为什么这两个查询的执行计划如此不同？

Question

我有一个包含以下现有表格的数据库：

CREATE TABLE [dbo].[sites]
(
    [site_id] [uniqueidentifier] NOT NULL,
    [description] [nvarchar](80) NOT NULL,
    [parent_id] [uniqueidentifier] NULL,
    [domain_id] [int] NOT NULL,

    CONSTRAINT [PK_sites] PRIMARY KEY NONCLUSTERED ( site_id] ASC
)

ALTER TABLE [dbo].[sites] WITH CHECK 
  ADD CONSTRAINT [FK_sites_sites] 
  FOREIGN KEY([parent_id]) REFERENCES [dbo].[sites] ([site_id])

ALTER TABLE [dbo].[sites] CHECK CONSTRAINT [FK_sites_sites]

我添加了另一个看起来像这样的表：

CREATE TABLE [dbo].[site_map]
(
    [force_id] [int] NOT NULL,
    [source_id] [int] NOT NULL,
    [camera_id] [int] NOT NULL,
    [site_id] [uniqueidentifier] NOT NULL,

    CONSTRAINT [PK_site_map] PRIMARY KEY CLUSTERED 
        ([force_id] ASC, [source_id] ASC, [camera_id] ASC)
)

ALTER TABLE [dbo].[site_map] WITH CHECK 
  ADD CONSTRAINT [FK_site_map_sites] 
  FOREIGN KEY([site_id]) REFERENCES [dbo].[sites] ([site_id])

ALTER TABLE [dbo].[site_map] CHECK CONSTRAINT [FK_site_map_sites]

现在，当我执行此查询时：

SELECT 
    m.[site_id], c.[description], s.[site_id] AS ReaderId, s.[domain_id]
FROM 
    [dbo].[site_map] AS m
LEFT OUTER JOIN 
    [dbo].[sites] AS c ON m.[site_id] = c.[site_id]
LEFT OUTER JOIN 
    [dbo].[sites] AS s ON c.[parent_id] = s.[site_id]
WHERE 
    [force_id] = @forceId 
    AND [source_id] = @sourceid 
    AND [camera_id] = @cameraId

我得到一个执行两个嵌套循环的查询计划。但是，当我更改查询时，它的内容如下：

SELECT 
    m.[site_id], c.[description], s.[site_id] AS ReaderId, s.[domain_d]
FROM 
    [dbo].[site_map] AS m
INNER JOIN 
    [dbo].[sites] AS c ON m.[site_id] = c.[site_id]
INNER JOIN 
    [dbo].[sites] AS s ON c.[parent_id] = s.[site_id]
WHERE 
    [force_id] = @forceId 
    AND [source_id] = @sourceid 
    AND [camera_id] = @cameraId

查询计划执行四个嵌套循环！唯一改变的是连接类型。为什么查询计划如此不同？

Answer 1

您是否真的注意到性能差异？

我的猜测是INNER JOIN仍然更有效率。看看＆＃34;执行次数＆＃34;查询计划中每个嵌套循环操作的数量。我猜测LEFT JOINS有天真的嵌套循环操作，或者至少是一个效率较低的扫描输入，并且INNER JOIN的执行次数较少，可能不是左边的完整笛卡尔循环在x侧右侧，它们可能只在左侧每行执行一次。很难说没有看到查询计划。

我将SET STATISTICS IO打开，刷新您的缓存，看看每次操作得到多少逻辑读取，看看有什么区别。

Answer 2

（在SQL Server Compact DB上，可能表现不同）：

SELECT m.[site_id], c.[description], s.[site_id] AS ReaderId, s.[domain_id]
FROM [site_map] AS m
LEFT OUTER JOIN [sites] AS c ON m.[site_id] = c.[site_id]
LEFT OUTER JOIN [sites] AS s ON c.[parent_id] = s.[site_id]
WHERE [force_id] = 1 AND [source_id] = 1 AND [camera_id] = 1

SELECT m.[site_id], c.[description], s.[site_id] AS ReaderId, s.[domain_id]
FROM [site_map] AS m
INNER JOIN [sites] AS c ON m.[site_id] = c.[site_id]
INNER JOIN [sites] AS s ON c.[parent_id] = s.[site_id]
WHERE [force_id] = 1 AND [source_id] = 1 AND [camera_id] = 1

抱歉，我在第二个执行计划中看不到四个循环。如果您提供执行计划的屏幕截图，那将会很有帮助。