在存储过程中使用合并需要3倍的时间才能执行

Question

我的应用程序检查帐号字段中是否存在帐号。如果不存在，则为@AccountNumber参数输入'000000'。在SQLSMS中进行测试以检查执行时间时，我注意到使用合并检查值所花费的时间几乎是执行时间的3倍。由于两个查询都使用帐号，为什么下面的示例中的执行时间不完全相同？请注意，在第一个示例中，我结合使用了参数，在第二个示例中，我仅使用了帐号。

查询返回18,000行，每行： 加载37秒：

   Declare @AccountNumber varchar(15) = '001234'
   Declare @EmployeeNumber varchar (15) = ''
   Declare @AccountType varchar (20) = 'Account Type'

    select DISTINCT AccountTable.ACCOUNT_TYPE, 
    PayrollTable.EmployeeNumber, 
    AccountTable.ACCOUNT_NUMBER 

    from AccountTable 
    LEFT JOIN PayrollTable
    ON AccountTable.ACCOUNT_NUMBER = PayrollTable.ACCOUNT_NUMBER

    where (coalesce(@AccountNumber,'') = '000000' or AccountTable.ACCOUNT_NUMBER = @AccountNumber)
    AND (coalesce(@EmployeeNumber,'') = '' or PayrollTable.EmployeeNumber = @EmployeeNumber)
    AND (coalesce(@AccountType,'') = 'Account Type' or AccountTable.ACCOUNT_TYPE = @AccountType)

加载12秒：

   Declare @AccountNumber varchar(15) = '001234'
   Declare @EmployeeNumber varchar (15) = ''
   Declare @AccountType varchar (20) = 'Account Type'

    select DISTINCT AccountTable.ACCOUNT_TYPE, 
    PayrollTable.EmployeeNumber, 
    AccountTable.ACCOUNT_NUMBER 

    from AccountTable 
    LEFT JOIN PayrollTable
    ON AccountTable.ACCOUNT_NUMBER = PayrollTable.ACCOUNT_NUMBER

    where (AccountTable.ACCOUNT_NUMBER = @AccountNumber)
    AND (coalesce(@EmployeeNumber,'') = '' or PayrollTable.EmployeeNumber = @EmployeeNumber)
    AND (coalesce(@AccountType,'') = 'Account Type' or AccountTable.ACCOUNT_TYPE = @AccountType)