相关疑难解决方法(0)

我有一个与此类似的数据库结构，

CREATE TABLE [dbo].[Dispatch](
    [DispatchId] [int] NOT NULL,
    [ContractId] [int] NOT NULL,
    [DispatchDescription] [nvarchar](50) NOT NULL,
CONSTRAINT [PK_Dispatch] PRIMARY KEY CLUSTERED 
(
    [DispatchId] ASC,
    [ContractId] 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

CREATE TABLE [dbo].[DispatchLink](
    [ContractLink1] [int] NOT NULL,
    [DispatchLink1] [int] NOT NULL,
    [ContractLink2] [int] NOT NULL,
    [DispatchLink2] [int] NOT NULL
) ON [PRIMARY]

GO
INSERT [dbo].[Dispatch] ([DispatchId], [ContractId], [DispatchDescription]) VALUES (1, 1, N'Test') …

为什么下面的查询很慢？

select count(*) 

from [dbo].[mt_dispatch_link] 
   , [dbo].[_mt_dispatch] [_mt_dispatch]

where   (mt_dispatch_link.contract_id_1 = _mt_dispatch.contract_id 
     and mt_dispatch_link.dispatch_id_1 = _mt_dispatch.dispatch_id)

   or   (mt_dispatch_link.contract_id_2 = _mt_dispatch.contract_id 
     and mt_dispatch_link.dispatch_id_2 = _mt_dispatch.dispatch_id)

这需要 10 多分钟，然后我倾向于在那个时候停止它。我的问题更多是关于如何理解查询计划。

查看查询计划，我可以看到底部聚集索引扫描返回大约 250000 条记录，但成本为 0% 并且它正在放入临时表中。

顶部索引扫描大约是 25000 条记录。

但是 95% 的成本来自嵌套连接。我应该从中得出什么结论？

上面的查询计划显示了两次索引扫描，是说它在做 25000 + 250000 次索引扫描，还是说它在做 25000 * 250000 次索引扫描？

如果我将查询更改为此（添加FORCESEEK）：

select count(*) 

from [dbo].[mt_dispatch_link] 
   , [dbo].[_mt_dispatch] [_mt_dispatch] 

    WITH (FORCESEEK)

where   (mt_dispatch_link.contract_id_1 = _mt_dispatch.contract_id 
     and mt_dispatch_link.dispatch_id_1 = _mt_dispatch.dispatch_id)

   or   (mt_dispatch_link.contract_id_2 = _mt_dispatch.contract_id 
     and mt_dispatch_link.dispatch_id_2 = _mt_dispatch.dispatch_id)

我最终得到了一个更好的计划，查询立即运行：

我在两个表上运行了更新统计信息。可惜没修好。表设计不是很好，所以我认为 SQL Server 并不真正理解，因此提出了一个糟糕的查询计划。有关 …