关于inner jion, left jion ,right jion之间的执行性能，期望高手一起探讨

关于inner jion, left jion ,right jion之间的执行性能，期望高手一起探讨

首先我们先创建两个表：

CREATE   TABLE   [ dbo ] . [ Roles ] (
[ Name ]   [ nvarchar ] ( 50 )  NOT   NULL ,
[ CreatedAt ]   [ datetime ]   NOT   NULL ,
[ UpdatedAt ]   [ datetime ]   NOT   NULL ,
[ Deleted ]   [ bit ]   NOT   NULL ,
[ RoleStatus ]   [ int ]   NOT   NULL ,
[ ID ]   [ uniqueidentifier ]   NOT   NULL ,
[ SystemID ]   [ uniqueidentifier ]   NOT   NULL ,
[ RoleKind ]   [ int ]   NOT   NULL ,
  CONSTRAINT   [ PK_Roles ]   PRIMARY   KEY   CLUSTERED  
(
[ ID ]   ASC
) WITH  (PAD_INDEX   =   OFF , STATISTICS_NORECOMPUTE   =   OFF , IGNORE_DUP_KEY  =   OFF , ALLOW_ROW_LOCKS   =   ON , ALLOW_PAGE_LOCKS   =   ON )  ON   [ PRIMARY ]
)  ON   [ PRIMARY ]


GO


EXEC  sys.sp_addextendedproperty  @name = N ' MS_Description ' ,  @value = N ' 1=默认管理员角色，2=普通角色 '  ,  @level0type = N ' SCHEMA ' , @level0name = N ' dbo ' ,  @level1type = N ' TABLE ' , @level1name = N ' Roles ' ,  @level2type = N ' COLUMN ' , @level2name = N ' RoleKind '
GO


ALTER   TABLE   [ dbo ] . [ Roles ]   ADD    CONSTRAINT   [ DF_Roles_Deleted ]    DEFAULT  (( 0 ))  FOR   [ Deleted ]
GO


ALTER   TABLE   [ dbo ] . [ Roles ]   ADD    CONSTRAINT   [ DF_Roles_RoleStatus ]    DEFAULT  (( 1 ))  FOR   [ RoleStatus ]
GO


ALTER   TABLE   [ dbo ] . [ Roles ]   ADD    CONSTRAINT   [ DF_Roles_RoleKind ]    DEFAULT  (( 2 ))  FOR   [ RoleKind ]
GO

CREATE   TABLE   [ dbo ] . [ RoleFunctions ] (
[ MappingID ]   [ int ]   IDENTITY ( 1 , 1 )  NOT   NULL ,
[ Status ]   [ int ]   NOT   NULL ,
[ RoleID ]   [ uniqueidentifier ]   NOT   NULL ,
[ FunctionID ]   [ uniqueidentifier ]   NOT   NULL ,
  CONSTRAINT   [ PK_RoleFunctions ]   PRIMARY   KEY   CLUSTERED  
(
[ MappingID ]   ASC
) WITH  (PAD_INDEX   =   OFF , STATISTICS_NORECOMPUTE   =   OFF , IGNORE_DUP_KEY  =   OFF , ALLOW_ROW_LOCKS   =   ON , ALLOW_PAGE_LOCKS   =   ON )  ON   [ PRIMARY ]
)  ON   [ PRIMARY ]


GO


ALTER   TABLE   [ dbo ] . [ RoleFunctions ]   ADD    CONSTRAINT   [ DF_RoleFunctions_Status ]    DEFAULT  (( 0 ))  FOR   [ Status ]
GO

以下是两个条SQL查询语句，但是不同的是一个是用inner jion，一个是用left jion，但是两个查询语句的执行计划差距非常大

/* ***** Script for SelectTopNRows command from SSMS  ***** */
SELECT   [ MappingID ]
      , [ Status ]
      , [ RoleID ]
      , [ FunctionID ]
   FROM   [ RoleFunctions ]
   left   join  roles  on  roles.id  =  rolefunctions.functionid


SELECT   [ MappingID ]
      , [ Status ]
      , [ RoleID ]
      , [ FunctionID ]
   FROM   [ RoleFunctions ]
   inner   join  roles  on  roles.id  =  rolefunctions.functionid

期望高手给予讲解……