test bind variable')    9vxdayk3yq1nn		 1	    4		  1
insert into t values(9958,'test bind variable')    9ptg2jd30k6d8		 1	    4		  1
insert into t values(9968,'test bind variable')    akt2u5gn1y9kp		 1	    4		  1
insert into t values(9992,'test bind variable')    ch4rx2b3ja9x8		 1	    4		  1
insert into t values(9962,'test bind variable')    a2p68fsk6abwz		 1	    4		  1
insert into t values(9997,'test bind variable')    f0474tah8ubzq		 1	    4		  1
insert into t values(9972,'test bind variable')    gzqpvbrsn6ggk		 1	    4		  1
insert into t values(9983,'test bind variable')    ah9r6ghzsugmp		 1	    4		  1
insert into t values(9979,'test bind variable')    2cvqu9h4wagva		 1	    4		  1
insert into t values(9996,'test bind variable')    3h90mc46sqmzr		 1	    4		  1
insert into t values(9961,'test bind variable')    7t8njvfx8fn4y		 1	    4		  1
insert into t values(9987,'test bind variable')    1qxhj0g7cuw8u		 1	    4		  1
insert into t values(9991,'test bind variable')    5n2jahrk5z258		 1	    4		  1
insert into t values(:"SYS_B_0",:"SYS_B_1")	   950r47takm3c4	      9953	    1		  1
insert into t values(9971,'test bind variable')    fyb5pvjuqz4d0		 1	    4		  1
insert into t values(9955,'test bind variable')    1adu3pctt76bp		 1	    4		  1
insert into t values(9990,'test bind variable')    62pp4zqc9r767		 1	    4		  1
insert into t values(9973,'test bind variable')    adb60k3nxr9mk		 1	    4		  1
insert into t values(9985,'test bind variable')    gz4hry47rzhvt		 1	    4		  1
insert into t values(9986,'test bind variable')    b54fdtcu47v0d		 1	    4		  1
insert into t values(9980,'test bind variable')    fvwh43nh6zvhk		 1	    4		  1
insert into t values(9956,'test bind variable')    1vcjq6rm9gx72		 1	    4		  1
insert into t values(9964,'test bind variable')    a06un7tf1rxgu		 1	    4		  1
insert into t values(9954,'test bind variable')    0nb4synx6bxqv		 1	    4		  1

48 rows selected.

和cursor_sharing=FORCE时，情况一样。

这两种方法都不推荐使用，有bug 。建议规范前台业务查询，尽量使用绑定变量。