SQL 查询得到很多超时

Question

我有一个大型数据库表 (SQL Server 2008)，其中存储了我的所有论坛消息（该表目前有超过 450 万个条目）。

这是表架构：

CREATE TABLE [dbo].[ForumMessage](
    [MessageId] [int] IDENTITY(1,1) NOT FOR REPLICATION NOT NULL,
    [ForumId] [int] NOT NULL,
    [MemberId] [int] NOT NULL,
    [Type] [tinyint] NOT NULL,
    [Status] [tinyint] NOT NULL,
    [Subject] [nvarchar](500) NOT NULL,
    [Body] [text] NOT NULL,
    [Posted] [datetime] NOT NULL,
    [Confirmed] [datetime] NULL,
    [ReplyToMessage] [int] NOT NULL,
    [TotalAnswers] [int] NOT NULL,
    [AvgRateing] [decimal](18, 2) NOT NULL,
    [TotalRated] [int] NOT NULL,
    [ReadCounter] [int] NOT NULL,
 CONSTRAINT [PK_GroupMessage] PRIMARY KEY CLUSTERED 
(
    [MessageId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

我看到不断出现的一个问题是，当我运行选择一条消息及其所有回复的存储过程时，我有时会从 SQL 服务器收到超时错误。

这是我的存储过程：

select fm1.[MessageId]
      ,fm1.[ForumId]
      ,fm1.[MemberId]
      ,fm1.[Type]
      ,fm1.[Status]
      ,fm1.[Subject]
    ,fm1.[Body]
      ,fm1.[Posted]
      ,fm1.[Confirmed]
      ,fm1.[ReplyToMessage]
      ,fm1.[TotalAnswers]
      ,fm1.[AvgRateing]
      ,fm1.[TotalRated]
      ,fm1.[ReadCounter],
     Member.NickName AS MemberNickName, Forum.Name as ForumName
from ForumMessage fm1 LEFT OUTER JOIN
                      Member ON fm1.MemberId = Member.MemberId INNER JOIN
                Forum On fm1.ForumId = Forum.ForumId
where MessageId = @MessageId or ReplyToMessage=@MessageId
order by MessageId 

我得到的错误如下所示：“超时已过期。在操作完成之前超时时间已过或服务器没有响应”

我正在查看执行计划，唯一看起来可疑的是，在 forummessage 表（我不明白为什么，因为我将它设置为集群，所以我希望它会更有效率）。我一直认为，当您搜索聚集索引时，查询应该非常有效。

有没有人知道我可以如何改进这个问题和这个查询以获取消息及其回复？

谢谢。

Answer 1

我想到了两个建议：

• 删除ugly OR 并为条件添加UNION（代码如下）
• 您必须在ReplyToMessage 上有一个非聚集索引

作为最后的手段，创建一个非聚集索引并将MessageId AND ReplyToMessage 放入其中。 （在这里查看我对另一个问题的回答Why does this Sql Statement (with 2 table joins) takes 5 mins to complete?）

---

代码：

select fm1.[MessageId]
      ,fm1.[ForumId]
      ,fm1.[MemberId]
      ,fm1.[Type]
      ,fm1.[Status]
      ,fm1.[Subject]
    ,fm1.[Body]
      ,fm1.[Posted]
      ,fm1.[Confirmed]
      ,fm1.[ReplyToMessage]
      ,fm1.[TotalAnswers]
      ,fm1.[AvgRateing]
      ,fm1.[TotalRated]
      ,fm1.[ReadCounter],
     Member.NickName AS MemberNickName, Forum.Name as ForumName
from ForumMessage fm1 LEFT OUTER JOIN
                      Member ON fm1.MemberId = Member.MemberId INNER JOIN
                Forum On fm1.ForumId = Forum.ForumId
where MessageId = @MessageId
UNION
select fm1.[MessageId]
      ,fm1.[ForumId]
      ,fm1.[MemberId]
      ,fm1.[Type]
      ,fm1.[Status]
      ,fm1.[Subject]
    ,fm1.[Body]
      ,fm1.[Posted]
      ,fm1.[Confirmed]
      ,fm1.[ReplyToMessage]
      ,fm1.[TotalAnswers]
      ,fm1.[AvgRateing]
      ,fm1.[TotalRated]
      ,fm1.[ReadCounter],
     Member.NickName AS MemberNickName, Forum.Name as ForumName
from ForumMessage fm1 LEFT OUTER JOIN
                      Member ON fm1.MemberId = Member.MemberId INNER JOIN
                Forum On fm1.ForumId = Forum.ForumId
where MessageId = @MessageId
order by MessageId 

Answer 2

根据您运行的 MS SQL Server 的版本，您还可以尝试使用分区表重新创建表以提高SELECT 的性能。

Answer 3

在ReplyToMessage上创建索引：

CREATE INDEX
        IX_ForumMessage_ReplyToMessage
ON      ForumMessage (ReplyToMessage)

这很可能会导致两个索引查找（通过 MessageId 上的 PRIMARY KEY 和 ReplyToMessage 上的索引）并包含合并或哈希连接，而不是您现在进行的全表扫描.

Answer 4

你为什么要ORDER BY MessageId，有必要订购吗？

尝试将您的SELECT 重构为SELECT FROM Forum，然后加入Member，最后加入LEFT JOIN ForumMessage。所以从小到大排列桌子