如何確定哪個查詢在SSMS 2014中效率更高

Question

使用SSMS，我如何確定哪個查詢更高效？我更喜歡A，但我被告知子查詢在Transmission中每行都執行一次，因此B是首選。

甲

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 

編輯：這是統計使用SET STATISTICS ON：

A.

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，據我所知，效率沒有差別。

編輯2：所以我現在得到它：由於子查詢不是correlated子查詢，它只執行一次。 https://en.wikipedia.org/wiki/Correlated_subquery（感謝@destination_data的鏈接）

Answer 1

這將需要檢查實際的執行計劃，但我懷疑這些將會是相同的。這些子查詢應該只執行一次，因爲它們不相關，引擎對於查看這些內容非常好。

如果你真的想進入細節檢查從Grant Fritchey免費電子書。 https://www.red-gate.com/library/sql-server-execution-plans-2nd-edition

Answer 2

一種方法來比較查詢在SSMS：

在SSMS，包括查詢一個標籤。選擇查詢菜單，然後選擇「包括客戶端統計」

註釋掉其中一個查詢，然後運行另一個查詢。選擇「客戶統計」標籤並查看統計。

現在註釋掉其他查詢，取消註釋第一個，然後再次運行。這兩個統計數據將顯示在客戶統計選項卡中，您可以輕鬆查看哪個更有效。