Question

CREATE TABLE [dbo].[Comment] 
(
  [Id] [int] IDENTITY(1,1) NOT NULL,
  [UserId] [int] NOT NULL,
  [Comment] [nvarchar](1024) NOT NULL,
  [Created] [datetime] NOT NULL CONSTRAINT [DF_Comment_Created]  DEFAULT (getdate()),
  [ContentId] [int] NOT NULL,
  [ParentId] [int] NULL,
  CONSTRAINT [PK_Comment] 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]

CREATE TABLE [dbo].[CommentLike]
(
  [CommentId] [int] NOT NULL,
  [UserId] [int] NOT NULL,
  CONSTRAINT [PK_CommentLike] PRIMARY KEY CLUSTERED 
  (
    [CommentId] ASC,
    [UserId] ASC
  )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
  ) ON [PRIMARY]

CREATE TABLE [dbo].[User]
(
  [Id] [int] IDENTITY(1,1) NOT NULL,
  [Username] [nvarchar](64) NOT NULL,
  [FirstName] [nvarchar](50) NOT NULL,
  [LastName] [nvarchar](50) NOT NULL,
  [Email] [nvarchar](255) NOT NULL,
CONSTRAINT [PK_User] 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]

Comment (ContentId, ParentId)上有一个非聚集索引。

简而言之，Comment表格包含Id和ParentId的评论和子评论。 CommentLike表包含评论和子评论的喜欢（每个用户一个）。

Comment表包含大约8000行，CommentLike表包含1800。

我创建了一个查询，列出了评论（只有顶级评论），子评论计数，如计数，还有一个值，表明提供的用户是否喜欢每个评论。整个查询在where子句中的ContentId上进行过滤（只是表示另一个系统中id的唯一整数值）

我有一个版本使用子查询而另一个版本加入（在子查询上）。

子查询版本：

select 
  c.Id,
  c.Comment,
  c.Created,
  c.ContentId,
  (select count(Id) from Comment where ParentId = c.Id) as SubComments,
  (select count(UserId) from CommentLike where CommentId = c.Id) as Likes,
  (select count(UserId) from CommentLike where CommentId = c.Id and UserId = @currentUserId) as CurrentUserIsLiking
from Comment c
where c.ContentId = @contentId and c.ParentId is null
group by
  c.Id, c.Comment, c.Created, c.ContentId

加入版本：

select
  c.Id,
  c.Comment,
  c.Created,
  c.ContentId,
  isnull(c2.SubComments, 0) as SubComments,
  isnull(cl.Likes, 0) as Likes,
  isnull(cl.CurrentUserIsLiking, 0) as CurrentUserIsLiking
from Comment c
left join
(
  select
    ParentId,
    count(Id) as SubComments
  from Comment 
  group by ParentId
) as c2
on c.Id = c2.ParentId
left join
(
  select
    CommentId,
    count(UserId) as Likes,
    count(case when UserId = @currentUserId then 1 else null end) as CurrentUserIsLiking
  from CommentLike 
  group by CommentId
) as cl
on c.Id = cl.CommentId
where c.ContentId = @contentId and c.ParentId is null
group by
  c.Id, c.Comment, c.Created, c.ContentId, 
  c2.SubComments, cl.Likes, cl.CurrentUserIsLiking

平均而言，两个版本的运行速度都低于600毫秒，但子查询版本的运行速度似乎总是比加入版本快20％。

问题：

无论表包含多少行，子查询版本总是比连接版本快。我一直认为，性能明智，连接比子查询更好，在这种情况下是不是真的？由于性能很重要，我想知道是否可以对任一版本进行任何优化，以使特定版本的性能优于另一个版本？

Answer 1

为什么要汇总join版本的外部查询？

select c.*, 
       p.SubComments,
       coalesce(cl.Likes, 0) as Likes,
       coalesce(cl.CurrentUserIsLiking, 0) as CurrentUserIsLiking
from Comment c left join
     (select ParentId, count(*) as SubComments
      from Comment
      group by ParentId
     ) p
     on p.ParentId = c.Id
     (select CommentId, count(UserId) as Likes,
             sum(case when UserId = @currentUserId then 1 else 0
                 end) as CurrentUserIsLiking
     from CommentLike 
     group by CommentId
    ) cl
    on c.Id = cl.CommentId
where c.ContentId = @contentId and c.ParentId is null;

因为外部查询限制了注释，所以使用相关子查询的版本更快是合理的。

在计算来自同一表的子项时的子查询与连接以及其他表

1 个答案: