Question

使用SSMS，如何确定哪个查询更有效？我更喜欢A，但我被告知子查询对Transmission中的每一行执行一次，因此B是首选。

A

Update t set t.transmission_status_id = 
(select transmission_status_id from transmission_status where code = 'TRANSLATED')
from transmission t
where t.transmission_status_id = 
(select transmission_status_id from transmission_status where code = 'RECEIVED')

乙

declare @transmission_status_TRANSLATED INT = (select transmission_status_id from transmission_status where code = 'TRANSLATED')
declare @transmision_status_RECEIVED INT = (select transmission_status_id from transmission_status where code = 'RECEIVED')

Update t set t.transmission_status_id = @transmission_status_TRANSLATED
from transmission t 
where t.transmission_status_id = @transmision_status_RECEIVED

编辑：这是使用SET STATISTICS ON的统计数据：

一个。

Table 'transmission_status'. Scan count 1, logical reads 2, physical reads 0, 
read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

Table 'transmission_status'. Scan count 1, logical reads 2, physical reads 0, 
read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

Table 'transmission'. Scan count 1, logical reads 778, physical reads 0,
read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

乙

Table 'transmission'. Scan count 1, logical reads 778, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'transmission_status'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

所以，据我所知，效率没有差别。

编辑2：所以我现在得到它：由于子查询不是correlated子查询，它只执行一次。 https://en.wikipedia.org/wiki/Correlated_subquery（感谢链接的@destination_data。）

Answer 1

这需要检查实际的执行计划，但我怀疑它们将是相同的。这些子查询应该只执行一次，因为它们没有相关性，引擎非常适合看到那些东西。

如果您真的想了解详情，请查看Grant Fritchey的这本免费电子书。 https://www.red-gate.com/library/sql-server-execution-plans-2nd-edition

Answer 2

与SSMS中的查询进行比较的方法：

在SSMS中，在单个选项卡上包含两个查询。选择“查询”菜单，然后选择“包含客户端统计信息”

注释掉其中一个查询，然后运行另一个查询。选择“客户统计”标签并查看统计信息。

现在注释掉另一个查询，取消注释第一个查询，然后再次运行。这两个统计信息都将显示在“客户端统计信息”选项卡中，您可以轻松查看哪个更有效。

如何确定哪个查询在SSMS 2014中更有效

2 个答案: