使用CASE作查询条件后，如何速度变得很慢，应怎样解决

使用CASE作查询条件后，怎么速度变得很慢，应怎样解决？
Select   *   from   vwOutHouseDsp
where   iDocStatus <=100   and   dDocDate> [email protected]   and   dDocDate <[email protected]
and   (cDocType= '销售发货 '   or   cDocType= '销售退货 ')
and   WhSysCode=case   when   @tcWhSysCode= ' '   then   WhSysCode   else   @tcWhSysCode   End  
and   ISysCode=case   when   @tcISysCode= ' '   then   ISysCode   else   @tcISysCode   End  
and   IBSysCode=case   when   @tcIBSysCode= ' '   then   IBSysCode   else   @tcIBSysCode  

以上这句在WHERE上使用CASE后查速度就变得很慢了，如果把
and   WhSysCode=case   when   @tcWhSysCode= ' '   then   WhSysCode   else   @tcWhSysCode   End  
and   ISysCode=case   when   @tcISysCode= ' '   then   ISysCode   else   @tcISysCode   End  
and   IBSysCode=case   when   @tcIBSysCode= ' '   then   IBSysCode   else   @tcIBSysCode  
这几句去掉，速度就正常了，怎么会这样呢？
另外，如果不用CASE，这种情况的条件查询怎样解决比较好呢？


------解决方案--------------------
把条件
@tcWhSysCode,@tcISysCode,@tcIBSysCode 默认设成null
create procedure protest
@tcWhSysCode int =null,
@tcISysCode int =null,
@tcIBSysCode int =null
as
Select * from vwOutHouseDsp
where iDocStatus <=100 and dDocDate> [email protected] and dDocDate <[email protected]
and (cDocType= '销售发货 ' or cDocType= '销售退货 ')
and WhSysCode=isnull(@tcWhSysCode,WhSysCode)
and ISysCode=isnull(@tcISysCode,ISysCode)
and IBSysCode=isnull(@tcIBSysCode,IBSysCode)