sql-server - SQL Server 查询使用参数比使用常量字符串花费更长的时间

Question

我遇到了 MS SQL Server 2008 的问题，即：

当我使用硬编码字符串作为参数执行查询时，我的查询运行得很快，但是当我使用字符串参数时，查询需要更长的时间！
常量字符串查询需要 1 秒，而另一个需要 11 秒。

下面是代码：

常量字符串（1 秒）：

     SELECT * 
FROM   VIEWCONTENTS 
WHERE  COUNTRY = 'ZA' 
       AND CONTENTTYPE = 'A' 
       AND TASK = 'R23562'; 

参数化（11 秒）：

DECLARE @country AS CHAR(2); 

SET @country = 'ZA'; 

SELECT * 
FROM   VIEWCONTENTS 
WHERE  COUNTRY = @country 
       AND CONTENTTYPE = 'A' 
       AND TASK = 'R23562' 

Answer 1

在查询结束时使用OPTION (RECOMPILE) 。所以：

DECLARE @country AS CHAR(2); 

SET @country = 'ZA'; 

SELECT * 
FROM   VIEWCONTENTS 
WHERE  COUNTRY = @country 
       AND CONTENTTYPE = 'A' 
       AND TASK = 'R23562'
OPTION (RECOMPILE)

Answer 2

这会产生什么？

DECLARE @country AS VARCHAR(2); 
SET @country = 'ZA'; 

SELECT * 
FROM   VIEWCONTENTS 
WHERE  COUNTRY = @country 
       AND CONTENTTYPE = 'A' 
       AND TASK = 'R23562' 

这个怎么样？

DECLARE @country AS CHAR(2); 
DECLARE @country1 AS VARCHAR(2); 
SET @country = 'ZA';
SET @country1 = @country; 

SELECT * 
FROM   VIEWCONTENTS 
WHERE  COUNTRY = @country 
       AND CONTENTTYPE = 'A' 
       AND TASK = 'R23562'