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HomeDatabaseMysql TutorialSQL参数化查询的另一个理由 命中执行计划

为了提高数据库运行的效率,我们需要尽可能的命中执行计划,这样就可以节省运行时间

1概述

SQL语言的本质就是一串伪代码,表达的是做什么,而不是怎么做的意思。如其它语言一样,SQL语句需要编译之后才能运行,所以每一条SQL是需要通过编译器解释才能运行的(在这之间还要做SQL的优化)。而这些步骤都是需要运行成本,所以在数据库中有一个叫做执行计划的东西,编译器会将编译过后的SQL存入执行计划当中,当遇到同样的SQL时,就直接调用执行计划来执行,而不需要再次编译。
通过对上面执行计划的认识,为了提高数据库运行的效率,我们需要尽可能的命中执行计划,这样就可以节省运行时间。

2相关SQL

2.1查看当前数据库中所有的执行计划:
代码如下:
SELECT cp.usecounts AS '使用次数'
,objtype AS '类型'
,st.[text] AS 'SQL文本'
,plan_handle AS '计划句柄'
FROM sys.dm_exec_cached_plans cp
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st
WHERE st.text not like '%sys%'

2.2删除执行计划
代码如下:
--删除所有计划
DBCC FREEPROCCACHE

2.3测试脚本(创建员工表,并向其插入1000条数据)
代码如下:
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[Employee]'))
DROP TABLE [dbo].Employee
GO
--人员表
CREATE TABLE dbo.Employee
(
id int,
name nvarchar(50)
);

--插入测试数据
DECLARE @I INT=0,@ENDI INT=1000;
WHILE(@IBEGIN
SET @I+=1;
INSERT dbo.Employee(id,name) VALUES(@I,'蒋大华'+CAST(@I AS NVARCHAR(20)));
END;

3测试执行计划

3.1 先执行删除所有执行计划,然后执行SELECT * FROM Employee ,最后查看执行计划(2.1中的查看执行计划脚本)如下图

即SQL SERVER会为每一条SQL建立一个执行计划,并将它缓存起来

3.2 再运行一次SQL: SELECT * FROM Employee,并查看执行计划

可以看到这个计划的重用次数为2,即这个计划被重用了;

3.3 修改SQL:SELECT * FROM Employee(在SELECT后多加一个空格),执行并查看执行计划

结果又新添加一个执行计划,即SQL SERVER认为这是两个不同的SQL语句并分别建立了执行计划;

4重用执行计划——使用参数化查询方法

4.1 未参数化SQL

代码如下:
string selectCmdText = string.Format(@"SELECT * FROM Employee WHERE name='{0}'",” 蒋大华1”);
SQLHelper.ExecuteNonQuery(SQLHelper.DefaulConnectiontString, System.Data.CommandType.Text, selectCmdText, null);

查看执行计划:

即当执行一个未参数化SQL时,SQL SERVER需要先将其转换成一个参数SQL并执行它。一共需要两执行计划

然后再执行下面的代码(查询的条件变了)
代码如下:
string selectCmdText = string.Format(@"SELECT * FROM Employee WHERE name='{0}'",” 蒋大华2”);
SQLHelper.ExecuteNonQuery(SQLHelper.DefaulConnectiontString, System.Data.CommandType.Text, selectCmdText, null);

查看执行计划

此时不需要再准备一个准备的SQL,但还是需要再产生一个执行计划,并缓存下来;

4.2 参数化SQL
代码如下:
SqlParameter[] param = { new SqlParameter("@name", txtEmployeeName.Text.Trim()) };
string selectCmdText = string.Format(@"SELECT * FROM Employee WHERE name=@name");
SQLHelper.ExecuteNonQuery(SQLHelper.DefaulConnectiontString, System.Data.CommandType.Text, selectCmdText, param);

输入参数并执行,然后查看执行计划:

只需要一个准备SQL,然后,输入不同的参数,并执行,再查看执行计划

重用执行计划,perfect...

5总结

总的来说,SQL语句在执行时,会生成执行计划并将它缓存起来,我们可以通过提高使用缓存中的执行计划次数,来减少数据库的压力。而使用参数化的SQL是一个很好的选择,参数化查询的作用不仅只有防止SQL注入,还可以提高缓存中执行计划使用次数。

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